[Lustre-discuss] MPI-IO / ROMIO support for Lustre

[Mark Dixon]

On Fri, 5 Nov 2010, Rob Latham wrote:
...
> Let's consider two process A and B who are doing non-contiguous
> writes.  A writes to the even bytes and B writes to the odd bytes.
> Data sieving means that both processes will read a big chunk of the
> file, modify the local copy of the buffer and then write it.  OK, so
> Lustre will ensure that either all of A's write or all of B's write
> will show up, but that still means one process stomps all over the
> other.
>
> so you need something to serialize the read-modify-write.
>
> Note that if you use *collective* i/o then much of this no longer
> becomes a problem.  Because it's collective the processes can use MPI
> to coordinate and eliminate this false sharing problem.
>
> Note also that if you never ever use a shared file, then I guess you
> can use local locks.  But if you are always doing one file per
> processor instead of a few shared files,  you will end up with at the
> very least a mess of files.
...

Brilliant, very informative - thanks Rob.

Hopefully this will help aid other people in my situation; this question 
has cropped-up a few times on this list and never seems to make it to a 
consensus - so different people have chosen different options.

Best wishes,

Mark
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Mark Dixon                       Email    : m.c.dixon at leeds.ac.uk
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