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From: Douglas Mayne on 17 Jan 2010 10:40
I had to zero some disk partitions yesterday. I used this command:

# dd if=/dev/zero of=/dev/sdb5 &
# P=$!

Then I checked its progress periodically,

# kill -USR1 $P

The progress checks showed significantly variable output speed, from 6 to
80 MB/s. I retested the same problem with a similar command,

# cat /dev/zero >/dev/sdb5

This does not allow progress monitoring, but it does complete with an
elapsed time consistent with the fastest disk I/O (i.e. 80 MB/s.)

I am curious to why there is such a big difference for what would seem to
be very similar I/O commands. Any comments?

Other information: I ran this on Slackware 13.0 with kernel 2.6.30.10.
The hardware was a Core 2 motherboard and Seagate SATA (3 Gb/s) hard disk.

TIA

--
Douglas Mayne
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