cluster/tier: add watermarks and policy driver

Backport fix 12039

This fix introduces infrastructure to support different
policies for promotion and demotion.

Currently the tier feature automatically promotes and demotes
files periodically based on access. This is good for testing
but too stringent for most real workloads. It makes it
difficult to fully utilize a hot tier- data will be demoted
before it is touched- its unlikely a 100GB hot SSD will have
all its data touched in a window of time.

A new parameter "mode" allows the user to pick promotion/demotion
polcies.

The "test mode" will be used for *.t and other general testing.
This is the current mechanism.

The "cache mode" introduces watermarks. The watermarks
represent levels of data residing on the hot tier.

"cache mode" policy:

The % the hot tier is full is called P.

Do not promote or demote more than D MB or F files.

A random number [0-100] is called R.

Rules for migration:

if (P < watermark_low) don't demote, always promote.

if (P >= watermark_low) && (P < watermark_hi) demote if R < P; promote if R > P.

if (P > watermark_hi) always demote, don't promote.

gluster volume set {vol} cluster.watermark-hi %
gluster volume set {vol} cluster.watermark-low %
gluster volume set {vol} cluster.tier-max-mb {D}
gluster volume set {vol} cluster.tier-max-files {F}
gluster volume set {vol} cluster.tier-mode {test|cache}

> Change-Id: I157f19667ec95aa1d53406041c1e3b073be127c2
> BUG: 1257911
> Signed-off-by: Dan Lambright <dlambrig@redhat.com>
> Reviewed-on: http://review.gluster.org/12039
> Tested-by: Gluster Build System <jenkins@build.gluster.com>
> Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Signed-off-by: Dan Lambright <dlambrig@redhat.com>

Signed-off-by: Dan Lambright <dlambrig@redhat.com>

Conflicts:
	xlators/cluster/dht/src/dht-rebalance.c
	xlators/cluster/dht/src/tier.c

Change-Id: Ibfe6b89563ceab98708325cf5d5ab0997c64816c
BUG: 1270527
Reviewed-on: http://review.gluster.org/12330
Tested-by: NetBSD Build System <jenkins@build.gluster.org>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Dan Lambright <dlambrig@redhat.com>
Tested-by: Dan Lambright <dlambrig@redhat.com>

6 files changed, 592 insertions, 110 deletions

diff --git a/xlators/cluster/dht/src/dht-common.h b/xlators/cluster/dht/src/dht-common.h
index b1d12c84a9f..26cf27a8676 100644
--- a/xlators/cluster/dht/src/dht-common.h
+++ b/xlators/cluster/dht/src/dht-common.h
@@ -337,6 +337,29 @@ struct dht_container {
         dict_t          *migrate_data;
 };
 
+typedef enum tier_mode_ {
+        TIER_MODE_NONE = 0,
+        TIER_MODE_TEST,
+        TIER_MODE_WM
+} tier_mode_t;
+
+typedef struct gf_tier_conf {
+        int                          is_tier;
+        int                          watermark_hi;
+        int                          watermark_low;
+        int                          watermark_last;
+        fsblkcnt_t                   blocks_total;
+        fsblkcnt_t                   blocks_used;
+        int                          percent_full;
+        uint64_t                     max_migrate_bytes;
+        int                          max_migrate_files;
+        tier_mode_t                  mode;
+        int                          tier_promote_frequency;
+        int                          tier_demote_frequency;
+        uint64_t                     st_last_promoted_size;
+        uint64_t                     st_last_demoted_size;
+} gf_tier_conf_t;
+
 struct gf_defrag_info_ {
         uint64_t                     total_files;
         uint64_t                     total_data;
@@ -357,8 +380,7 @@ struct gf_defrag_info_ {
         gf_boolean_t                 stats;
         uint32_t                     new_commit_hash;
         gf_defrag_pattern_list_t    *defrag_pattern;
-        int                          tier_promote_frequency;
-        int                          tier_demote_frequency;
+        gf_tier_conf_t               tier_conf;
 
         /*Data Tiering params for scanner*/
         uint64_t                     total_files_promoted;
@@ -1093,5 +1115,7 @@ int32_t dht_set_local_rebalance (xlator_t *this, dht_local_t *local,
                                  struct iatt *stbuf,
                                  struct iatt *prebuf,
                                  struct iatt *postbuf, dict_t *xdata);
+void
+dht_build_root_loc (inode_t *inode, loc_t *loc);
 
 #endif/* _DHT_H */
diff --git a/xlators/cluster/dht/src/dht-rebalance.c b/xlators/cluster/dht/src/dht-rebalance.c
index 6471d54cdfe..69c64816909 100644
--- a/xlators/cluster/dht/src/dht-rebalance.c
+++ b/xlators/cluster/dht/src/dht-rebalance.c
@@ -1331,6 +1331,15 @@ dht_migrate_file (xlator_t *this, loc_t *loc, xlator_t *from, xlator_t *to,
                 }
         }
 
+        /* store size of previous migrated file  */
+        if (defrag->tier_conf.is_tier) {
+                if (from == conf->subvolumes[0]) {
+                        defrag->tier_conf.st_last_promoted_size = stbuf.ia_size;
+                } else {
+                        defrag->tier_conf.st_last_demoted_size = stbuf.ia_size;
+                }
+        }
+
         /* The src file is being unlinked after this so we don't need
            to clean it up */
         clean_src = _gf_false;
diff --git a/xlators/cluster/dht/src/dht-shared.c b/xlators/cluster/dht/src/dht-shared.c
index 5fff3e8f793..dc5211a55fd 100644
--- a/xlators/cluster/dht/src/dht-shared.c
+++ b/xlators/cluster/dht/src/dht-shared.c
@@ -983,17 +983,32 @@ struct volume_options options[] = {
         { .key  = {"write-freq-threshold"},
           .type = GF_OPTION_TYPE_INT,
           .default_value = "0",
-          .description = "Defines the write fequency "
-                        "that would be considered hot"
         },
 
