Re: [PATCH] O_DIRECT with hardware blocksize alignment

[Joel Becker <jlbec@evilplan.org>]

On Tue, Jan 15, 2002 at 01:20:26PM +0100, Andrea Arcangeli wrote:
> > 	How so?  All I/O is at the computed blocksize.  In every
> > request, the size of each I/O in the kiovec is the same.  The
> 
> in the kiovec yes, but in the same request queue there will be also the
> concurrent requests from the filesystem, and they will have different
> b_size, see Jens's mail about different b_size merged in the same
> request.

	Ok, I've read over Badri's latest patch, and it would seem he
assumes that the kiovec coming into brw_kiovec has buffer_heads of
512bytes (as raw.c would prepare).  He then submits the I/O in differing
chunks (eg 2048 + 4096 + 4096 + 2048 for a 2048-aligned buffer).
Correct me if I am wrong (I can't see anything in raw.c that would
change the sizes in the kiovec).
	For O_DIRECT, the fallback is s_blocksize, not the hardware
minimum of 512.  So the kiovec would be coming into brw_kiovec with a
b_size of 4096 or so.  My patch lets b_size be anything between 512 and
s_blocksize.
	To work with Badri's code, would not the O_DIRECT path want to
submit a kiovec that is entirely b_size = 512 and let brw_kiovec handle
expanding it to larger sizes?  This makes my patch simpler, but I wonder
what issues that presents.

Joel

-- 

"When choosing between two evils, I always like to try the one
 I've never tried before."
        - Mae West

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