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

[Christian Kujau]

On Fri, 12 Jan 2007, Maurice Volaski wrote:
> 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.

Strange indeed. Only thing that comes to mind is: you're specifying the 
output file not as an absolute path, but relative: the directories (and 
its contents) are distributed all over the disk: some may "live" in the 
inner part of the plattern, some in the outer part - and different areas 
have different speeds. I've never encountered this and I could be dead 
wrong, but I'd suggest to specify the same 'of=/path/to/output' - I 
could imagine that it's more likely that for the next benchmark the 
filesystem uses the same on-disk location...no?

Christian.
-- 
BOFH excuse #12:

dry joints on cable plug