Re: [Qemu-devel] [PATCHv2 07/11] iscsi: let bdrv_create conditionally zero out the device

[Peter Lieven]

Am 02.07.2013 um 12:49 schrieb Paolo Bonzini <address@hidden>:

> Il 02/07/2013 12:36, Peter Lieven ha scritto:
>>>>> Perhaps we can have a new "discard_zeroes" field in bdrv_get_info, and
>>>>> the unmap functionality can be moved up to qemu-img convert?
>>>> 
>>>> Is there any other storage protocol out there that could benefit from it?
>>> 
>>> Definitely LVM.  Perhaps in the future gluster too, though right now it
>>> only supports discard on files, not block devices.
>> 
>> Is discards on LVM sth that is already implemented in qemu?
> 
> Yes, it supports BLKDISCARD (see handle_aiocb_discard in
> block/raw-posix.c).  Of course there is no way to query the host
> discard_zeroes setting yet.

No way in qemu or no way at all?

> 
> But even if it weren't implemented in QEMU, you should aim at making it
> easier (if it's not too much work, which it isn't), not harder.  If you
> do it in block/iscsi.c, the next person who comes will have to basically
> undo your work and reimplement+retest it with the right API.
> 
>> Would you mind if we postpone the general approach to a later point.
>> It seems that this is much more complex than the iSCSI approach.
> 
> It shouldn't be more complex at all, actually.  You just need to pass
> the maximum unmap sectors and lbprz parameters through bdrv_get_info.
> 
> I'm not asking you to add support for BLKDISCARDZEROES and all that.
> I'm asking you to do the work at the right level.

You are right, if its possible in other protocols also it should be done at a 
higher level.
I will at least look into integrating it into bdrv_get_info for iSCSI and 
implement the
unmapping logic in qemu-img for the case that has_zero_init is 0.

Peter