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problem with disks on SAN

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Operating Systems Linux problem with disks on SAN
# 1  
Old 06-04-2005
problem with disks on SAN


I have a linux box attched to a SAN storage from EMC with RAID 5 .I understand that it has 3g cache howver a 20gb file creation takes too much time here are my results any ideas why

time dd if=/dev/zero of=disk.img bs=1048576 count=20000
20000+0 records in
20000+0 records out
997.59s real 0.01s user 268.61s system

time dd if=/dev/zero of=disk.img bs=1048576 count=20000
20000+0 records in
20000+0 records out
1093.96s real 0.04s user 269.08s system

How do i go about and pin point where the problem is.

# 2  
Old 06-06-2005
hmm.. Why would you consider it "too much"? I see it reasonable. The peak transfer rate to SAN (assuming you are using fiber link) is 100MB/s which is the ceiling. That leads you to 200s for 20GB data. Then, there are latencies, raid 5 parity generation, other application using SAN, etc.

So a bit slow but not unreasonable. In order to speed up, you may want to try smaller block size, say 8k or 16k. I would expect a ~100s less in real time with smaller block size.

Hope it helpful,
# 3  
Old 06-15-2005
I agree the blocksize may be part of the issue but you may also want to check your EMC. EMCs are funny beasts, if your RAID group contains too few disks you may have a problem. We ran into this in our AIX 5.3->EMC Clariion environment.

Other things worth checking are your firmware and driver versions for your HBA.

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