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SubjectRe: [Bench] New benchmark showing fileserver problem in 2.4.12
On Oct 17, 2001  23:06 +1000, Robert Cohen wrote:
> Factor 1: the performance problems only occur when you are rewriting an
> existing file in place. That is writing to an existing file which is
> opened without O_TRUNC. Equivalently, if you have written a file and
> then seek'ed back to the beginning and started writing again.
> Evidence: in the report I posted yesterday, the test I was using
> involved 5 clients rewriting 30 Meg files on a 128 Meg machine. The
> symptom was that after about 10 seconds, the throughput as shown by
> vmstat "bo" drops sharply and we start getting reads occuring as shown
> by the "bi" figure.

Just a guess - if you are getting reads that are about the same as writes,
then it would indicate that the code is doing "read-modify-write" for the
existing file data rather than just "write". This would be caused by not
writing only full-sized aligned blocks to the files.

As to why this is happening only over the network - it may be that you are
are unable to send an even multiple of the blocksize over the network (MTU)
and this is causing fragmented writes. Try using a smaller block size like
4k or so to see if it makes a difference.

Another possibility is that with 8k chunks you are needing order-1
allocations to receive the data and this is causing a lot of searching for
buffers to free.

Cheers, Andreas
Andreas Dilger \ "If a man ate a pound of pasta and a pound of antipasto,
\ would they cancel out, leaving him still hungry?" -- Dogbert

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