On 28 October, 2012 - Edward Ned Harvey (opensolarisisdeadlongliveopensolaris) 
sent me these 1,0K bytes:

> > From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
> > boun...@opensolaris.org] On Behalf Of Jim Klimov
> > 
> > I tend to agree that parity calculations likely
> > are faster (even if not all parities are simple XORs - that would
> > be silly for double- or triple-parity sets which may use different
> > algos just to be sure).
> Even though parity calculation is faster than fletcher, which is
> faster than sha256, it's all irrelevant, except in the hugest of file
> servers.  Go write to disk or read from disk as fast as you can, and
> see how much CPU you use.  Even on moderate fileservers that I've done
> this on (a dozen disks in parallel) the cpu load is negligible.  
> If you ever get up to a scale where the cpu load becomes significant,
> you solve it by adding more cpu's.  There is a limit somewhere, but
> it's huge.

For just the parity thing, this is an older linux on a quite old cpu
(first dual core athlon64's):

[961655.168961] xor: automatically using best checksumming function: generic_sse
[961655.188007]    generic_sse:  6128.000 MB/sec
[961655.188010] xor: using function: generic_sse (6128.000 MB/sec)
[961655.256025] raid6: int64x1   1867 MB/s
[961655.324020] raid6: int64x2   2372 MB/s
[961655.392027] raid6: int64x4   1854 MB/s
[961655.460019] raid6: int64x8   1672 MB/s
[961655.528062] raid6: sse2x1     834 MB/s
[961655.596047] raid6: sse2x2    1273 MB/s
[961655.664028] raid6: sse2x4    2116 MB/s
[961655.664030] raid6: using algorithm sse2x4 (2116 MB/s)

So raid6 at 2Gbyte/s and raid5 at 6Gbyte/s should be enough on a 6+ year
old low-end desktop machine..

Tomas Forsman, st...@acc.umu.se, http://www.acc.umu.se/~stric/
|- Student at Computing Science, University of UmeƄ
`- Sysadmin at {cs,acc}.umu.se
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