Maurice Volaski
2007-Jan-13 02:31 UTC
[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 at aecom.yu.edu Computing Support, Rose F. Kennedy Center Albert Einstein College of Medicine of Yeshiva University
Christian Kujau
2007-Jan-14 03:12 UTC
[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