Re: Speed parameter for 'dd' command

[sdrb]

Pádraig Brady pisze:
> On 17/12/09 12:20, sdrb wrote:
> > Hello,
> >
> > I'd like to ask if it would be possible to add new parameter to "dd"
> > command. I was thinking about "speed" parameter. Sometimes it is needed
> > to use "dd" with very large amount of data and the whole process can
> > cause quite high load average for long time.
> > Maybe it would be good idea to add some kind of limitation for "dd"
> > command - and limits I/O operations to given kB or MB per seconds?
> > For example:
> >
> > dd if=/dev/zero of=/dev/sda speed=4096k
> >
> > and it will limits I/O operations to 4MB/s?
> >
> > Of course the whole "dd" process will take more time to complete in this
> > case, but at the same time it will be possible to keep load average in
> > more acceptable values and thus make using system more comfortable.
> >
> > What do you think?
>
> Well you would only slow things down so as to not take resources
> from other processes, but they in turn might not be constantly
> using those resources. I.E. it would be better to tell the
> system the priorities and let it sort it out optimally.
>
> Have you tried ionice?
>
> cheers,
> Pádraig.

You mean - have I tried to use ionice with IDLE class?
Yes, I tried ionice with IDLE parameter, but if I have to run more than 
one "dd" simultaneously - even if I use IDLE class - I have not any 
influence on this how the IDLE class will share I/O resources into those 
dd commands.

My intention was to limit the I/O operations to given value - so it 
could be possible to do I/O at this speed or a bit slower but not faster 
that the set value. It doesn't have to keep given average I/O speed for 
the whole time with 1 byte accuracy.

In the simplest version it could be done by sleeping for some time after 
transmitting one block of data for as long as the average I/O data speed 
is equal to set one with "speed" parameter. This "speed" parameter could 
be optional - not obligatory.

I've made some patch for doing this and it works quite good. It allows 
to keep very low load average, but of course the whole "dd" process 
takes much more time to finish.