Jeff Bonwick wrote:
http://blogs.sun.com/roller/page/roch?entry=when_to_and_not_to
thanks, that is very useful information. it pretty much rules out raid-z
for this workload with any reasonable configuration I can dream up
with only 12 disks available. it looks like mirroring is going to
provide higher write IOPS and increased redundancy, obviously at the
expense of the available space.
There's an important caveat I want to add to this. When you're
doing sequential I/Os, or have a write-mostly workload, the issues
that Roch explained so clearly won't come into play. The trade-off
between space-efficient RAID-Z and IOP-efficient mirroring only
exists when you're doing lots of small random reads.
If your I/Os are large, sequential, or write-mostly, then ZFS's
I/O scheduler will aggregate them in such a way that you'll get
very efficient use of the disks regardless of the data replication
model. It's only when you're doing small random reads that the
difference between RAID-Z and mirroring becomes significant.
For such workloads, everything that Roch said is spot on.
An other data point that comes into play here, more so with enterprise
customers then SMB, is, "What happens when a failure occurs?" The
performance difference between, for example, a HW raid array running R5
and a ZFS pool running RZ would be good to have tested and documented.
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