         { .key  = {"read-freq-threshold"},
           .type = GF_OPTION_TYPE_INT,
           .default_value = "0",
-          .description = "Defines the read fequency "
-                        "that would be considered hot"
         },
-
+        { .key         = {"watermark-hi"},
+          .type = GF_OPTION_TYPE_PERCENT,
+          .default_value = "90",
+        },
+        { .key         = {"watermark-low"},
+          .type = GF_OPTION_TYPE_PERCENT,
+          .default_value = "75",
+        },
+        { .key         = {"tier-mode"},
+          .type = GF_OPTION_TYPE_STR,
+          .default_value = "test",
+        },
+        { .key         = {"tier-max-mb"},
+          .type = GF_OPTION_TYPE_INT,
+          .default_value = "1000",
+        },
+        { .key         = {"tier-max-files"},
+          .type = GF_OPTION_TYPE_INT,
+          .default_value = "5000",
+        },
         /* switch option */
         { .key  = {"pattern.switch.case"},
           .type = GF_OPTION_TYPE_ANY
diff --git a/xlators/cluster/dht/src/tier.c b/xlators/cluster/dht/src/tier.c
index c93281bc785..397ac6b86ad 100644
--- a/xlators/cluster/dht/src/tier.c
+++ b/xlators/cluster/dht/src/tier.c
@@ -118,6 +118,120 @@ out:
         return ret;
 }
 
+int
+tier_do_migration (xlator_t *this, int promote)
+{
+        gf_defrag_info_t       *defrag = NULL;
+        dht_conf_t             *conf   = NULL;
+        long                    rand = 0;
+        int                     migrate = 0;
+        gf_tier_conf_t         *tier_conf = NULL;
+
+        conf = this->private;
+        if (!conf)
+                goto exit;
+
+        defrag = conf->defrag;
+        if (!defrag)
+                goto exit;
+
+        if (defrag->tier_conf.mode != TIER_MODE_WM) {
+                migrate = 1;
+                goto exit;
+        }
+
+        tier_conf = &defrag->tier_conf;
+
+        switch (tier_conf->watermark_last) {
+        case TIER_WM_LOW:
+                migrate = promote ? 1 : 0;
+                break;
+        case TIER_WM_HI:
+                migrate = promote ? 0 : 1;
+                break;
+        case TIER_WM_MID:
+                rand = random() % 100;
+                if (promote) {
+                        migrate = (rand > tier_conf->percent_full);
+                } else {
+                        migrate = (rand <= tier_conf->percent_full);
+                }
+                break;
+        }
+
+exit:
+        return migrate;
+}
+
+int
+tier_check_watermark (xlator_t *this, loc_t *root_loc)
+{
+        tier_watermark_op_t     wm = TIER_WM_NONE;
+        int                     ret = -1;
+        gf_defrag_info_t       *defrag = NULL;
+        dht_conf_t             *conf   = NULL;
+        dict_t                 *xdata  = NULL;
+        struct statvfs          statfs = {0, };
+        gf_tier_conf_t         *tier_conf = NULL;
+
+        conf = this->private;
+        if (!conf)
+                goto exit;
+
+        defrag = conf->defrag;
+        if (!defrag)
+                goto exit;
+
+        tier_conf = &defrag->tier_conf;
+
+        if (tier_conf->mode != TIER_MODE_WM) {
+                ret = 0;
+                goto exit;
+        }
+
+        /* Find how much free space is on the hot subvolume. Then see if that value */
+        /* is less than or greater than user defined watermarks. Stash results in */
+        /* the tier_conf data structure. */
+        ret = syncop_statfs (conf->subvolumes[1], root_loc, &statfs,
+                             xdata, NULL);
+        if (ret) {
+                gf_msg (this->name, GF_LOG_ERROR, -ret,
+                        DHT_MSG_LOG_TIER_STATUS,
+                        "Unable to obtain statfs.");
+                goto exit;
+        }
+
+        pthread_mutex_lock (&dm_stat_mutex);
+
+        tier_conf->blocks_total = statfs.f_blocks;
+        tier_conf->blocks_used = statfs.f_blocks - statfs.f_bfree;
+
+        tier_conf->percent_full = (100 * tier_conf->blocks_used) /
+                statfs.f_blocks;
+        pthread_mutex_unlock (&dm_stat_mutex);
+
+        if (tier_conf->percent_full < tier_conf->watermark_low) {
+                wm = TIER_WM_LOW;
+
+        } else if (tier_conf->percent_full < tier_conf->watermark_hi) {
+                wm = TIER_WM_MID;
+
+        } else {
+                wm = TIER_WM_HI;
+        }
+
+        if (wm != tier_conf->watermark_last) {
+
+                tier_conf->watermark_last = wm;
+                gf_msg (this->name, GF_LOG_INFO, 0,
+                        DHT_MSG_LOG_TIER_STATUS,
+                        "Tier watermark now %d", wm);
+        }
+
+exit:
+        return ret;
+}
+
 static int
 tier_migrate_using_query_file (void *_args)
 {
@@ -145,6 +259,8 @@ tier_migrate_using_query_file (void *_args)
         char *link_str                          = NULL;
         xlator_t *src_subvol                    = NULL;
         dht_conf_t   *conf                      = NULL;
+        uint64_t total_migrated_bytes           = 0;
+        int total_files                         = 0;
 
         GF_VALIDATE_OR_GOTO ("tier", query_cbk_args, out);
         GF_VALIDATE_OR_GOTO ("tier", query_cbk_args->this, out);
@@ -159,14 +275,20 @@ tier_migrate_using_query_file (void *_args)
 
         queryFILE = query_cbk_args->queryFILE;
 
-        query_record = gfdb_query_record_init();
+        query_record = gfdb_query_record_init ();
         if (!query_record) {
+                gf_msg (this->name, GF_LOG_ERROR, 0,
+                        DHT_MSG_LOG_TIER_ERROR,
+                        "Call to gfdb_query_record_init() failed.");
                 goto out;
         }
 
         query_record->_link_info_str = GF_CALLOC (1, DB_QUERY_RECORD_SIZE,
                                                   gf_common_mt_char);
         if (!query_record->_link_info_str) {
+                gf_msg (this->name, GF_LOG_ERROR, 0,
+                        DHT_MSG_LOG_TIER_ERROR,
+                        "Allocating query record link info string failed.");
                 goto out;
         }
         link_buffer = query_record->_link_info_str;
@@ -195,13 +317,14 @@ tier_migrate_using_query_file (void *_args)
                         continue;
                 }
 
+                if (!tier_do_migration (this, query_cbk_args->is_promotion))
+                        continue;
+
                 gf_uuid_parse (gfid_str, query_record->gfid);
 
-                if (dict_get(migrate_data, GF_XATTR_FILE_MIGRATE_KEY))
-                        dict_del(migrate_data, GF_XATTR_FILE_MIGRATE_KEY);
+                dict_del (migrate_data, GF_XATTR_FILE_MIGRATE_KEY);
 
