On 07/02/2010 20:56, Christo Kutrovsky wrote:
Darren, thanks for reply.
Still not clear to me thought.
Based on what you wrote below you do understand it.
The only purpose of the slog is to serve the ZIL. There may be many "ZIL"s on a
single slog.
Correct, and correct.
From Milek's blog:
logbias=latency - data written to slog first
logbias=throughtput - data written directly to dataset.
Roughly yes but there is slightly more to it than that, but those
are implementation details.
Here's my problem. I have raidz device with SATA drives. I use it to serve
iSCSI that is used for NTFS devices (bootable).
Windows is constantly writing something to the devices and all writes are
"synchronous". The result is that cache flushes are so often that the NCQ
(queue dept) hardly goes above 0.5 resulting in very poor read/write performance.
Disabling the ZIL (globally unfortunately) yields huge performance benefits for me
as now my ZFS server is acting as a buffer, and Windows is far more snappy. And
now I see queue> 3 occasionally and write performance doesn't suck big time.
That hints that an SSD that is fast to right to would be a good addition
to your system.
I am fine with loosing 5-10 even 15 seconds of data in the event of the crash,
as far as the data is consistent.
The "never turn off the ZIL" sounds scary, but if the only consequences are 15
(even 45) seconds of data loss .. i am willing to take this for my home environment.
Opinions?
You have done a risk analysis and if you are happy that your NTFS
filesystems could be corrupt on those ZFS ZVOLs if you lose data then
you could consider turning off the ZIL. Note though that it isn't
just those ZVOLs you are serving to Windows that lose access to a ZIL
but *ALL* datasets on *ALL* pools and that includes your root pool.
For what it's worth I personally run with the ZIL disabled on my home
NAS system which is serving over NFS and CIFS to various clients, but I
wouldn't recommend it to anyone. The reason I say never to turn off the
ZIL is because in most environments outside of home usage it just isn't
worth the risk to do so (not even for a small business).
--
Darren J Moffat
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