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
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