Frank Cusack wrote:
> On July 14, 2008 9:54:43 PM -0700 Frank Cusack <[EMAIL PROTECTED]> wrote:
>
>> On July 14, 2008 7:49:58 PM -0500 Bob Friesenhahn
>>
> <[EMAIL PROTECTED]> wrote:
>
>>>> It sounds like they're talking more about traditional hardware RAID
>>>> but is this also true for ZFS? Right now I've got four 750GB drives
>>>> that I'm planning to use in a raid-z 3+1 array. Will I get markedly
>>>> better performance with 5 drives (2^2+1) or 6 drives 2*(2^1+1)
>>>> because the parity calculations are more efficient across N^2
>>>> drives?
>>>>
>>> With ZFS and modern CPUs, the parity calculation is surely in the noise
>>> to the point of being unmeasurable.
>>>
>> I would agree with that. The parity calculation has *never* been a
>> factor in and of itself. The problem is having to read the rest of
>> the stripe and then having to wait for a disk revolution before writing.
>>
>
> oh, you know what though? raid-z had this bug, or maybe we should just
> call it a behavior, where you only want an {even,odd} number of drives
> in the vdev. I can't remember if it was even or odd. Or maybe it was
> that you wanted only N^2+1 disks, choose any N. Otherwise you had
> suboptimal performance in certain cases. I can't remember the exact
> details but it wasn't because of "more efficient parity calculations".
> Maybe something about block sizes having to be powers of two and the
> wrong number of disks forcing a read?
>
> Anybody know what I'm referring to? Has it been fixed? I see the
> zfs best practices guide says to use only odd numbers of disks, but
> it doesn't say why. (don't you hate that?)
>
See the "Metaslab alignment" thread.
http://www.opensolaris.org/jive/thread.jspa?messageID=60241
-- richard
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