On Monday August 28, raid@bymeinc.com wrote:

quoted

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?

I assume you meant raid5 there.

md/raid5 shouldn't use that much CPU when reading.
It does use more than raid0 as it reads data in the stripe-cache and
then copies the data from the stripe cache into the read-buffer.  But
I wouldn't expect that to come anywhere near 50%.

Are you really seeing 'raid5d' using 50% of CPU in 'top' or similar?

NeilBrown

Sorry for the long response time -- email got lost.

top - 16:45:21 up 10 days, 17:41,  2 users,  load average: 0.58, 0.17, 0.05
Tasks: 113 total,   2 running, 111 sleeping,   0 stopped,   0 zombie
Cpu(s):  1.7% us, 87.7% sy,  6.3% ni,  0.0% id,  0.0% wa,  0.0% hi,  4.3% si
Mem:   2061564k total,  2044784k used,    16780k free,  1193384k buffers
Swap:  4257016k total,      552k used,  4256464k free,    24348k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
  945 root      10  -5     0    0    0 S 44.2  0.0   7:27.73 md11_raid5

quoted

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.

Thanks,
Rob

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