On Sat, 24 Oct 2009, Albert Chin wrote:
5-disk RAIDZ2 - doesn't that equate to only 3 data disks?
Seems pointless - they'd be much better off using mirrors,
which is a better choice for random IO...
Is it really pointless? Maybe they want the insurance RAIDZ2
provides. Given the choice between insurance and performance, I'll
take insurance, though it depends on your use case. We're using
5-disk RAIDZ2 vdevs. While I want the performance a mirrored vdev
would give, it scares me that you're just one drive away from a
failed pool. Of course, you could have two mirrors in each vdev but
I don't want to sacrifice that much space. However, over the last
two years, we haven't had any demonstratable failures that would
give us cause for concern. But, it's still unsettling.
I am using duplex mirrors here even though if a drive fails, the pool
is just one drive away from failure. I do feel that it is safer than
raidz1 because resilvering is much less complex so there is less to go
wrong and the resilver time should be the best possible.
For heavy multi-user use (like this Sun customer has) it is impossible
to beat the mirrored configuration for performance. If the I/O load
is heavy and the storage is "an intermediate holding place for data"
then it makes sense to use mirrors. If it was for long term archival
storage, then raidz2 would make more sense.
iostat shows queued I/O and I'm not happy about the total latencies -
wsvc_t in excess of 75ms at times. Average of ~60KB per read and only
~2.5KB per write. Evil Tuning guide tells me that RAIDZ2 is happiest
for long reads and writes, and this is not the use case here.
~2.5KB per write is definitely problematic. NFS writes are usually
synchronous so this is using up the available IOPS, and consuming them
at a 5X elevated rate with a 5 disk raidz2. It seems that a SSD for
the intent log would help quite a lot for this situation so that zfs
can aggregate the writes. If the typical writes are small, it would
also help to reduce the filesystem blocksize to 8K.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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