On Tue, Apr 25 2006, Hua Zhong wrote:
> It seems that new BIOs do not have BIO_SEG_VALID set. So when you do
> sequential IO, the IO being back-merged should always have not had
> valid segments.
>
> I ran bonnie++ and it shows the same thing.
But blk_recount_segments() sets the BIO_SEG_VALID flag. Ugh ok
__bio_add_page() basically kills the flag. James, I think you are the
author of that addition, does it really need to be so restrictive?
/* If we may be able to merge these biovecs, force a recount */
if (bio->bi_vcnt && (BIOVEC_PHYS_MERGEABLE(bvec-1, bvec) ||
BIOVEC_VIRT_MERGEABLE(bvec-1, bvec)))
bio->bi_flags &= ~(1 << BIO_SEG_VALID);
with that in place, we may as well just remove ->bi_phys_segments
and ->bi_hw_segments since we'll be calculating the values over and over
again while building up a bio.
--
Jens Axboe
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