[OpenIndiana-discuss] ZIL write cache performance

[Open Indiana]

There are several online descriptions on how to tune MySQL in combination
with ZFS. One thing you need to tune or look at is the blocksize of the ZFS
drive where the MySQL data is stored. As far as I can imagine you could also
tune the ZIL blocksize to see what that changes.
Some usefull links:
http://blogs.oracle.com/roch/entry/proper_alignment_for_extra_performance
http://blogs.oracle.com/realneel/entry/the_zfs_intent_log


-----Original Message-----
From: Steve Gonczi [mailto:gonczi at comcast.net]
Sent: vrijdag 13 januari 2012 0:35
To: Discussion list for OpenIndiana
Subject: Re: [OpenIndiana-discuss] ZIL write cache performance

Hi Matt,

The ZIL is not a performance enhancer. (This is a common misunderstanding,
people sometimes view the ZIL as a write cache) .

It is a way to simulate "sync" semantics on files where you really need
that, instead of the coarser ganularity guarantee that zfs gives you without
it. (txg level, where the in-progress transaction group may roll back if you
crash).

If I am reading your post correctly, you are comparing 2 scenarios

1) Zil is enabled, and goes to the main storage pool
2) Zil is enabled but it goes to a dedicated SSD instead.
Please verify that this is indeed the case.

You should not expect having an SSD based zil performing better than when
turning the ZIL off altogether.

The latter of course will have better
performance, but you have to live with the possibility of losing some data.

Given that the case is (1) and (2), it all depends on how much performance
headroom your pool has, vs. the write performance of the SSD.

A fast SSD ( e.g.: DRAM based, and preferably dedicated to Zil and not
split) would work best. It does not have to be huge, just large enough to
store (say)
5 seconds worth of your planned peak data inflow.

You need to be aware of a recent performance regression discovered
pertaining to ZIL ( George Wilson has just posted the fix for review on the
illumos dev list) This has been in Illumos for a while, so it is possible,
that it is biting you.

Steve

----- Original Message -----
Hi, I've installed an SSD drive in my OI machine and have it partitioned
(sliced) with a main slice to boot from and a smaller slice to use as a
write cache (ZIL) for our data pool.

I've noticed that for many tasks, using the ZIL actually slows many tasks at
hand (operation within a qemu-kvm virtual machine, mysql loading importing a
dump file, etc). I know I bought a cheap SSD to play with so I wasn't
expected the best performance, but I would have expected some improvement,
not a slow down.

In one particular test, I have mysql running in a zone and loading a test
data set takes about 40 seconds without the ZIL and about 60 seconds with
ZIL. I certainly wasn't expecting a 50% slow down.

Is this to be expected?

Are there any best practices for testing an SSD to see if it will actually
improve performance of a zfs pool?


Thanks,
Matt


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