[Dirvish] slow writing to external USB2 hard disc

[Gert Brinkmann]

Hello,

sorry that this posting is somewhat off topic. But maybe someone had the
same problems when using dirvish and did find a solution?

I am using dirvish to backup files from my Linux system (debian/sid) to
an external USB2 drive (a Seagate disc). I am running kernel 2.6.18. The
hard disc is connected to an USB2 controller. The Problem: data transfer
is very slow. It makes approx. 1 MByte/s only, but it should be more
than 20 MByte/s. The hard disc is formatted as ext3fs and is mounted as

> /dev/sde2 on /media/bak1 type ext3 (rw,nosuid,nodev,sync,noatime,data=ordered)

I have tried several things:

The fastest transfer is 23 MB/s by using dd to write a file directly to
the disc:

sync; time (dd if=/dev/zero of=foo count=100 bs=1048576; sync)
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 4,45075 seconds, 23,6 MB/s

real    0m4.559s

-------------------------------

If I am copying the 100MB file from my internal hard disc to the
external it takes much longer:

sync; time (cp foo /media/bak1/test/;sync)

real    1m24.978s
user    0m0.008s
sys     0m1.444s

I have tried the same with rsync that was at similar speed. Trying it
via rsync again after remounting the device was much faster!?!?!

sync; time (rsync foo /media/bak1/test/;sync)

real    0m11.064s

-------------------------------

Because cp AFAIK cannot be configured to use different block sizes I
have tried tar instead (the -b value is 20 per default):

sync; time (tar cf /media/bak1/test/foo.tar foo; sync)

real    0m40.504s
user    0m0.016s
sys     0m1.284s

Using -b with different values does give different times:

-b	time (sec)
------------------
2	4m52.585s
4	2m32.798s
8	1m25.332s
16	0m46.627s
20	0m40.504s
32	0m29.584s
64	0m19.463s
256	0m7.635s
512	0m6.128s


I have also tried to mount the device as ext2fs or via data=writeback
but that did not change anything.

Do you have an idea what the problem might be? Ist there a solution?

Thanks,
Gert