[Gluster-devel] self-heal behavior

[Anand Avati]

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.

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.

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).

thanks,
avati

2007/7/4, Gerry Reno <greno at verizon.net>:
>
>   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