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

[Krutika Dhananjay]

----- Original Message -----

> From: "A Ghoshal" <a.ghoshal at tcs.com>
> To: gluster-users at gluster.org
> Sent: Tuesday, February 3, 2015 12:00:15 AM
> Subject: [Gluster-users] A few queries on self-healing and AFR (glusterfs
> 3.4.2)

> Hello,

> 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...
In 3.4.x, even files that are currently undergoing modification will be listed in heal-info output. So this could be the reason why the file(s) disappear from the output after a while, in which case reducing cluster.heal-timeout might not solve the problem. Since 3.5.1, heal-info _only_ reports those files which are truly undergoing heal. 

> 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.
AFR has always returned write-completion status to the application only _after_ the data is written to all replicas. The appearance of files under modification in heal-info output might have led you to think the changes have not (yet) been synced to the other replica(s). 

> 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.
There is an infra piece called syncop in gluster using which multiple heal jobs are handled by handful of threads. The maximum it can scale up to is 16 depending on the load. It is safe to assume that there will be one healer thread per replica set. But if the load is not too high, just 1 thread may do all the healing. 

-Krutika 

> Thanks,
> Anirban

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