Re: Problem using dd within bash script

[Sebastian Rückerl]

On Mon, Apr 28, 2014 at 05:53:44PM +0200, Bernhard Voelker wrote:
> On 04/28/2014 05:01 PM, Pádraig Brady wrote:
> >Note the kernel may cache lots more than 4M before it
> >even starts writing out to the device.
> >I think there maybe some very recent improvements
> >to the kernel to address at least the excessive
> >caching part of the issue, though if there are
> >large pending writes to slow devices the kernel
> >can still block for a long time.
> 
> Okay, that sounds reasonable.
> I.e., in average the blocking time could be smaller,
> but if many writes are pending to be flushed to disk
> as in this case, then ~1min is reasonable for a slow
> device.
> 
> Thanks & have a nice day,
> Berny

This is in fact the case here. If i wait for some seconds before killing it,
it takes some (long) time to finish and after that states that it copied
about 500MB to 1GB. So I guess it is in fact a hughe buffer to ram that
is implicitly used by the OS. And of course if you have to wait for the
pending 500MB to 1GB it takes a while to finish.

The solution in my case is in fact to use oflag=direct. This way no caching
(at least not that much caching) is done.

Thanks,
Sebastian

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