Hi Daniel,
Am 08.02.10 05:45, schrieb Daniel Carosone:
On Mon, Feb 08, 2010 at 04:58:38AM +0100, Felix Buenemann wrote:
I have some questions about the choice of SSDs to use for ZIL and L2ARC.
I have one answer. The other questions are mostly related to your
raid controller, which I can't answer directly.
- Is it safe to run the L2ARC without battery backup with write cache
enabled?
Yes, it's just a cache, errors will be detected and re-fetched from
the pool. Also, it is volatile-at-reboot (starts cold) at present
anyway, so preventing data loss at power off is not worth spending any
money or time over.
Thanks for clarifying this.
- Does it make sense to use HW RAID10 on the storage controller or would
I get better performance out of JBOD + ZFS RAIDZ2?
A more comparable alternative would be using the controller in jbod
mode and a pool of zfs mirror vdevs. I'd expect that gives similar
performance to the controller's mirroring (unless higher pci bus usage
is a bottleneck) but gives you the benefits of zfs healing on disk
errors.
I was under the impression, that using HW RAID10 would save me 50% PCI
bandwidth and allow the controller to more intelligently handle its
cache, so I sticked with it. But I should run some benchmarks in RAID10
vs. JBOD with ZFS mirrors to see if this makes a difference.
Performance of RaidZ/5 vs mirrors is a much more workload-sensitive
question, regardless of the additional implementation-specific
wrinkles of either kind.
Your emphasis on lots of slog and l2arc suggests performance is a
priority. Whether all this kit is enough to hide the IOPS penalty of
raidz/5, or whether you need it even to make mirrors perform
adequately, you'll have to decide yourself.
So it seems right to assume, that RAIDZ1/2 has about the same
performance hit as HW RAID5/6 with Write Cache. I wasn't aware that ZFS
can do RAID10 style multiple mirrors, so that seems to be the better
option anyways.
--
Dan.
- Felix
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