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[Q] How can the directory location to dd output affect performance?



I have two Opteron-based Tyan systems being supported by PCI-e Areca cards. There is definitely an issue going on in the two systems that is causing significantly degraded performance of these cards. It appeared, initially, that the SATA backplane on the Tyan chassis was wholly to blame.

But then I made an odd discovery.

I'm running from the Ubuntu LiveCD for 64-bit. It uses kernel 2.6.19-7 and the RAID drives are formatted as ext3. The benchmark command is dd if=/dev/zero of=output oflag=sync bs=100M count=1

My root is organized has a /maurice directory and a /maurice/drbd directory and initially I had changed to that directory to run the benchmark. In here, the speeds were slow, averaging about 40 MB/second. When I happened to run it from /, I suddenly began getting about 70 MB/second. So in some bizarre fashion, the location to where the output of dd is directed to dramatically impacts the performance. I have run from other directories and the performance varies depending on which directory I'm in.

Can anyone explain this?
--

Maurice Volaski, mvolaski aecom yu edu
Computing Support, Rose F. Kennedy Center
Albert Einstein College of Medicine of Yeshiva University


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