> Rob Bray wrote:
>
>>This might be a dumb question, but what causes md to use a large amount
>> of
>>cpu resources when reading a large amount of data from a raid1 array?
>>Examples are on a 2.4GHz AMD64, 2GB, 2.6.15.1 (I realize there are md
>>enhancements to later versions; I had some other unrelated issues and
>>rolled back to one I've run on for several months).
>>
>>A given 7-disk raid0 array can read 450MB/s (using cat > null) and use
>>virtually no CPU resources. (Although cat and kswapd use quite a bit
>> [60%]
>>munching on the data)
>>
>>A raid5 array on the same drive set pulls in at 250MB/s, but md uses
>>roughly 50% of the CPU (the other 50% is spent dealing with the data,
>>saturating the processor).
>>
>>A consistency check on the raid5 array uses roughly 3% of the cpu. It is
>>otherwise ~97% idle.
>>md11 : active raid5 sdi2[5] sdh2[4] sdf2[3] sde2[2] sdd2[1] sdc2[6]
>> sdb2[0]
>> 248974848 blocks level 5, 256k chunk, algorithm 2 [7/7] [UUUUUUU]
>> [==============>......] resync = 72.2% (29976960/41495808)
>>finish=3.7min speed=51460K/sec
>>(~350MB/s aggregate throughput, 50MB/s on each device)
>>
>>Just a friendly question as to why CPU utilization is significantly
>>different between a check and a real-world read on raid5? I feel like if
>>there was vm overhead getting the data into userland, the slowdown would
>>be present in raid0 as well. I assume parity calculations aren't done on
>> a
>>read of the array, which leaves me at my question.
>>
>>
> What are you stripe and cache sizes?
>
> --
> bill davidsen <[EMAIL PROTECTED]>
> CTO TMR Associates, Inc
> Doing interesting things with small computers since 1979
>
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md11 : active raid5 sdi2[5] sdh2[4] sdf2[3] sde2[2] sdd2[1] sdc2[6] sdb2[0]
248974848 blocks level 5, 256k chunk, algorithm 2 [7/7] [UUUUUUU]
stripe_cache_size = 256
I've tried increasing it with the same result
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