-                if (dict_get(migrate_data, "from.migrator"))
-                        dict_del(migrate_data, "from.migrator");
+                dict_del (migrate_data, "from.migrator");
 
                 token_str = strtok (link_buffer, delimiter);
                 if (token_str != NULL) {
@@ -239,6 +362,7 @@ tier_migrate_using_query_file (void *_args)
 
                 }
                 per_link_status = 0;
+
                 /* Per link of file */
                 while (token_str != NULL) {
 
@@ -274,9 +398,9 @@ tier_migrate_using_query_file (void *_args)
                         ret = syncop_lookup (this, &p_loc, &par_stbuf, NULL,
                                              NULL, NULL);
                         if (ret) {
-                                gf_msg (this->name, GF_LOG_ERROR, 0,
+                                gf_msg (this->name, GF_LOG_ERROR, -ret,
                                         DHT_MSG_LOG_TIER_ERROR,
-                                        " ERROR in parent lookup\n");
+                                        " Error in parent lookup\n");
                                 per_link_status = -1;
                                 goto abort;
                         }
@@ -288,7 +412,7 @@ tier_migrate_using_query_file (void *_args)
                         gf_uuid_copy (loc.gfid, query_record->gfid);
                         loc.inode = inode_new (defrag->root_inode->table);
                         gf_uuid_copy (loc.pargfid, link_info->pargfid);
-                        loc.parent = inode_ref(p_loc.inode);
+                        loc.parent = inode_ref (p_loc.inode);
 
                         loc.name = gf_strdup (link_info->file_name);
                         if (!loc.name) {
@@ -329,7 +453,10 @@ tier_migrate_using_query_file (void *_args)
                          * should be. It means another brick moved the file
                          * so is not an error.
                          */
-                        src_subvol = dht_subvol_get_cached(this, loc.inode);
+                        src_subvol = dht_subvol_get_cached (this, loc.inode);
+
+                        if (src_subvol == NULL)
+                                goto abort;
 
                         if (query_cbk_args->is_promotion &&
                              src_subvol == conf->subvolumes[1]) {
@@ -367,18 +494,48 @@ tier_migrate_using_query_file (void *_args)
                                 goto abort;
                         }
 
-                        if (query_cbk_args->is_promotion)
+                        if (query_cbk_args->is_promotion) {
                                 defrag->total_files_promoted++;
-                        else
+                                total_migrated_bytes +=
+                                        defrag->tier_conf.st_last_promoted_size;
+                                pthread_mutex_lock (&dm_stat_mutex);
+                                defrag->tier_conf.blocks_used +=
+                                        defrag->tier_conf.st_last_promoted_size;
+                                pthread_mutex_unlock (&dm_stat_mutex);
+                        } else {
                                 defrag->total_files_demoted++;
+                                total_migrated_bytes +=
+                                        defrag->tier_conf.st_last_demoted_size;
+                                pthread_mutex_lock (&dm_stat_mutex);
+                                defrag->tier_conf.blocks_used -=
+                                        defrag->tier_conf.st_last_demoted_size;
+                                pthread_mutex_unlock (&dm_stat_mutex);
+                        }
+                        if (defrag->tier_conf.blocks_total) {
+                                pthread_mutex_lock (&dm_stat_mutex);
+                                defrag->tier_conf.percent_full =
+                                        (100 * defrag->tier_conf.blocks_used) /
+                                        defrag->tier_conf.blocks_total;
+                                pthread_mutex_unlock (&dm_stat_mutex);
+                        }
 abort:
-
                         loc_wipe(&loc);
                         loc_wipe(&p_loc);
 
                         token_str = NULL;
                         token_str = strtok (NULL, delimiter);
                         GF_FREE (link_str);
+
+                        if ((++total_files > defrag->tier_conf.max_migrate_files) ||
+                            (total_migrated_bytes > defrag->tier_conf.max_migrate_bytes)) {
+                                gf_msg (this->name, GF_LOG_INFO, 0,
+                                        DHT_MSG_LOG_TIER_STATUS,
+                                        "Reached cycle migration limit."
+                                        "migrated bytes %"PRId64" files %d",
+                                        total_migrated_bytes,
+                                        total_files);
+                                goto out;
+                        }
                 }
                 per_file_status = per_link_status;
 per_file_out:
@@ -421,7 +578,7 @@ tier_gf_query_callback (gfdb_query_record_t *gfdb_query_record,
         GF_VALIDATE_OR_GOTO ("tier", query_cbk_args->queryFILE, out);
 
         gf_uuid_unparse (gfdb_query_record->gfid, gfid_str);
-        fprintf (query_cbk_args->queryFILE, "%s|%s|%ld\n", gfid_str,
+        fprintf (query_cbk_args->queryFILE, "%s|%s|%zd\n", gfid_str,
                  gfdb_query_record->_link_info_str,
                  gfdb_query_record->link_info_size);
 
@@ -439,7 +596,7 @@ out:
 
 /*Create query file in tier process*/
 static int
-tier_process_self_query (brick_list_t *local_brick, void *args)
+tier_process_self_query (tier_brick_list_t *local_brick, void *args)
 {
         int ret                                         = -1;
         char *db_path                                   = NULL;
@@ -480,7 +637,7 @@ tier_process_self_query (brick_list_t *local_brick, void *args)
                                 db_path, ret, out);
 
         /*Get the db connection*/
-        conn_node = gfdb_methods.init_db((void *)params_dict, dht_tier_db_type);
+        conn_node = gfdb_methods.init_db ((void *)params_dict, dht_tier_db_type);
         if (!conn_node) {
                 gf_msg (this->name, GF_LOG_ERROR, 0,
                         DHT_MSG_LOG_TIER_ERROR,
@@ -489,8 +646,8 @@ tier_process_self_query (brick_list_t *local_brick, void *args)
         }
 
         /*Query for eligible files from db*/
-        query_cbk_args->queryFILE = fopen(GET_QFILE_PATH
-                                (gfdb_brick_dict_info->_gfdb_promote), "a+");
+        query_cbk_args->queryFILE = fopen (
+                GET_QFILE_PATH (gfdb_brick_dict_info->_gfdb_promote), "a+");
         if (!query_cbk_args->queryFILE) {
                 gf_msg (this->name, GF_LOG_ERROR, errno,
                         DHT_MSG_LOG_TIER_ERROR,
@@ -593,7 +750,7 @@ out:
 
