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SubjectRe: Writing on raw device with software RAID 0 is slow
Hi,

On Thu, Mar 01, 2001 at 10:44:38AM -0500, Ben LaHaise wrote:
>
> On Thu, 1 Mar 2001, Stephen C. Tweedie wrote:
>
> > Raw IO is always synchronous: it gets flushed to disk before the write
> > returns. You don't get any write-behind with raw IO, so the smaller
> > the blocksize you write in, the slower things get.
>
> More importantly, the mainstream raw io code only writes in 64KB chunks
> that are unpipelined, which can lead to writes not hitting the drive
> before the sector passes under the rw head. You can work around this to
> some extent by issuing multiple writes (via threads, or the aio work I've
> done) at the expense of atomicity. Also, before we allow locking of
> arbitrary larger ios in main memory, we need bean counting to prevent the
> obvious DoSes.

Yep. There shouldn't be any problem increasing the 64KB size, it's
only the lack of accounting for the pinned memory which stopped me
increasing it by default.

--Stephen
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