Oops - couple of corrections and clarifications below...
On Sat, May 19, 2012 at 9:47 AM, Jan Nielsen <jan.sture.nielsen@xxxxxxxxx> wrote:
After seeing less much performance during pg_dump and pg_restore operations from a 10x15k SAN RAID1+1 XFS mount
10x15k RAID1+0 on a SAN with XFS on /dev/sdc
(allocsize=256m,attr2,logbufs=8,logbsize=256k,noatime,nobarrier) than the local-storage 2x15k RAID1 EXT4 mount,
2x15k RAID1 on local-storage with EXT4 on /dev/sda
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?
Is this variance, as contrasted to the local-storage drive, and drop in performance in relation to the local-storage typical of SAN?
Cheers,Jan
