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Re: bug: dd copies a random number of bytes


From: Robb Matzke
Subject: Re: bug: dd copies a random number of bytes
Date: Mon, 23 Feb 2009 14:11:40 -0500

Ah, thanks. I get it now. The full and partial blocks do indeed always add
up to what I would expect. It might be good if this were explicit in the man
page--I was expecing dd to try harder to fill its buffer each time no matter
how large a value for "bs=N".

Thanks,
--Robb

On Mon, 23 Feb 2009 19:46:39 +0100
Jim Meyering <address@hidden> wrote:

> Robb Matzke wrote:
> > I'm actually not concerned with the SIGPIPE and expect it. What concerns me
> > is that the final counts reported by *dd* are wrong. It should report 1024
> > records copied, but instead reports a seemingly random and variable amount.
> >
> > Why is dd closing its stdin earlier than I asked it to? Why is dd 
> > transfering
> > a random amount of data?
> 
> You chose a block size of 1MB.
> That is way larger than will ever be filled via a pipe,
> so every read gets a partial "block".
> dd stops after 1024 of those.
> 
> The output you see,
> 
>     0+1024 records in
>     0+1024 records out
>     4259840 bytes (4.3 MB) copied, 0.00729445 s, 584 MB/s
> 
> tells you that there were 0 full blocks read and 1024 partials.
> 
> ...
> >> >    cat /dev/zero |dd bs=1MB count=1024 of=/dev/null
> >> >    cat /dev/null |dd bs=1MB count=1024 >/dev/null
> >> >
> >> > An strace on dd shows that the last read/write pair both succeed and 
> >> > then dd
> >> > inexplicably closes both files.  None of the reads return zero or 
> >> > failure.
> >> > An strace on cat shows that its last write to stdout gets a sigpipe, so 
> >> > it
> >> > really is trying to send the data to dd and it's dd that's closing the 
> >> > pipe.
> >>
> >> That's expected and in fact required.
> >> You're telling dd to exit after reading 1024*1MB.
> >> Once dd exits, it closes its side of the pipe, but cat is still writing
> >> to the other end.  Writing to a closed pipe provokes a SIGPIPE, by default.
> >> How much cat actually writes before it's killed by the
> >> SIGPIPE depends on kernel buffering, hence the variability.
> >>
> >> > Replacing /dev/null with a real file exhibits the behavior one would 
> >> > expect.
> >> > Substituting some other I/O-counting command for dd works fine.


-- 
Robb Matzke L-159   | Ph:  812-967-6325  | I/O, I/O, it's off to disk we go,
Lawrence Livermore  | Fax: 812-967-6326  | A petabyte to read or write,
National Laboratory | Hrs: 3am-11am PDT  | I/O, I/O.




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