From ddc044bfa2840981de4003c3b9efcac84387dc2b Mon Sep 17 00:00:00 2001 From: Jeff Darcy Date: Mon, 12 Mar 2012 09:32:40 -0400 Subject: replicate: add hashed read-child method. Both the first-to-respond method and the round-robin method are susceptible to clients repeatedly choosing the same servers across a series of opens, creating hot spots. Also, the code to handle a replica being down will ignore both methods and just choose the first remaining (which is not an issue for two-way but can be otherwise). The hashed method more reliably avoids such hot spots. There are three values/modes. 0: use the old (broken) methods. 1: select a read-child based on a hash of the file's GFID, so all clients will choose the same subvolume for a file (ensuring maximum consistency) but will distribute load for a set of files. 2: select a read-child based on a hash of the file's GFID plus the client's PID, so different children will distribute load even for one file. Mode 2 will probably be optimal for most cases. Using response time when we open the file is problematic, both because a single sample might not have been representative even then and because load might have shifted in the hours or days since (for long-lived files). Trying to use more current load information can lead to "herd following" behavior which is just as bad. Pseudo-random distribution is likely to be the best we can reasonably do, just as it is for DHT. Change-Id: I798c2760411eacf32e82a85f03bb7b08a4a49461 BUG: 802513 Signed-off-by: Jeff Darcy Reviewed-on: http://review.gluster.com/2926 Tested-by: Gluster Build System Reviewed-by: Anand Avati --- xlators/cluster/afr/src/afr-self-heal-data.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) (limited to 'xlators/cluster/afr/src/afr-self-heal-data.c') diff --git a/xlators/cluster/afr/src/afr-self-heal-data.c b/xlators/cluster/afr/src/afr-self-heal-data.c index acc96697f..93b645295 100644 --- a/xlators/cluster/afr/src/afr-self-heal-data.c +++ b/xlators/cluster/afr/src/afr-self-heal-data.c @@ -796,7 +796,8 @@ afr_sh_data_fxattrop_fstat_done (call_frame_t *frame, xlator_t *this) int afr_lookup_select_read_child_by_txn_type (xlator_t *this, afr_local_t *local, dict_t **xattr, - afr_transaction_type txn_type) + afr_transaction_type txn_type, + uuid_t gfid) { afr_private_t *priv = NULL; int read_child = -1; @@ -855,7 +856,8 @@ afr_lookup_select_read_child_by_txn_type (xlator_t *this, afr_local_t *local, priv->child_count, prev_read_child, config_read_child, - sources); + sources, + priv->hash_mode, gfid); out: afr_matrix_cleanup (pending_matrix, priv->child_count); gf_log (this->name, GF_LOG_DEBUG, "returning read_child: %d", -- cgit