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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 18/07/2013 12:44, Peter Lieven ha scritto:
> 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).

I also thought so, but I like Kevin's idea of not shoehorning it in
bdrv_discard().  Extending bdrv_write_zeroes is a better API.

So far we've avoided "discard zeroes" semantics in QEMU (device models
are also careful not to expose that).  Since the only sane way to
implement what you want is to use the SCSI WRITE SAME command, adding
flags to bdrv_write_zeroes will even be easier because the mapping with
SCSI is natural: discard = UNMAP, write_zeroes = WRITE SAME (either
without or with the UNMAP bit).

> 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.

But if you set BDRV_DISCARD_WRITE_ZEROES, then you always need a
fallback to bdrv_write_zeroes.  Why not just call bdrv_write_zeroes to
begin with?  That's why extending bdrv_write_zeroes is preferable.

> 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.

No, you cannot do that unconditionally.  Some operations can use it,
some cannot.  Think of SCSI disk emulation: it must not discard is WRITE
SAME is sent without the UNMAP bit!

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

I think a flag is not needed here.  You can add BDRV_BLOCK_ZERO directly
in iscsi_co_get_block_status.

Paolo