 /*Ask CTR to create the query file*/
 static int
-tier_process_ctr_query (brick_list_t *local_brick, void *args)
+tier_process_ctr_query (tier_brick_list_t *local_brick, void *args)
 {
         int ret                                         = -1;
         query_cbk_args_t *query_cbk_args                = NULL;
@@ -721,7 +878,7 @@ out:
  * It picks up each bricks db and queries for eligible files for migration.
  * The list of eligible files are populated in appropriate query files*/
 static int
-tier_process_brick (brick_list_t *local_brick, void *args) {
+tier_process_brick (tier_brick_list_t *local_brick, void *args) {
         int ret = -1;
         dict_t *ctr_ipc_in_dict = NULL;
         dict_t *ctr_ipc_out_dict = NULL;
@@ -835,7 +992,7 @@ tier_build_migration_qfile (demotion_args_t *args,
         _gfdb_brick_dict_info_t         gfdb_brick_dict_info;
         gfdb_time_t                     time_in_past;
         int                             ret = -1;
-        brick_list_t                    *local_brick = NULL;
+        tier_brick_list_t                    *local_brick = NULL;
 
         /*
          *  The first time this function is called, query file will
@@ -930,8 +1087,8 @@ tier_demote (void *args)
         query_cbk_args.is_promotion = 0;
 
         /*Build the query file using bricklist*/
-        ret = tier_build_migration_qfile(demotion_args, &query_cbk_args,
-                                    _gf_false);
+        ret = tier_build_migration_qfile (demotion_args, &query_cbk_args,
+                                          _gf_false);
         if (ret)
                 goto out;
 
@@ -968,8 +1125,8 @@ static void
         query_cbk_args.is_promotion = 1;
 
         /*Build the query file using bricklist*/
-        ret = tier_build_migration_qfile(promotion_args, &query_cbk_args,
-                                         _gf_true);
+        ret = tier_build_migration_qfile (promotion_args, &query_cbk_args,
+                                          _gf_true);
         if (ret)
                 goto out;
 
@@ -995,7 +1152,7 @@ tier_get_bricklist (xlator_t *xl, struct list_head *local_bricklist_head)
         char           *brickname = NULL;
         char            db_name[PATH_MAX] = "";
         int             ret = 0;
-        brick_list_t    *local_brick = NULL;
+        tier_brick_list_t    *local_brick = NULL;
 
         GF_VALIDATE_OR_GOTO ("tier", xl, out);
         GF_VALIDATE_OR_GOTO ("tier", local_bricklist_head, out);
@@ -1007,19 +1164,19 @@ tier_get_bricklist (xlator_t *xl, struct list_head *local_bricklist_head)
          * those running on the same node as the tier daemon.
          */
         if (strcmp(xl->type, "protocol/client") == 0) {
-                ret = dict_get_str(xl->options, "remote-host", &rh);
+                ret = dict_get_str (xl->options, "remote-host", &rh);
                 if (ret < 0)
                         goto out;
 
-               if (gf_is_local_addr (rh)) {
+                if (gf_is_local_addr (rh)) {
 
-                       local_brick = GF_CALLOC (1, sizeof(brick_list_t),
+                       local_brick = GF_CALLOC (1, sizeof(tier_brick_list_t),
                                                 gf_tier_mt_bricklist_t);
                         if (!local_brick) {
                                 goto out;
                         }
 
-                        ret = dict_get_str(xl->options, "remote-subvolume",
+                        ret = dict_get_str (xl->options, "remote-subvolume",
                                            &rv);
                         if (ret < 0)
                                 goto out;
@@ -1052,7 +1209,7 @@ tier_get_bricklist (xlator_t *xl, struct list_head *local_bricklist_head)
         }
 
         for (child = xl->children; child; child = child->next) {
-                ret = tier_get_bricklist(child->xlator, local_bricklist_head);
+                ret = tier_get_bricklist (child->xlator, local_bricklist_head);
                 if (ret) {
                         goto out;
                 }
@@ -1071,11 +1228,50 @@ out:
         return ret;
 }
 
+int
+tier_get_freq_demote (gf_tier_conf_t *tier_conf)
+{
+        if ((tier_conf->mode == TIER_MODE_WM) &&
+            (tier_conf->watermark_last == TIER_WM_HI))
+                return DEFAULT_DEMOTE_DEGRADED;
+        else
+                return tier_conf->tier_demote_frequency;
+}
+
+int
+tier_get_freq_promote (gf_tier_conf_t *tier_conf)
+{
+        return tier_conf->tier_promote_frequency;
+}
+
+static int
+tier_check_demote (gfdb_time_t  current_time,
+                   int freq_demote)
+{
+        return ((current_time.tv_sec % freq_demote) == 0) ?
+                _gf_true : _gf_false;
+}
+
+static gf_boolean_t
+tier_check_promote (gf_tier_conf_t   *tier_conf,
+                    gfdb_time_t  current_time,
+                    int freq_promote)
+{
+        if ((tier_conf->mode == TIER_MODE_WM) &&
+            (tier_conf->watermark_last == TIER_WM_HI))
+                return _gf_false;
+
+        else
+                return ((current_time.tv_sec % freq_promote) == 0) ?
+                        _gf_true : _gf_false;
+}
+
+
 void
 clear_bricklist (struct list_head *brick_list)
 {
-        brick_list_t  *local_brick      = NULL;
-        brick_list_t  *temp             = NULL;
+        tier_brick_list_t  *local_brick      = NULL;
+        tier_brick_list_t  *temp             = NULL;
 
         if (list_empty(brick_list)) {
                 return;
@@ -1106,7 +1302,11 @@ tier_start (xlator_t *this, gf_defrag_info_t *defrag)
         pthread_t promote_thread;
         pthread_t demote_thread;
         gf_boolean_t  is_promotion_triggered = _gf_false;
-        gf_boolean_t  is_demotion_triggered = _gf_false;
+        gf_boolean_t  is_demotion_triggered  = _gf_false;
+        xlator_t                *any         = NULL;
+        xlator_t                *xlator      = NULL;
+        gf_tier_conf_t    *tier_conf   = NULL;
+        loc_t      root_loc = { 0 };
 
         conf   = this->private;
 
