[Gluster-users] A few queries on self-healing and AFR (glusterfs 3.4.2)

A Ghoshal a.ghoshal at tcs.com
Mon Feb 2 22:41:20 UTC 2015

It seems I found out what goes wrong here - and this was useful learning 
to me:

On one of the replica servers, the client mount did not have an open port 
to communicate with the other krfsd process. To illustrate:

root at serv1:/root> ps -ef | grep replicated_vol
root     30627     1  0 Jan29 ?        00:17:30 /usr/sbin/glusterfs 
--volfile-id=replicated_vol --volfile-server=serv1 /mnt/replicated_vol
root     31132 18322  0 23:04 pts/1    00:00:00 grep 
root     31280     1  0 06:32 ?        00:09:10 /usr/sbin/glusterfsd -s 
serv1 --volfile-id replicated_vol.serv1.mnt-bricks-replicated_vol-brick -p 
-S /var/run/4d70e99b47c1f95cc2eab1715d3a9b67.socket --brick-name 
/mnt/bricks/replicated_vol/brick -l 
--xlator-option *-posix.glusterd-uuid=c7930be6-969f-4f62-b119-c5bbe4df22a3 
--brick-port 49172 --xlator-option replicated_vol.listen-port=49172

root at serv1:/root> netstat -p | grep 30627
tcp        0      0 serv1:715           serv1:24007         ESTABLISHED 
30627/glusterfs <= client<->local glusterd
tcp        0      0 serv1:863           serv1:49172         ESTABLISHED 
30627/glusterfs <= client<->local brick
root at serv1:/root> 

However, the client on the other server did have a port open to the mount, 
and so whatever one wrote on the other server synced over immediately.

root at serv0:/root> ps -ef | grep replicated_vol
root     12761  7556  0 23:05 pts/1    00:00:00 replicated_vol
root     15067     1  0 06:32 ?        00:04:50 /usr/sbin/glusterfsd -s 
serv1 --volfile-id replicated_vol.serv1.mnt-bricks-replicated_vol-brick -p 
-S /var/run/f642d7dbff0ab7a475a23236f6f50b33.socket --brick-name 
/mnt/bricks/replicated_vol/brick -l 
--xlator-option *-posix.glusterd-uuid=13df1bd2-6dc8-49fa-ade0-5cd95f6b1f19 
--brick-port 49209 --xlator-option replicated_vol.listen-port=49209
root     30587     1  0 Jan30 ?        00:12:17 /usr/sbin/glusterfs 
--volfile-id=serv --volfile-server=serv0 /mnt/replicated_vol

root at serv0:/root> netstat -p | grep 30587
tcp        0      0 serv0:859           serv1:49172         ESTABLISHED 
30587/glusterfs <= client<->remote brick
tcp        0      0 serv0:746           serv0:24007         ESTABLISHED 
30587/glusterfs <= client<->glusterd
tcp        0      0 serv0:857           serv0:49209         ESTABLISHED 
30587/glusterfs <= client<->local brick
root at serv0:/root> 

So, the client has no open tcp link with the mate brick - which is why it 
cannot write to the mate brick directly, and instead has to rely on the 
self-heal daemon instead to do the job. Of course, I now need to debug why 
the connection fails, but at least we are clean on AFR. 

Thanks everyone.

From:   A Ghoshal <a.ghoshal at tcs.com>
To:     gluster-users at gluster.org
Date:   02/03/2015 12:00 AM
Subject:        [Gluster-users] A few queries on self-healing and AFR 
(glusterfs      3.4.2)
Sent by:        gluster-users-bounces at gluster.org


I have a replica-2 volume in which I store a large number of files that 
are updated frequently (critical log files, etc). My files are generally 
stable, but one thing that does worry me from time to time is that files 
show up on one of the bricks in the output of gluster v <volname> heal 
info. These entries disappear on their own after a while (I am guessing 
when cluster.heal-timeout expires and another heal by the self-heal daemon 
is triggered). For certain files, this could be a bit of a bother - in 
terms of fault tolerance... 

I was wondering if there is a way I could force AFR to return 
write-completion to the application only _after_ the data is written to 
both replicas successfully (kind of, like, atomic writes) - even if it 
were at the cost of performance. This way I could ensure that my bricks 
shall always be in sync. 

The other thing I could possibly do is reduce my cluster.heal-timeout (it 
is 600 currently). Is it a bad idea to set it to something as small as 
say, 60 seconds for volumes where redundancy is a prime concern? 

One question, though - is heal through self-heal daemon accomplished using 
separate threads for each replicated volume, or is it a single thread for 
every volume? The reason I ask is I have a large number of replicated 
file-systems on each volume (17, to be precise) but I do have a reasonably 
powerful multicore processor array and large RAM and top indicates the 
load on the system resources is quite moderate. 

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