On Mon, Feb 08, 2010 at 09:06:21AM -0500, Sean Kennedy wrote:
> Moving this to m...@...
>
> Would part of this discussion usefully related to such issues like using 'dd'
> for diskwipes/copies/reformatting and slow data movement speeds?
>
> There are times when I am wiping (for reuse) hard disks using 'dd' and I set
> the BlockSize to > 512 (like 1M or so sometimes)
> and the transfer speeds are quite a lot slower than for using 'dd' on some
> other Operating systems. (Linux or Windows)
>
> Mind you, for a lot of this, I am using oBSD RamDISK, so I am not anticipating
> a full-fledged OS support for the ATA or SCSI or USB2 platforms. But for those
> systems where I am using -stable or -current, the speeds are still comparably
> slow.
>
> I concur with Theo's point on portability and making a sysctl for kernel is
> hazardous, but what am I seeing in the above for 'dd' that would be causing
> the poor performance?
> (* BTW, I am using if=/dev/zero for the baseline, other if=/...'es may have
> lower performance as an input for compare*)
>
>
> Just my 2 cents.
>
> -sean
Well, for starters, different os, different caching behaviour. You
should read from and write to the raw device, sync and use the actual
elapsed time. Even then your measurement might be influenced by
caching.
# sync
# time (dd if=/dev/rsd0a of=/dev/null bs=1m && sync)
133+1 records in
133+1 records out
139797504 bytes transferred in 1.558 secs (89709129 bytes/sec)
0m1.56s real 0m0.00s user 0m0.07s system
#
OTOH, we do use a smaller size of the physical writes than other OSes.
So a lower performance for modern hardware is not really a big
surprise.
-Otto
