Re: discard and barriers

[Christoph Hellwig <hch@lst.de>]

On Sat, Aug 14, 2010 at 11:46:36AM -0400, Chris Mason wrote:
> Another way to say this is we have to be 100% sure that if we write
> something after a discard, that storage will do that write after it does
> the discard.

Once we don't have barriers anymore the only way to do that is to wait
for the discard to finish before submitting that I/O.

> I care a lot about the write after the discards though.  If the discards
> themselves become async, that's ok too as long as we have some way to do
> end_io processing on them.

You can just submit the discard bio yourself.  That's what I do in the
current XFS code.  It's a bit awkward that you have to do all the size
checking yourself currently, but we could make that a common helper,
I just don't want to do that now as it would create all kinds of
dependencies for merging the trees in the .37 window.