Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert

[Peter Lieven]

On 18.07.2013 12:24, Paolo Bonzini wrote:
> Il 18/07/2013 11:23, Kevin Wolf ha scritto:
> > Am 17.07.2013 um 19:48 hat Peter Lieven geschrieben:
> > > Am 17.07.2013 um 19:04 schrieb Paolo Bonzini <address@hidden>:
> > >
> > > > Il 17/07/2013 19:02, Peter Lieven ha scritto:
> > > > > For Disks we always use read/write16 so i think we Should also use writesame16. 
> > > > > Or not?
> > > > Yes.
> > > >
> > > > Remember you can still use UNMAP if LBPRZ=0.
> > > I can always use it if writesame is not available, but in this case 
> > > bdi->discard_zeroes must be 0.
> > >
> > > Maybe we should call it discard_writes_zeroes or similar.
> > >
> > > Discard_zeroes is sth that should only indicate if lbprz == 1. At least if we 
> > > refer to the Linux ioctl. We could include both in BDI.
> > Maybe what we really should do is to define different operations (with
> > an exact behaviour) instead of having one bdrv_discard() and then adding
> > flags everywhere to tell what the operation is doing exactly.
> A BDRV_MAY_UNMAP flag for bdrv_write_zeroes?

I thought that we wanted to add a paramter to the BDI (call it 
write_zeroes_w_discard).
If this is set the bdrv MUST accept a flag to bdrv_discard() lets call it 
BDRV_DISCARD_WRITE_ZEROES
and he has to ensure that all sectors specified in bdrv_discard() read as zero 
after the operation.

If this flag is not set (e.g. when the OS issues a normal discard) the 
operation may still silently fail with
undefined provisioning state and content of the specified sectors.

With the above flag a general optimization of bdrv_write_zeroes would be 
possible, that
calls bdrv_discard with BDRV_DISCARD_WRITE_ZEROES if 
bdi->write_zeroes_w_discard == 1.

For iscsi this would mean it sets bdi->write_zeroes_w_discard = 1 iff 
BDRV_O_UNMAP == 1 and lbpme == 1 and lbpws == 1 and lbprz == 1.

The second flag that could be added is bdi->discard_zeroes. But this only says 
unmapped blocks
read back as zero.

Peter