Writing over a drive using /dev/zero

[Daniel Wayne Armstrong]

On Mon, Nov 29, 2010 at 12:09 PM, Lennart Sorensen
<lsorense-1wCw9BSqJbv44Nm34jS7GywD8/FfD2ys at public.gmane.org> wrote:
> On Mon, Nov 29, 2010 at 11:55:16AM -0500, Daniel Wayne Armstrong wrote:
>> 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!
>
> Did you remember to sync to finish the write to disk as part of our
> measurement?
>
> For 1TB though, it probably becomes just noise in the end given how long
> it already takes.
>
> If it is USB2, then you can probably figure about 25MB/s, so 40 seconds
> per GB, so 40000 seconds for a TB.  So about 11 or 12 hours.  Might do
> it in 10 if you get lucky.  eSATA can probably do 80MB/s on a decent
> 1TB drive, so maybe 4 hours.

USB2 connection is a limiting factor... I see. Thanks Lennart!
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