[TAG] 2c tip: filtering in-place

[Jay R. Ashworth]

On Mon, Mar 22, 2004 at 11:05:41AM -0500, Ben Okopnik wrote:
> On Mon, Mar 22, 2004 at 11:21:52AM +0530, Kapil Hari Paranjape wrote:
> > As far reading and writing to pipes is concerned, here is how I
> > understand it---please correct me if I am wrong. The kernel has its
> > own *internal* settings on how much data is buffered before a writing
> > process is put at the back of the queue and the reading process is woken
> > up. Thus killing any one process in the "dd" pipeline could only result
> > in *less* data being written than was read---an error from which one can
> > recover as described above.
> 
> Since the source and the target file are the same, wouldn't you end up
> with some truncated version of your data (i.e., the source being gone no
> matter what)? It seems to me that the difference between complete
> destruction of data and truncation of it at some random point can only
> matter theoretically, except in a vanishingly small number of
> situations.

No, you wouldn't.

The target side dd is doing *random access*.

It writes the blocks sequentially, but it writes them *into* the
standing file, one at a time, without touching the blocks around them.  
Likewise on the read side.  The killer is the *redirection*, which his
approach does not use, at all.  Not the pipe.

Cheers,
-- jra
-- 
Jay R. Ashworth                                                jra at baylink.com
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