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 <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.com/2926
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/xlators/cluster/afr/src/afr.h b/xlators/cluster/afr/src/afr.h
index 27cb83a57..a1a30562b 100644
--- a/xlators/cluster/afr/src/afr.h
+++ b/xlators/cluster/afr/src/afr.h
@@ -130,6 +130,7 @@ typedef struct _afr_private {
         gf_boolean_t entry_change_log;      /* on/off */
 
         int read_child;               /* read-subvolume */
+        unsigned int hash_mode;       /* for when read_child is not set */
         int favorite_child;  /* subvolume to be preferred in resolving
                                          split-brain cases */
 
@@ -936,12 +937,13 @@ afr_first_up_child (unsigned char *child_up, size_t child_count);
 int
 afr_select_read_child_from_policy (int32_t *fresh_children, int32_t child_count,
                                    int32_t prev_read_child,
-                                   int32_t config_read_child, int32_t *sources);
+                                   int32_t config_read_child, int32_t *sources,
+                                   unsigned int hmode, uuid_t gfid);
 
 void
 afr_set_read_ctx_from_policy (xlator_t *this, inode_t *inode,
                               int32_t *fresh_children, int32_t prev_read_child,
-                              int32_t config_read_child);
+                              int32_t config_read_child, uuid_t gfid);
 
 int32_t
 afr_get_call_child (xlator_t *this, unsigned char *child_up, int32_t read_child,