After seeing less much performance during pg_dump and pg_restore operations
from a 10x15k SAN RAID1+1 XFS mount (
allocsize=256m,attr2,logbufs=8,logbsize=256k,noatime,nobarrier) than the
local-storage 2x15k RAID1 EXT4 mount, I ran the following test of the
effect of read-ahead (RA):

for t in `seq 1 1 10`
do
  for drive in `ls /dev/sd[b-z]`
  do
    for ra in 256 512 `seq 1024 1024 70000`
    do
      echo benchmark-test: $drive $ra
      blockdev --setra $ra $drive
      hdparm -t $drive
      hdparm -T $drive
      echo benchmark-test-complete: $drive $ra
    done
  done
done

In this test, the local mount's buffered reads perform best around RA~10k @
150MB/sec then starts a steady decline. The SAN mount has a similar but
more subtle decline with a maximum around RA~5k @ 80MB/sec but with much
greater variance. I was surprised at the 80MB/sec for the SAN - I was
expecting 150MB/sec - and I'm also surprised at the variance. I understand
that there are many more elements involved for the SAN: more drives,
network overhead & latency, iscsi, etc. but I'm still surprised.

Is this expected behavior for a SAN mount or is this a hint at some
misconfiguration? Thoughts?


Cheers,

Jan

<<attachment: readahead-sdc.png>>

<<attachment: readahead-sda.png>>

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