Quoting Terje Elde <te...@elde.net>:
On 16. sep. 2011, at 16:18, free...@top-consulting.net wrote:
Got a measly 74MB/sec.
You can't ask for advice, get it, do something completely different,
and then complain that it didn't work.
Neither can you ask people to donate their time, if you won't spend yours.
In other words: if you won't listen, there's no point in us talking.
However:
Don't disable ZIL. Just don't. It's not the way to go. If you want
to know why, google will help.
Also, you're making some assumptions, such as the ZIL being bad for
performance. That's not always the case. ZIL-writes are a rather
nice load for spinning metal storage. Even if you write through
cache, that can give you a boost on your real world workload.
Which brings us to the third bit. You're benchmarking, not trying
real world loads. That's the load you'll have to worry about, and
it's the load zfs shines at.
Thanks to the ZIL (the thing you're trying to kill, remember?) you
can convert seek heavy writes to sequential zil-writes, freeing up
disk bandwith for concurrent reads.
If you want to test before spending money, try what Svein said. Set
up a small logical volume (preferrably smaller than your controller
cache, if it's large enough), then try that as a dedicated zil-device.
Never tried that, but worth a shot.
Terje
It's not about spending money or not. I really want to use ZFS for
some of its features ( journaled, snapshots, etc ) but it has to be a
good fit for me. I'm not ignoring the advice I am given, just taking
it with a grain of salt disabling the ZIL is recommended - sometimes -
for NFS.
As per hundreds of messages I've read from the Archive along with this
page, http://wiki.freebsd.org/ZFSTuningGuide, it does appear that
disabling the ZIL is a solution for NFS. Yes, they still recommend
SSD drives and I fully understand that. My point was the following:
Why is a sequential write test like dd slower on ZFS than on UFS ? The
writes is already serialized so enabling/disabling the ZIL should have
very little impact - which is indeed the case.
I even went as far as disabling the cache flush option of ZFS through
this variable: vfs.zfs.cache_flush_disable: 1, since I already have
the write cache of the controller. I've also set some other variables
as per the Tuning guide but according to several benchmarks ( iozone,
bonnie++, dd ) ZFS still comes in slower than UFS at pretty much
everything.
Either I am missing something or there is something wrong with my setup.
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