On 10 Aug 2010, at 08:49, Ian Collins <i...@ianshome.com> wrote:
On 08/10/10 06:21 PM, Terry Hull wrote:
I am wanting to build a server with 16 - 1TB drives with 2 – 8 dri
ve RAID Z2 arrays striped together. However, I would like the capa
bility of adding additional stripes of 2TB drives in the future. W
ill this be a problem? I thought I read it is best to keep the str
ipes the same width and was planning to do that, but I was wonderi
ng about using drives of different sizes. These drives would all b
e in a single pool.
It would work, but you run the risk of the smaller drives becoming
full and all new writes doing to the bigger vdev. So while usable,
performance would suffer.
Almost by definition, the 1TB drives are likely to be getting full
when the new drives are added (presumably because of running out of
space).
Performance can only be said to suffer relative to a new pool built
entirely with drives of the same size. Even if he added 8x 2TB drives
in a RAIDZ3 config it is hard to predict what the performance gap will
be (on the one hand: RAIDZ3 vs RAIDZ2, on the other: an empty group vs
an almost full, presumably fragmented, group).
One option would be to add 2TB drives as 5 drive raidz3 vdevs. That
way your vdevs would be approximately the same size and you would
have the optimum redundancy for the 2TB drives.
I think you meant 6, but I don't see a good reason for matching the
group sizes. I'm for RAIDZ3, but I don't see much logic in mixing
groups of 6+2 x 1TB and 3+3 x 2TB in the same pool (in one group I
appear to care most about maximising space, in the other I'm
maximising availability)
The other issue is that of hot spares. In a pool of mixed size drives
you either waste array slots (by having spares of different sizes) or
plan to have unavailable space when small drives are replaced by large
ones.
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