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?)
-frank
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