geo-rep: geo-replication in Tiering based volume

Adding design document for supporting Geo-replication
in a Tiering based volume.

Change-Id: Ia0287c17e6911a4df439e1c9f523cc7654761111
Signed-off-by: Saravanakumar Arumugam <sarumuga@redhat.com>
Reviewed-on: http://review.gluster.org/13515
Reviewed-by: Aravinda VK <avishwan@redhat.com>
Tested-by: Niels de Vos <ndevos@redhat.com>

1 files changed, 131 insertions, 0 deletions

diff --git a/under_review/Tiering_georeplication.md b/under_review/Tiering_georeplication.md
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+Feature
+-------
+Geo-replication setup in a Tiering based volume.
+
+Summary
+-------
+
+To be able to create and use a geo-replication session in a Tiering based
+volume.
+
+Owners
+------
+
+Saravanakumar Arumugam <sarumuga@redhat.com>
+
+Current status
+--------------
+
+Due to frequent movement of files between hot/cold tier(due to rebalance)
+changes gets recorded in both the subvolumes.
+
+Now, for the same files, changes are recorded in both tiers.
+There is a race condition possible while processing changelogs(*) due to which
+files may get into inconsistent state (at the slave side) in a geo-replication
+session.
+
+(*)- changelog contains all changes in file like CREATE, RENAME, DELETE, DATA, METADATA.
+
+Related Feature Requests and Bugs
+---------------------------------
+
+https://bugzilla.redhat.com/show_bug.cgi?id=1266875
+
+https://bugzilla.redhat.com/show_bug.cgi?id=1288027
+
+https://bugzilla.redhat.com/show_bug.cgi?id=1301032
+
+
+Detailed Description
+--------------------
+
+1. Consider, tier is attached to a volume and file operations are carried out
+in the volume.
+Currently, any namespace operation carried out, is getting recorded in both
+hot tier and cold tier.
+
+Note: cold tier is the default HASH for a file.
+
+So, in cold tier, all namespace operations (ENTRY operations like CREATE,
+UNLINK, RENAME) are recorded(for the linkto file).
+Also, it is recorded in hot tier for actual files.
+
+So, process entry operations(only) from changelogs in cold tier and ignore
+hot tier bricks while processing changelogs(for namespace operation).
+Carry out only DATA/METADATA operations in hot tier.
+
+So,
+before attaching the tier, all operations are carried out and recorded
+in cold tier.
+
+After attaching the tier, all namespace operations are carried out and
+recorded in cold tier(using linkto file).
+Also, we carry out and record DATA/METADATA operations in hot tier.
+
+Hot tier bricks will process changelogs, but ignore ENTRY operations.
+
+This way, we can get both tier's operation and race conditions can be avoided.
+
+Note, if a file is modified when it was demoted, DATA/METADATA operations
+will be recorded in Cold tier(Hashed).
+Also, once the file is demoted (means file is present only in cold tier),
+all namespace operations will be recorded only in cold tier.
+
+2. Record MKNOD, if tier-dht linkto file is encountered (which happens in
+cold tier).
+
+3. If MKNOD with Sticky bit is present, record it as LINK.
+This way, Geo-rep will create a file if not exists, OR create a link if file
+exists.
+
+4. All the rebalance operation like DELETE from one brick and CREATION in
+another brick needs to be ignored and need not be RECORDED in changelog.
+
+    How?
+Use GF_CLIENT_PID_TIER_DEFRAG  pid to ignore all related operations in
+ changelog.
+
+Benefit to GlusterFS
+--------------------
+
+In a geo-replication session, able to properly sync files to Slave with a
+Tiering based volume as Master.
+
+Scope
+-----
+Changes are mainly in geo-replication and changelog code.
+
+
+How To Test
+-----------
+
+Create a geo-rep session for Tiering based volume and check whether files are
+synced properly to slave.
+
+User Experience
+---------------
+No change in geo-replication setup commands.
+
+Dependencies
+------------
+NIL
+
+Status
+------
+Completed.
+
+http://review.gluster.org/#/c/12326
+
+http://review.gluster.org/#/c/12355
+
+http://review.gluster.org/#/c/12239
+
+http://review.gluster.org/#/c/12417
+
+http://review.gluster.org/#/c/12844
+
+http://review.gluster.org/#/c/13281
+
+Comments and Discussion
+-----------------------
+