@@ -1121,9 +1321,26 @@ tier_start (xlator_t *this, gf_defrag_info_t *defrag)
                         " demote %d", freq_promote, freq_demote);
 
         defrag->defrag_status = GF_DEFRAG_STATUS_STARTED;
+        tier_conf = &defrag->tier_conf;
+
+        dht_build_root_loc (defrag->root_inode, &root_loc);
 
         while (1) {
 
+                /*
+                 * Check if a graph switch occured. If so, stop migration
+                 * thread. It will need to be restarted manually.
+                 */
+                any = THIS->ctx->active->first;
+                xlator = xlator_search_by_name (any, this->name);
+
+                if (xlator != this) {
+                        gf_msg (this->name, GF_LOG_INFO, 0,
+                                DHT_MSG_LOG_TIER_STATUS,
+                                "Detected graph switch. Exiting migration daemon.");
+                        goto out;
+                }
+
                 sleep(1);
 
                 if (defrag->defrag_status != GF_DEFRAG_STATUS_STARTED) {
@@ -1146,10 +1363,6 @@ tier_start (xlator_t *this, gf_defrag_info_t *defrag)
                         goto out;
                 }
 
-                freq_promote = defrag->tier_promote_frequency;
-                freq_demote  = defrag->tier_demote_frequency;
-
-
                 /* To have proper synchronization amongst all
                  * brick holding nodes, so that promotion and demotions
                  * start atomicly w.r.t promotion/demotion frequency
@@ -1164,18 +1377,29 @@ tier_start (xlator_t *this, gf_defrag_info_t *defrag)
                         goto out;
                 }
 
-                is_demotion_triggered = ((current_time.tv_sec %
-                                        freq_demote) == 0) ? _gf_true :
-                                        _gf_false;
-                is_promotion_triggered = ((current_time.tv_sec %
-                                        freq_promote) == 0) ? _gf_true :
-                                        _gf_false;
+                freq_demote = tier_get_freq_demote (tier_conf);
+
+                is_demotion_triggered = tier_check_demote (current_time,
+                                                           freq_demote);
+
+                freq_promote = tier_get_freq_promote(tier_conf);
+
+                is_promotion_triggered = tier_check_promote (tier_conf,
+                                                             current_time,
+                                                             freq_promote);
 
                 /* If no promotion and no demotion is
-                 * scheduled/triggered skip a iteration */
+                 * scheduled/triggered skip an iteration */
                 if (!is_promotion_triggered && !is_demotion_triggered)
                         continue;
 
+                ret = tier_check_watermark (this, &root_loc);
+                if (ret != 0) {
+                        gf_msg (this->name, GF_LOG_CRITICAL, errno,
+                                DHT_MSG_LOG_TIER_ERROR,
+                                "Failed to get watermark");
+                        goto out;
+                }
 
                 ret_promotion = -1;
                 ret_demotion = -1;
@@ -1283,8 +1507,8 @@ tier_migration_get_dst (xlator_t *this, dht_local_t *local)
         int32_t                  ret = -1;
         gf_defrag_info_t        *defrag = NULL;
 
-        GF_VALIDATE_OR_GOTO("tier", this, out);
-        GF_VALIDATE_OR_GOTO(this->name, this->private, out);
+        GF_VALIDATE_OR_GOTO ("tier", this, out);
+        GF_VALIDATE_OR_GOTO (this->name, this->private, out);
 
         conf = this->private;
 
@@ -1318,10 +1542,10 @@ tier_search (xlator_t *this, dht_layout_t *layout, const char *name)
         int                      layout_cold = 0;
         int                      layout_hot = 1;
 
-        GF_VALIDATE_OR_GOTO("tier", this, out);
-        GF_VALIDATE_OR_GOTO(this->name, layout, out);
-        GF_VALIDATE_OR_GOTO(this->name, name, out);
-        GF_VALIDATE_OR_GOTO(this->name, this->private, out);
+        GF_VALIDATE_OR_GOTO ("tier", this, out);
+        GF_VALIDATE_OR_GOTO (this->name, layout, out);
+        GF_VALIDATE_OR_GOTO (this->name, name, out);
+        GF_VALIDATE_OR_GOTO (this->name, this->private, out);
 
         conf = this->private;
 
@@ -1375,7 +1599,7 @@ tier_load_externals (xlator_t *this)
         char *libpathfull = (LIBDIR "/libgfdb.so.0");
         get_gfdb_methods_t get_gfdb_methods;
 
-        GF_VALIDATE_OR_GOTO("this", this, out);
+        GF_VALIDATE_OR_GOTO ("this", this, out);
 
         libhandle = dlopen (libpathfull, RTLD_NOW);
         if (!libhandle) {
@@ -1406,6 +1630,20 @@ out:
         return ret;
 }
 
+static
+int tier_validate_mode (char *mode)
+{
+        int ret = -1;
+
+        if (strcmp (mode, "test") == 0) {
+                ret = TIER_MODE_TEST;
+        } else {
+                ret = TIER_MODE_WM;
+        }
+
+        return ret;
+}
+
 int
 tier_init (xlator_t *this)
 {
@@ -1414,10 +1652,11 @@ tier_init (xlator_t *this)
         dht_conf_t       *conf           = NULL;
         gf_defrag_info_t *defrag         = NULL;
         char             *voldir         = NULL;
+        char             *mode           = NULL;
 
-        ret = dht_init(this);
+        ret = dht_init (this);
         if (ret) {
-                gf_msg(this->name, GF_LOG_ERROR, 0,
+                gf_msg (this->name, GF_LOG_ERROR, 0,
                        DHT_MSG_LOG_TIER_ERROR,
                        "dht_init failed");
                 goto out;
@@ -1428,7 +1667,7 @@ tier_init (xlator_t *this)
         conf->methods = &tier_methods;
 
         if (conf->subvolume_cnt != 2) {
-                gf_msg(this->name, GF_LOG_ERROR, 0,
+                gf_msg (this->name, GF_LOG_ERROR, 0,
                        DHT_MSG_LOG_TIER_ERROR,
                        "Invalid number of subvolumes %d", conf->subvolume_cnt);
                 goto out;
@@ -1441,7 +1680,7 @@ tier_init (xlator_t *this)
         }
 
         /* if instatiated from server side, load db libraries */
-        ret = tier_load_externals(this);
+        ret = tier_load_externals (this);
         if (ret) {
                 gf_msg(this->name, GF_LOG_ERROR, 0,
                        DHT_MSG_LOG_TIER_ERROR,
@@ -1451,13 +1690,15 @@ tier_init (xlator_t *this)
 
         defrag = conf->defrag;
 
