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 <greno@xxxxxxxxxxx <mailto:greno@xxxxxxxxxxx>>:
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
