15 minutes to do a fdsync is way outside the slowdown usually seen.
The footprint for 6413510 is that when a huge amount of
data is being written non synchronously and a fsync comes in for the
same filesystem then all the non-synchronous data is also forced out
synchronously. So is there a lot of data being written during the vi?
vi will write the whole file (in 4K) chunks and fsync it.
(based on a single experiment).
So for a largefile vi , on quit, we have lots of data to
sync in and of itself. But because 6413510 we potentially
have to sync lots of other data written by other
applications.
Now take a Niagara with lots of available CPUs and lots
of free memory (32GB maybe?) running some 'tar x' in
parallel. A huge chunk of the 32GB can end up as dirty.
I say too much so because of lack of throttling:
http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6429205
6429205 each zpool needs to monitor it's throughput and throttle heavy
writers
Then vi :q; fsyncs; and all of the pending data must
sync. So we have extra data to sync because of:
http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6413510
zfs: writing to ZFS filesystem slows down fsync() on other files in the
same FS
Furthermore, we can be slowed by this:
http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6440499
zil should avoid txg_wait_synced() and use dmu_sync() to issue parallel
IOs...
Note: 6440499 is now fixed in the gate.
And finally all this data goes to a single disk. Worse a
slice of a disk. Since it's just a slice ZFS can't enable
the write cache. Then if there is no tag queue (is there ?) we
will handle everything one I/O at a time. If it's a SATA
drive we have other issues...
I think we've hit is all here. So can this lead to 15 min
fsync ? I can't swear, Actually I won't be convinced myself
before I convince you, but we do have things to chew on
already.
Do I recall that this is about a 1GB file in vi ?
:wq-uitting out of a 1 GB vi session on a 50MB/sec disk will
take 20sec when everything hums and there are no other
traffic involved. With no write cache / no tag queue , maybe
10X more.
-r
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