Re: [Qemu-devel] [PATCH 4/4] block: add bdrv_co_discard and bdrv_aio_discard support

[Paolo Bonzini]

On 10/14/2011 04:23 PM, Kevin Wolf wrote:
> >  This similarly adds support for coroutine and asynchronous discard.
> >
> >  Signed-off-by: Paolo Bonzini<address@hidden>
>
> Do we really need bdrv_discard and bdrv_aio_discard in the backends? I
> think it makes sense to have a bdrv_aio_discard() in block.h as AIO
> generally fits well for device models, but I would just require
> bdrv_co_discard for any block drivers implementing discard.

bdrv_discard is needed for now since I wouldn't like to conflate this 
patch with the qcow2 patch.

I can certainly drop aio_discard from the backends, but I'm not sure how 
heavy can fallocate be (with FALLOC_FL_PUNCH_HOLE).  Probably not much, 
but I think there's no guarantee of O(1) behavior especially with 
filesystems like ecryptfs.  So you would need to go through the thread 
pool and aio_discard would come in handy.

Paolo