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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 18/07/2013 15:29, Peter Lieven ha scritto:
>>> If the driver would have a better method of writing zeroes than
>>> discard it simply should not set bdi->write_zeroes_w_discard = 1.
>> If the driver had a better method of writing zeroes than discard, it
>> simply should ignore the BDRV_MAY_UNMAP (or BDRV_MAY_DISCARD) flag in
>> its bdrv_write_zeros implementation.
>
> ok, but this would require an individual patch in every driver, wouldn't
> it.  i am ok with that.

Yes (making the drivers return the flag in the BDI would also require
per-driver patches).

BDRV_MAY_UNMAP is an advisory flag, that the driver is free to ignore
(just like bdrv_discard can be ignored altogether).

block.c probably should avoid passing down BDRV_MAY_UNMAP altogether if
the BDRV_O_UNMAP flag is not set.

> the BDRV_MAY_DISCARD flag is at the end a hint if the driver can optimize
> writing zeroes by a discard or if real zeroes should be written e.g. to
> sanitize the device?
> 
> talking for iscsi:
> bdrv->discard can remain to use UNMAP and silently fail if lbpu == 0.
> bdrv->write_zeroes will use writesame16 and set the unmap flag only if
> BDRV_MAY_DISCARD == 1 and BDRV_O_UNMAP == 1 and lbprz == 1.
> in case lbpws == 0 it will return -ENOSUP.

Almost: for LBPWS == 0 it can use WRITE SAME(16) without the unmap flag.
 If that doesn't work either it can return -ENOTSUP.

Paolo

> 
> Correct?
> 
> Peter
>