Question about a ' dd ' command.

[James Gray]

On 08/03/2008, at 10:37 AM, Nils Kassube wrote:
> elmo wrote:
>> I read somewhere that these two commands are special application of
>> the basic 'dd' command that will copy and display the transfer  
>> action.
>>
>> $ dd if=/dev/zero of=/dev/null& pid=$!
>> $ kill -USR1 $pid; sleep 1; kill $pid
>>
>> I don't understand the null& and pid=$! in the first line and I don't
>> understand any of the second line.
>
> Now you still want to read the man page for the dd command which  
> hopefully
> explains the use of the USR1 signal.

Indeed, it does (for the archives):

Sending a USR1 signal to a running `dd' process makes it print I/O  
statistics to standard error and then resume copying.

       $ dd if=/dev/zero of=/dev/null& pid=$!
       $ kill -USR1 $pid; sleep 1; kill $pid

       18335302+0 records in 18335302+0 records out 9387674624 bytes  
(9.4 GB) copied, 34.6279 seconds, 271 MB/s

So, as you alluded to in your message Nils, the dumping of /dev/zero  
to /dev/null is quite useless for anything other than benchmarking.   
However, it would be interesting to see where the bottle-neck ended  
being in this situation; my guess would be that you'd end up CPU  
bound ;)  I couldn't be bothered digging any deeper; I have to sort  
out some SNMP weirdness elsewhere for work :(

Cheers,

James
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