FS of choice for max random iops ( Maildir )
freebsd at top-consulting.net
freebsd at top-consulting.net
Fri Sep 16 21:39:59 UTC 2011
Quoting Terje Elde <terje at elde.net>:
> On 16. sep. 2011, at 16:18, freebsd at 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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