Re: Raid5 reads and cpu

[Bill Davidsen <davidsen@tmr.com>]

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 <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979