Re: Writing on raw device with software RAID 0 is slow

["Stephen C. Tweedie" <sct@redhat.com>]

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