+        defrag->tier_conf.is_tier = 1;
+
         ret = dict_get_int32 (this->options,
                               "tier-promote-frequency", &freq);
         if (ret) {
                 freq = DEFAULT_PROMOTE_FREQ_SEC;
         }
 
-        defrag->tier_promote_frequency = freq;
+        defrag->tier_conf.tier_promote_frequency = freq;
 
         ret = dict_get_int32 (this->options,
                               "tier-demote-frequency", &freq);
@@ -1465,7 +1706,23 @@ tier_init (xlator_t *this)
                 freq = DEFAULT_DEMOTE_FREQ_SEC;
         }
 
-        defrag->tier_demote_frequency = freq;
+        defrag->tier_conf.tier_demote_frequency = freq;
+
+        ret = dict_get_int32 (this->options,
+                              "watermark-hi", &freq);
+        if (ret) {
+                freq = DEFAULT_WM_HI;
+        }
+
+        defrag->tier_conf.watermark_hi = freq;
+
+        ret = dict_get_int32 (this->options,
+                              "watermark-low", &freq);
+        if (ret) {
+                freq = DEFAULT_WM_LOW;
+        }
+
+        defrag->tier_conf.watermark_low = freq;
 
         ret = dict_get_int32 (this->options,
                               "write-freq-threshold", &freq);
@@ -1483,7 +1740,38 @@ tier_init (xlator_t *this)
 
         defrag->read_freq_threshold = freq;
 
-        ret = gf_asprintf(&voldir, "%s/%s",
+        ret = dict_get_int32 (this->options,
+                              "tier-max-mb", &freq);
+        if (ret) {
+                freq = DEFAULT_TIER_MAX_MIGRATE_MB;
+        }
+
+        defrag->tier_conf.max_migrate_bytes = freq * 1024 * 1024;
+
+        ret = dict_get_int32 (this->options,
+                              "tier-max-files", &freq);
+        if (ret) {
+                freq = DEFAULT_TIER_MAX_MIGRATE_FILES;
+        }
+
+        defrag->tier_conf.max_migrate_files = freq;
+
+        ret = dict_get_str (this->options,
+                            "tier-mode", &mode);
+        if (ret) {
+                defrag->tier_conf.mode = DEFAULT_TIER_MODE;
+        } else {
+                ret = tier_validate_mode (mode);
+                if (ret < 0) {
+                        gf_msg(this->name, GF_LOG_ERROR, 0,
+                               DHT_MSG_LOG_TIER_ERROR,
+                               "tier_init failed - invalid mode");
+                        goto out;
+                }
+                defrag->tier_conf.mode = ret;
+        }
+
+        ret = gf_asprintf (&voldir, "%s/%s",
                           DEFAULT_VAR_RUN_DIRECTORY,
                           this->name);
         if (ret < 0)
@@ -1491,7 +1779,7 @@ tier_init (xlator_t *this)
 
         ret = mkdir_p(voldir, 0777, _gf_true);
         if (ret == -1 && errno != EEXIST) {
-                gf_msg(this->name, GF_LOG_ERROR, 0,
+                gf_msg (this->name, GF_LOG_ERROR, 0,
                        DHT_MSG_LOG_TIER_ERROR,
                        "tier_init failed");
 
@@ -1501,37 +1789,37 @@ tier_init (xlator_t *this)
 
         GF_FREE(voldir);
 
-        ret = gf_asprintf(&promotion_qfile, "%s/%s/%s-%s",
-                          DEFAULT_VAR_RUN_DIRECTORY,
-                          this->name,
-                          PROMOTION_QFILE,
-                          this->name);
+        ret = gf_asprintf (&promotion_qfile, "%s/%s/%s-%s",
+                           DEFAULT_VAR_RUN_DIRECTORY,
+                           this->name,
+                           PROMOTION_QFILE,
+                           this->name);
         if (ret < 0)
                 goto out;
 
-        ret = gf_asprintf(&demotion_qfile, "%s/%s/%s-%s",
-                          DEFAULT_VAR_RUN_DIRECTORY,
-                          this->name,
-                          DEMOTION_QFILE,
-                          this->name);
+        ret = gf_asprintf (&demotion_qfile, "%s/%s/%s-%s",
+                           DEFAULT_VAR_RUN_DIRECTORY,
+                           this->name,
+                           DEMOTION_QFILE,
+                           this->name);
         if (ret < 0) {
-                GF_FREE(promotion_qfile);
+                GF_FREE (promotion_qfile);
                 goto out;
         }
 
-        unlink(promotion_qfile);
-        unlink(demotion_qfile);
+        unlink (promotion_qfile);
+        unlink (demotion_qfile);
 
-        gf_msg(this->name, GF_LOG_INFO, 0,
-               DHT_MSG_LOG_TIER_STATUS,
+        gf_msg (this->name, GF_LOG_INFO, 0,
+                DHT_MSG_LOG_TIER_STATUS,
                "Promote/demote frequency %d/%d "
                "Write/Read freq thresholds %d/%d",
-               defrag->tier_promote_frequency,
-               defrag->tier_demote_frequency,
+               defrag->tier_conf.tier_promote_frequency,
+               defrag->tier_conf.tier_demote_frequency,
                defrag->write_freq_threshold,
                defrag->read_freq_threshold);
 
-        gf_msg(this->name, GF_LOG_INFO, 0,
+        gf_msg (this->name, GF_LOG_INFO, 0,
                DHT_MSG_LOG_TIER_STATUS,
                "Promote file %s demote file %s",
                promotion_qfile, demotion_qfile);
@@ -1549,18 +1837,19 @@ tier_reconfigure (xlator_t *this, dict_t *options)
 {
         dht_conf_t       *conf           = NULL;
         gf_defrag_info_t *defrag         = NULL;
-
+        char             *mode           = NULL;
+        int               migrate_mb     = 0;
         conf = this->private;
 
         if (conf->defrag) {
                 defrag = conf->defrag;
                 GF_OPTION_RECONF ("tier-promote-frequency",
-                                  defrag->tier_promote_frequency, options,
-                                  int32, out);
+                                  defrag->tier_conf.tier_promote_frequency,
+                                  options, int32, out);
 
                 GF_OPTION_RECONF ("tier-demote-frequency",
-                                  defrag->tier_demote_frequency, options,
-                                  int32, out);
+                                  defrag->tier_conf.tier_demote_frequency,
+                                  options, int32, out);
 
