Re: [PATCH 1/5] [BLOCK] Add 'discard' request handling

[David Woodhouse <dwmw2@infradead.org>]

On Thu, 2008-08-07 at 01:19 +0900, OGAWA Hirofumi wrote:
> David Woodhouse <dwmw2@infradead.org> writes:
> 
> > Some block devices benefit from a hint that they can forget the contents
> > of certain sectors. Add basic support for this to the block core, along
> > with a 'blkdev_issue_discard()' helper function which issues such
> > requests.
> >
> > Although blkdev_issue_discard() can take an end_io function, it's
> > acceptable to leave that as NULL and in that case the allocated bio will
> > be automatically freed. Most of the time, it's expected that callers
> > won't care about when, or even _if_, the request completes. It's only a
> > hint to the device anyway. By definition, the file system doesn't _care_
> > about these sectors any more.
> 
> Looks like good start. Thanks. Although I'm not quite sure it helps, is
> there any plan to merge bios?

Yes, it would be very good to merge bios. In fact, we can also allow the
elevator to let discards cross reads and writes -- there's no _real_
need for them to be a soft barrier. And we can _drop_ a discard request
if there's a later write to the same sectors.

I was planning to do that work in the elevator later. Or preferably,
leave it to someone more familiar with that code.

> If fs merges contiguous blocks to one bio, bi_size will overflow.
> Will we have to limit (of the device?) and separate for the device?

Or just let the device deal with it in multiple requests if it needs to?
I have no strong opinion.

> Or how about to have #define for discard bio for BIO_RW_DISCARD users?

Que?

> The following seems good to convert to bio_dataless?

Yes, that seems appropriate; thanks.
_
-- 
dwmw2