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

[Peter Lieven <pl@kamp.de>]

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 <pbonzini@redhat.com>:
>>>
>>>> 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