Roman Mamedov wrote:
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.
Unless the notrunc option is used, the file is truncated and allocated
all over again. I'm pretty sure the inode is reused, that's down in the
noise.
--
Bill Davidsen <davidsen@xxxxxxx>
"We can't solve today's problems by using the same thinking we
used in creating them." - Einstein
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