On Thu, 1 Jul 2010 14:40:44 +0800 Shaochun Wang <scwang@xxxxxxxxx> wrote: > -bash-4.1$ sudo dd if=/dev/zero of=test.dd bs=1M count=5000 > conv=fdatasync,notrunc Password: > 5000+0 records in > 5000+0 records out > 5242880000 bytes (5.2 GB) copied, 63.497 s, 82.6 MB/s > > -bash-4.1$ sudo dd if=/dev/zero of=test.dd bs=1M count=5000 > conv=fdatasync,notrunc 5000+0 records in > 5000+0 records out > 5242880000 bytes (5.2 GB) copied, 18.1033 s, 290 MB/s > > I don't know why the second dd becomes 290MB/s and the first 82.6MB/s. That's because the first time the filesystem had to increase the file's size 5000 times by allocating additional 1 MB, and the second time it was just writing to an already allocated file. If you see such a big difference here, run that test 3 or more times, and discard the first run's results. -- With respect, Roman
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