Chris Dunbar - Earthside, LLC wrote:
Hello,
After being immersed in this list and other ZFS sites for the past few weeks I am having
some doubts about the zpool layout on my new server. It's not too late to make a change
so I thought I would ask for comments. My current plan to to have 12 x 1.5 TB disks in a
what I would normally call a RAID 10 configuration. That doesn't seem to be the right
term here, but there are 6 sets of mirrored disks striped together. I know that
"smaller" sets of disks are preferred, but how small is small? I am wondering
if I should break this into two sets of 6 disks. I do have a 13th disk available as a hot
spare. Would it be available for either pool if I went with two? Finally, would I be
better off with raidz2 or something else instead of the striped mirrored sets?
Performance and fault tolerance are my highest priorities.
Thank you,
Chris Dunbar
There's not much benefit I can see to having two pools if both are using
the same configuration (i.e all mirrors or all raidz). There are reasons
to do so, but I don't see that they would be of any real benefit for
what you describe. A Hot spare disk can be assigned to multiple pools
(often referred to as a "global" hot spare)
Preferences for raidz[123] configs is to have 4-6 data disks in the vdev.
Realistically speaking, you have several different (practical)
configurations possible, in order of general performance:
(a) 6 x 2-way mirrors + 1 pool hot spare -> 9TB usable
(b) 4 x 3-ways mirrors + 1 pool hot spare -> 6TB usable
(c) 1 6-disk raidz + 1 7-disk raidz -> 16.5TB usable
(d) 2 6-disk raidz + 1 pool hot spare -> 15TB usable
(e) 1 6-disk raidz2 + 1 7-disk raidz2 -> 13.5TB usable
(f) 2 6-disk raidz2 + 1 pool hot spare -> 12TB usable
(g) 1 6-disk raidz3 + 1 7-disk raidz3 -> 10.5TB usable
(h) 1 13-disk raidz3 -> 15TB usable
Given the size of your disks, resilvering is likely to have a
significant time problem in any RAIDZ[123] configuration. That is,
unless you are storing (almost exclusively) very large files, resilver
time is going to be significant, and can potentially be radically higher
than a mirrored config.
The mirroring configs will out-perform raidz[123] on everything except
large streaming write/reads, and even then, it's a toss-up.
Overall, the (a), (d), and (f) configurations generally offer the best
balance of redundancy, space, and performance.
Here's the chances to survive disk failures (assuming hot spares are
unable to be used; that is, all disk failures happen in a short period
of time) - note that all three can always survive a single disk failure:
(a) 90% for 2, 73% for 3, 49% for 4, 25% for 5.
(d) 55% for 2, 27% for 3, 0% for 4 or more
(f) 100% for 2, 80% for 3, 56% for 4, 0% for 5.
Depending on your exact requirements, I'd go with (a) or (f) as the best
choices - (a) if performance is more important, (f) if redundancy
overrides performance.
--
Erik Trimble
Java System Support
Mailstop: usca22-123
Phone: x17195
Santa Clara, CA
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