Writing over a drive using /dev/zero

[Daniel Wayne Armstrong]

I wanted to estimate how much time it would take to write over a 1TB
external hard drive connected by USB using "dd" and /dev/zero ... so I
generated a sample 1GB file using:

touch zero_erase && /usr/bin/time -av -o zero_erase dd if=/dev/zero
of=file1g_zero.tmp bs=1M count=1024

User time (seconds): 0.00
System time (seconds): 0.93
Percent of CPU this job got: 15%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:06.06

Extrapolating from the sample I estimated it would take roughly 2
hours to fill a 1TB drive. But the actual result was:

/usr/bin/time -av -o zero_erase dd if=/dev/zero of=/dev/sdb bs=1M

User time (seconds): 1.11
System time (seconds): 889.01
Percent of CPU this job got: 2%
Elapsed (wall clock) time (h:mm:ss or m:ss): 10:09:45

Quite a difference. Two factors I think might account for it:

* test sample was created on the local disk ... external disk might be
slower due to the USB connection?

* CPU usage is much lower writing to the external disk vs the sample
file... would increasing the priority and CPU usage increase the speed
of the dd command?

Any thoughts? Thanks!
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