                 GF_OPTION_RECONF ("write-freq-threshold",
                                   defrag->write_freq_threshold, options,
@@ -1569,6 +1858,28 @@ tier_reconfigure (xlator_t *this, dict_t *options)
                 GF_OPTION_RECONF ("read-freq-threshold",
                                   defrag->read_freq_threshold, options,
                                   int32, out);
+
+                GF_OPTION_RECONF ("watermark-hi",
+                                  defrag->tier_conf.watermark_hi, options,
+                                  int32, out);
+
+                GF_OPTION_RECONF ("watermark-low",
+                                  defrag->tier_conf.watermark_low, options,
+                                  int32, out);
+
+                GF_OPTION_RECONF ("tier-mode",
+                                  mode, options,
+                                  str, out);
+                defrag->tier_conf.mode = tier_validate_mode (mode);
+
+                GF_OPTION_RECONF ("tier-max-mb",
+                                  migrate_mb, options,
+                                  int32, out);
+                defrag->tier_conf.max_migrate_bytes = migrate_mb*1024*1024;
+
+                GF_OPTION_RECONF ("tier-max-files",
+                                  defrag->tier_conf.max_migrate_files, options,
+                                  int32, out);
         }
 
 out:
@@ -1579,10 +1890,10 @@ void
 tier_fini (xlator_t *this)
 {
         if (libhandle)
-                dlclose(libhandle);
+                dlclose (libhandle);
 
-        GF_FREE(demotion_qfile);
-        GF_FREE(promotion_qfile);
+        GF_FREE (demotion_qfile);
+        GF_FREE (promotion_qfile);
 
         dht_fini(this);
 }
diff --git a/xlators/cluster/dht/src/tier.h b/xlators/cluster/dht/src/tier.h
index d5fbba655e5..b840f339d2e 100644
--- a/xlators/cluster/dht/src/tier.h
+++ b/xlators/cluster/dht/src/tier.h
@@ -25,10 +25,6 @@
 #include <fnmatch.h>
 #include <signal.h>
 
-#define DEFAULT_PROMOTE_FREQ_SEC 120
-#define DEFAULT_DEMOTE_FREQ_SEC  120
-#define DEFAULT_WRITE_FREQ_SEC 0
-#define DEFAULT_READ_FREQ_SEC 0
 /*
  * Size of timer wheel. We would not promote or demote less
  * frequently than this number.
@@ -65,7 +61,7 @@ typedef struct brick_list {
         xlator_t          *xlator;
         char              *brick_db_path;
         struct list_head  list;
-} brick_list_t;
+} tier_brick_list_t;
 
 typedef struct _dm_thread_args {
         xlator_t                *this;
@@ -75,4 +71,22 @@ typedef struct _dm_thread_args {
         int                     return_value;
 } promotion_args_t, demotion_args_t;
 
+typedef enum tier_watermark_op_ {
+        TIER_WM_NONE = 0,
+        TIER_WM_LOW,
+        TIER_WM_HI,
+        TIER_WM_MID
+} tier_watermark_op_t;
+
+#define DEFAULT_PROMOTE_FREQ_SEC       120
+#define DEFAULT_DEMOTE_FREQ_SEC        120
+#define DEFAULT_DEMOTE_DEGRADED        10
+#define DEFAULT_WRITE_FREQ_SEC         0
+#define DEFAULT_READ_FREQ_SEC          0
+#define DEFAULT_WM_LOW                 75
+#define DEFAULT_WM_HI                  90
+#define DEFAULT_TIER_MODE              TIER_MODE_TEST
+#define DEFAULT_TIER_MAX_MIGRATE_MB    1000
+#define DEFAULT_TIER_MAX_MIGRATE_FILES 5000
+
 #endif
diff --git a/xlators/mgmt/glusterd/src/glusterd-volume-set.c b/xlators/mgmt/glusterd/src/glusterd-volume-set.c
index b90d3f1ef57..b4bf16da074 100644
--- a/xlators/mgmt/glusterd/src/glusterd-volume-set.c
+++ b/xlators/mgmt/glusterd/src/glusterd-volume-set.c
@@ -24,6 +24,10 @@ validate_tier (glusterd_volinfo_t *volinfo, dict_t *dict, char *key,
         int                  ret           = 0;
         xlator_t            *this          = NULL;
         int                  origin_val    = -1;
+        char                *current_wm_hi = NULL;
+        char                *current_wm_low = NULL;
+        uint64_t             wm_hi = 0;
+        uint64_t             wm_low = 0;
 
         this = THIS;
         GF_ASSERT (this);
@@ -39,12 +43,20 @@ validate_tier (glusterd_volinfo_t *volinfo, dict_t *dict, char *key,
                 goto out;
         }
 
+        if (strstr (key, "cluster.tier-mode")) {
+                if (strcmp(value, "test") &&
+                    strcmp(value, "cache")) {
+                        ret = -1;
+                        goto out;
+                }
+                goto out;
+        }
+
         /*
-         * All the volume set options for tier are expecting a positive
+         * Rest of the volume set options for tier are expecting a positive
          * Integer. Change the function accordingly if this constraint is
          * changed.
          */
-
         ret = gf_string2int (value, &origin_val);
         if (ret) {
                 snprintf (errstr, sizeof (errstr), "%s is not a compatible "
@@ -56,13 +68,55 @@ validate_tier (glusterd_volinfo_t *volinfo, dict_t *dict, char *key,
                 ret = -1;
                 goto out;
         }
+        if (strstr (key, "watermark-hi") ||
+            strstr (key, "watermark-low")) {
+                if ((origin_val < 1) || (origin_val > 99)) {
+                        snprintf (errstr, sizeof (errstr), "%s is not a compatible"
+                                  "value. %s expects a percentage from 1-99.",
+                                  value, key);
+                        gf_msg (this->name, GF_LOG_ERROR, EINVAL,
+                                GD_MSG_INCOMPATIBLE_VALUE, "%s", errstr);
+                        *op_errstr = gf_strdup (errstr);
+                        ret = -1;
+                        goto out;
+                }
+
+                if (strstr (key, "watermark-hi")) {
+                        wm_hi = origin_val;
+                } else {
+                        glusterd_volinfo_get (volinfo,
+                                              "cluster.watermark-hi",
+                                              &current_wm_hi);
+                        gf_string2bytesize_uint64 (current_wm_hi,
+                                                   &wm_hi);
+                }
 
