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SubjectRe: [PATCH] Speed up the cdrw packet writing driver
Peter Osterlund <petero2@telia.com> wrote:
>
> Is this a general VM limitation? Has anyone been able to saturate the
> write bandwidth of two different block devices at the same time, when
> they operate at vastly different speeds (45MB/s vs 5MB/s), and when
> the writes are large enough to cause memory pressure?

I haven't explicitly tested the pdflush code in a while, and I never tested
on devices with such disparate bandwidth. But it _should_ work.

The basic deign of the pdflush writeback path is:

for ( ; ; ) {
for (each superblock) {
if (no pdflush thread is working this sb's queue &&
the superblock's backingdev is not congested) {
do some writeout, up to congestion, trying
to not block on request queue exhaustion
}
}
blk_congestion_wait()
}

So it basically spins around all the queues keeping them full in a
non-blocking manner.

There _are_ times when pdflush will accidentally block. Say, doing a
metadata read. In that case other pdflush instances will keep other queues
busy.

I tested it up to 12 disks. Works OK.
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