Re: [Gluster-devel] self-heal behavior

[Gerry Reno]

Avati,
 Comments inline...

Anand Avati wrote:
> Gerry,
>  your question is appropriate, but the answer to 'when to resync' is 
> not very simple. when a brick which was brought down is brought up 
> later, it may be a completely new (empty) brick. In that case starting 
> to sync every file would most likely be the wrong decision. (we should 
> rather sync the file which the user needs than some unused file). Even 
> if we chose to sync files without user accessing them it would be very 
> sluggish too since it would be intervening in other operations.
Self-heal should start immediately to sync files but not at full speed 
but rather at some throttled nice level that would not impact operations.


> The current approach is to sync files on the next open() on it. This 
> is usually a good balance since, during open() if we were to sync a 
> file, even if it was a GB it would take 10-15 secs, and for normal 
> files (in the order of few MBs) it is almost not noticable. But if 
> this were to happen together for all files whether the user accessed 
> them or not there would be a lot of traffic and be very sluggish.
Again this should be done at a throttled level if there were other 
operations happening, if not then throttle it up.


> This approach of syncing on open() is what even other filesystems 
> which support redundancy do.
>
> Detecting 'idle time' and beginning sync-up and pausing the sync-up 
> when user begins activity is a very tricky job, but that is definitely 
> what we aim at finally. It is not enough if AFR detects the client is 
> free, because the servers may be busy serving files to another client 
> and syncing at that time may not be the most apprpriate time. The 
> following versions of AFR will have more options to tune 'when' to 
> sync. Currently it is only at open(). We plan to add options to make 
> it sync on lookup() (happens on ls). Later versions would have 
> pro-active syncing (detecting that both server and clients are idle etc).
That will be great.

Gerry





> thanks,
> avati
>
> 2007/7/4, Gerry Reno <address@hidden <mailto:address@hidden>>:
>
>       I've been doing some testing of self-heal.  Basically taking
>     down one
>     brick and then copying some files to one of the client mounts, then
>     bringing the downed brick back up.  What I see is that when I
>     bring the
>     downed brick back up, no activity occurs.  It's only when I start
>     doing
>     something in one of the client mounts that something occurs to rebuild
>     the out-of-sync brick.  My concern with this is that if I have four
>     applications on different client nodes (separate machines) using the
>     same data set (mounted on GlusterFS).  The brick on one of these nodes
>     is out-of-sync, and it is not until some user is trying to use the
>     application that the brick starts to resync.  This results in
>     sluggish
>     performance to the user as all the data has to be brought over the
>     network from other bricks since the local brick is out-of-sync.  Now
>     there may have been ten minutes of idle time prior to this user trying
>     to access the data but glusterfs did not make any use of this time to
>     rebuild the out-of-sync brick but rather waited until a user tried to
>     access data.  To me, it appears that glusterfs should be making use of
>     such opportunity and this would diminish the overall impact to
>     users of
>     the out-of-sync condition.
>
>     Regards,
>     Gerry
>
>
>
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> --
> Anand V. Avati