On Tue, Sep 2, 2008 at 15:39, Barton Fisk <[EMAIL PROTECTED]> wrote:
> Hi,
> Forgive my ignorance of ZFS, but I have a customer that would like to set up
> three 14+2 raidz2 groups on a new thor with 48 1TB drives (updated thumper)
> so that 42TB for data could be achieved. What performance or other technical
> issues with a stripe 14 disks wide would he likely see? He does not want a
> hot spare.
Well, first, you need a boot disk of some sort. Thor does have a CF
slot, so that's an option, but if the customer plans to do, say, home
directories on the root directory, using disks might be a good place
to start.
Second, random read/write performance is going to suck a lot compared
to narrower stripes. Raidz{,2} groups need to read from N-{1,2} disks
so that they can verify on reads that the checksum matches. Thus,
with a 16-disk raidz2 group, you must wait for 14 IOs to complete
before you can decide whether you've got junk back from any disk.
Smaller groups would help this quite a bit, both because there are
more groups and because they're smaller. Even four 10+2 groups (for a
still-respectable 40 TB) would be better, and six 6+2 groups (for 36
TB) would be my recommendation. If space is really that much of a
concern, or you're doing sequential transfers to large files, 14+2
groups will probably be survivable... but I wouldn't use them.
Will
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