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



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


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