-        if (strstr ("cluster.tier-promote-frequency", key) ||
-            strstr ("cluster.tier-demote-frequency", key)) {
+                if (strstr (key, "watermark-low")) {
+                        wm_low = origin_val;
+                } else {
+                        glusterd_volinfo_get (volinfo,
+                                              "cluster.watermark-low",
+                                              &current_wm_low);
+                        gf_string2bytesize_uint64 (current_wm_low,
+                                                   &wm_low);
+                }
+                if (wm_low > wm_hi) {
+                        snprintf (errstr, sizeof (errstr), "lower watermark"
+                                  " cannot exceed upper watermark.");
+                        gf_msg (this->name, GF_LOG_ERROR, EINVAL,
+                                GD_MSG_INCOMPATIBLE_VALUE, "%s", errstr);
+                        *op_errstr = gf_strdup (errstr);
+                        ret = -1;
+                        goto out;
+                }
+        } else if (strstr (key, "tier-promote-frequency") ||
+                   strstr (key, "tier-max-mb") ||
+                   strstr (key, "tier-max-files") ||
+                   strstr (key, "tier-demote-frequency")) {
                 if (origin_val < 1) {
                         snprintf (errstr, sizeof (errstr), "%s is not a "
-                                  "compatible value. %s expects a positive "
-                                  "integer value.",
+                                  " compatible value. %s expects a positive "
+                                  "integer value greater than 0.",
                                   value, key);
                         gf_msg (this->name, GF_LOG_ERROR, EINVAL,
                                 GD_MSG_INCOMPATIBLE_VALUE, "%s", errstr);
@@ -70,10 +124,12 @@ validate_tier (glusterd_volinfo_t *volinfo, dict_t *dict, char *key,
                         ret = -1;
                         goto out;
                 }
+
         } else {
+                /* check write-freq-threshold and read-freq-threshold. */
                 if (origin_val < 0) {
                         snprintf (errstr, sizeof (errstr), "%s is not a "
-                                   "compatible value. %s expects a non-negative"
+                                   "compatible value. %s expects a positive"
                                    " integer value.",
                                    value, key);
                          gf_msg (this->name, GF_LOG_ERROR, EINVAL,
@@ -1872,6 +1928,7 @@ struct volopt_map_entry glusterd_volopt_map[] = {
         /* tier translator - global tunables */
         { .key         = "cluster.write-freq-threshold",
           .voltype     = "cluster/tier",
+          .value       = "0",
           .option      = "write-freq-threshold",
           .op_version  = GD_OP_VERSION_3_7_0,
           .flags       = OPT_FLAG_CLIENT_OPT,
@@ -1883,6 +1940,7 @@ struct volopt_map_entry glusterd_volopt_map[] = {
         },
         { .key         = "cluster.read-freq-threshold",
           .voltype     = "cluster/tier",
+          .value       = "0",
           .option      = "read-freq-threshold",
           .op_version  = GD_OP_VERSION_3_7_0,
           .flags       = OPT_FLAG_CLIENT_OPT,
@@ -1894,23 +1952,74 @@ struct volopt_map_entry glusterd_volopt_map[] = {
         },
         { .key         = "cluster.tier-promote-frequency",
           .voltype     = "cluster/tier",
+          .value       = "120",
           .option      = "tier-promote-frequency",
           .op_version  = GD_OP_VERSION_3_7_0,
           .flags       = OPT_FLAG_CLIENT_OPT,
           .validate_fn = validate_tier,
-          .description = "Defines how often the promotion should be triggered "
-                         "i.e. periodicity of promotion cycles. The value is in "
-                         "secs."
         },
         { .key         = "cluster.tier-demote-frequency",
           .voltype     = "cluster/tier",
+          .value       = "120",
           .option      = "tier-demote-frequency",
           .op_version  = GD_OP_VERSION_3_7_0,
           .flags       = OPT_FLAG_CLIENT_OPT,
           .validate_fn = validate_tier,
-          .description = "Defines how often the demotion should be triggered "
-                         "i.e. periodicity of demotion cycles. The value is in "
-                         "secs."
+        },
+        { .key         = "cluster.watermark-hi",
+          .voltype     = "cluster/tier",
+          .value       = "90",
+          .option      = "watermark-hi",
+          .op_version  = GD_OP_VERSION_3_7_6,
+          .flags       = OPT_FLAG_CLIENT_OPT,
+          .validate_fn = validate_tier,
+          .description = "Upper % watermark for promotion. If hot tier fills"
+          " above this percentage, no promotion will happen and demotion will "
+          "happen with high probability."
+        },
+        { .key         = "cluster.watermark-low",
+          .voltype     = "cluster/tier",
+          .value       = "75",
+          .option      = "watermark-low",
+          .op_version  = GD_OP_VERSION_3_7_6,
+          .flags       = OPT_FLAG_CLIENT_OPT,
+          .validate_fn = validate_tier,
+          .description = "Lower % watermark. If hot tier is less "
+          "full than this, promotion will happen and demotion will not happen. "
+          "If greater than this, promotion/demotion will happen at a probability "
+          "relative to how full the hot tier is."
+        },
+        { .key         = "cluster.tier-mode",
+          .voltype     = "cluster/tier",
+          .option      = "tier-mode",
+          .value       = "test",
+          .op_version  = GD_OP_VERSION_3_7_6,
+          .flags       = OPT_FLAG_CLIENT_OPT,
+          .validate_fn = validate_tier,
+          .description = "Either 'test' or 'cache'. Test mode periodically"
+          " demotes or promotes files automatically based on access."
+          " Cache mode does so based on whether the cache is full or not,"
+          " as specified with watermarks."
+        },
+        { .key         = "cluster.tier-max-mb",
+          .voltype     = "cluster/tier",
+          .option      = "tier-max-mb",
+          .value       = "1000",
+          .op_version  = GD_OP_VERSION_3_7_6,
+          .flags       = OPT_FLAG_CLIENT_OPT,
+          .validate_fn = validate_tier,
+          .description = "The maximum number of MB that may be migrated"
+          " in any direction in a given cycle."
+        },
+        { .key         = "cluster.tier-max-files",
+          .voltype     = "cluster/tier",
+          .option      = "tier-max-files",
+          .value       = "5000",
+          .op_version  = GD_OP_VERSION_3_7_6,
+          .flags       = OPT_FLAG_CLIENT_OPT,
+          .validate_fn = validate_tier,
+          .description = "The maximum number of files that may be migrated"
+          " in any direction in a given cycle."
         },
         { .key         = "features.ctr-enabled",
           .voltype     = "features/changetimerecorder",