On 01/25/2016 02:17 AM, Richard Wareing wrote:
Hello all,
Just gave a talk at SCaLE 14x today and I mentioned our new locks
revocation feature which has had a significant impact on our GFS
cluster reliability. As such I wanted to share the patch with the
community, so here's the bugzilla report:
https://bugzilla.redhat.com/show_bug.cgi?id=1301401
=====
Summary:
Mis-behaving brick clients (gNFSd, FUSE, gfAPI) can cause cluster
instability and eventual complete unavailability due to failures in
releasing entry/inode locks in a timely manner.
Classic symptoms on this are increased brick (and/or gNFSd) memory
usage due the high number of (lock request) frames piling up in the
processes. The failure-mode results in bricks eventually slowing down
to a crawl due to swapping, or OOMing due to complete memory
exhaustion; during this period the entire cluster can begin to fail.
End-users will experience this as hangs on the filesystem, first in a
specific region of the file-system and ultimately the entire
filesystem as the offending brick begins to turn into a zombie (i.e.
not quite dead, but not quite alive either).
Currently, these situations must be handled by an administrator
detecting & intervening via the "clear-locks" CLI command.
Unfortunately this doesn't scale for large numbers of clusters, and
it depends on the correct (external) detection of the locks piling up
(for which there is little signal other than state dumps).
This patch introduces two features to remedy this situation:
1. Monkey-unlocking - This is a feature targeted at developers (only!)
to help track down crashes due to stale locks, and prove the utility
of he lock revocation feature. It does this by silently dropping 1%
of unlock requests; simulating bugs or mis-behaving clients.
The feature is activated via:
features.locks-monkey-unlocking <on/off>
You'll see the message
"[<timestamp>] W [inodelk.c:653:pl_inode_setlk] 0-groot-locks: MONKEY
LOCKING (forcing stuck lock)!" ... in the logs indicating a request
has been dropped.
2. Lock revocation - Once enabled, this feature will revoke a
*contended*lock (i.e. if nobody else asks for the lock, we will not
revoke it)either by the amount of time the lock has been held, how
many other lock requests are waiting on the lock to be freed, or some
combination of both. Clients which are losing their locks will be
notified by receiving EAGAIN (send back to their callback function).
The feature is activated via these options:
features.locks-revocation-secs <integer; 0 to disable>
features.locks-revocation-clear-all [on/off]
features.locks-revocation-max-blocked <integer>
Recommended settings are: 1800 seconds for a time based timeout (give
clients the benefit of the doubt, or chose a max-blocked requires some
experimentation depending on your workload, but generally values of
hundreds to low thousands (it's normal for many ten's of locks to be
taken out when files are being written @ high throughput).
I really like this feature. One question though, self-heal, rebalance
domain locks are active until self-heal/rebalance is complete which can
take more than 30 minutes if the files are in TBs. I will try to see
what we can do to handle these without increasing the revocation-secs
too much. May be we can come up with per domain revocation timeouts.
Comments are welcome.
Pranith
=====
The patch supplied will patch clean the the v3.7.6 release tag, and
probably to any 3.7.x release & master (posix locks xlator is rarely
touched).
Richard
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