Re: [PATCH, RFC] Use WRITE_SYNC in __block_write_full_page() if WBC_SYNC_ALL

[Theodore Tso <tytso@mit.edu>]

On Sun, Jan 04, 2009 at 03:19:27PM -0800, Andrew Morton wrote:
> >  Still, if we are
> > submitting I/O which we are going to end up waiting on, we really
> > should submit it with WRITE_SYNC, and this patch should optimize
> > writes in other situations; for example, if we fsync() a file, we will
> > also end up calling block_write_full_page(), and so supplying the
> > WRITE_SYNC hint to the block layer would be a Good Thing.
> 
> Is it?  WRITE_SYNC means "unplug the queue after this bh/BIO".  By setting
> it against every bh, don't we risk the generation of more BIOs and
> the loss of merging opportunities?

Good point, yeah, that's a problem.  Some of IO schedulers also use
REQ_RW_SYNC to prioritize the I/O's above non-sync I/O's.  That's an
orthognal issue to unplugging the queue; it would be useful to be able
to mark an I/O as "this is bio is one that we will eventually end up
waiting to complete", separately from "please unplug the the queue
after this bio submitted".

BTW, I notice that the CFQ io scheduler prioritizes REQ_RW_META bio's
behind REQ_RW_SYNC bio's, but ahead of normal bio requeuss.  But as
far as I can tell nothing is actually marking requests REQ_RW_META.
What is the intended use for this, and are there plans to make other
I/O schedulers honor REQ_RW_META?

							- Ted

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