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

[Maurice Volaski]

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