Dear Benjamin,

In message [ref] you wrote:

On several more RAID specific SoC (like the PowerPC 440x/460x) a XOR 
engine is included to speed up RAID calculations (effectively a hardware 
RAID engine).

I think Intel Atom CPUs were planned/got a similar engine added, but 
with the advance in CPU efficiencies and frequencies, these XOR engines 
are practically obsolete: You will barely use 5% of your CPU (if a high 
end CPU) for the parity calculation on RAID5.

These days it seems that the overhead from the parity calculations are 
so small that they become insignificant.

I fully agree here.

Even on a not so fresh desktop CPU (Intel Core2 Quad CPU Q9550 at
2.83GHz) I see this in the kernel logs:

...
[   15.944466] xor: measuring software checksum speed
[   15.983008]    prefetch64-sse: 11592.000 MB/sec
[   16.020008]    generic_sse: 10232.000 MB/sec
[   16.045623] xor: using function: prefetch64-sse (11592.000 MB/sec)
...

So for each percent of CPU bandwith I offer I get 115 MB/s bandwith
for parity calculations.  None of the RAID arrays I have running would
ever need even close to 10 % of my CPU (in theory, at least).

Best regards,

Wolfgang Denk

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