geo-rep: rename handling in dht volume

Background:

Glusterfs changelogs are stored in each brick, which records the changes
happened in that brick. Georep will run in all the nodes of master and
processes changelogs "independently".
Processing changelogs is in brick level, but all the fops will be replayed
on "slave mount" point.

Problem:
With a DHT volume, in changelog "internal fops" are NOT recorded.
For Rename case, Rename is recorded in "hashed" brick changelog.
(DHT's internal fops like creating linkto file, unlink is NOT recorded).
This lead us to inconsistent rename operations.

For example,
Distribute volume created with Two bricks B1, B2.

//Consider master volume mounted @ /mnt/master
and following operations executed:
cd /mnt/master
touch f1 // f1 falls on B1 Hash
mv f1 f2 // f2 falls on B2 Hash

// Here, Changelogs are recorded as below:
@B1
CREATE f1
@B2
RENAME f1 f2

Here, race exists between Brick B1 and B2, say B2 will get executed first.
Source file f1 itself is "NOT PRESENT", so it will go ahead and create
f2 (Current implementation).
We have this problem When rename falls in another brick and
file is unlinked in Master.

Similar kind of issue exists in following case too(multiple rename):
CREATE f1
RENAME f1 f2
RENAME f2 f1

Solution:

Instead of carrying out "changelogging" at "HASHED volume",
carry out  at the "CACHED volume".
This way we have rename operations carried out where actual files are present.
So,Changelog recorded as :
@B1
CREATE f1
RENAME f1 f2

credit: sarumuga@redhat.com

PS: Some of the races as the one below are _NOT_ fixed by this patch

* f1 and f2 exist. B1 and B2 are their respective cached subvols. For 
both files hashed-subvol == cached-subvol
* mv f1 f2 on master.
* B1 has change-log entry of rename f1 f2
* rebalance migrates f2 from B1 and B2
* mv f2 f1 on master.
* B2 has change-log entry of rename f2 f1

Since changelog entries (rename f1 f2) and (rename f2 f1) are processed 
independently by gsyncds, which of either f1 and f2 survives on slave 
is subject to race. Note that on master its file f1 with name f1 which 
survived. On slave it can be either file f1 with name f1 or file f2 
with name f2 based on who wins the race of processing changelog.


Change-Id: Iebc222f582613924c3a7cba37fb6d3e2d8332eda
BUG: 1141379
Signed-off-by: Nithya Balachandran <nbalacha@redhat.com>
Reviewed-on: http://review.gluster.org/10410
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Tested-by: NetBSD Build System
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Tested-by: Raghavendra G <rgowdapp@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/libglusterfs/src/glusterfs.h b/libglusterfs/src/glusterfs.h
index ba59e4d54db..3373ef6f7b8 100644
--- a/libglusterfs/src/glusterfs.h
+++ b/libglusterfs/src/glusterfs.h
@@ -140,6 +140,7 @@
 #define GLUSTERFS_VERSION_XCHG_KEY     "glusterfs.version.xchg"
 
 #define GLUSTERFS_INTERNAL_FOP_KEY  "glusterfs-internal-fop"
+#define DHT_CHANGELOG_RENAME_OP_KEY   "changelog.rename-op"
 
 #define ZR_FILE_CONTENT_STR     "glusterfs.file."
 #define ZR_FILE_CONTENT_STRLEN 15