On Tue, 2015-08-04 at 13:36 +0200, Christoph Hellwig wrote:
> On Sun, Aug 02, 2015 at 10:58:22PM -0700, Ming Lin wrote:
> > I think the important thing is the late splitting for regular bio.
> > For discard/write_same bio, how about just don't do late splitting?
>
> I'd hate having to special case them even more. Especially as the
> discard splitting is nasty and we really don't want to send giant
> discards by default anyway (see Jens' patches to limit discard size
> by default).
>
> So I'd recommend to keep everything as-is, just make sure we don't
> overflow bi_size.
Did you mean to remove "PATCH 4 block: remove split code in
blkdev_issue_discard" or to keep it?
Which of below 2 solutions you prefer?
- Solution 1
remove splits in blkdev_issue_{discard,write_same} and keep
blk_bio_{discard,write_same}_split
But for blkdev_issue_discard(), it's not enough if only make sure
bi_size not overflow, for example, discard 4G
4G bytes = 8388608 sectors
UINT_MAX = 8388607 sectors
So blkdev_issue_discard() will send 2 discard bios.
First bio: sector 0 .. 8388606
Second bio: sector 8388607 .. 8388607
In this case, the 2 discard tests in device-mapper-test-suite still
fail, probably because the second bio start sector is not aligned with
discard_granularity.
So I have to take into account discard_granularity(assume 32 sectors),
then blkdev_issue_discard() will send 2 discard bios, as
First bio: sector 0 .. 8388575
Second bio: sector 8388576 .. 8388607
In this case, both discard tests passed.
- Solution 2
special case discard/write_same bios(You said you hate it).
That is to keep splits in blkdev_issue_{discard,write_same} and remove
blk_bio_{discard,write_same}_split
I think this is more clean way because blkdev_issue_{discard,write_same}
already make sure we don't overflow bi_size.
And blk_bio_{discard,write_same}_split are actually duplicated with the
splits in blkdev_issue_{discard,write_same}. It's OK to remove it.
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