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

From: Peter Lieven
Subject: Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert
Date: Thu, 18 Jul 2013 12:44:23 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130623 Thunderbird/17.0.7

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?

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 
If this is set the bdrv MUST accept a flag to bdrv_discard() lets call it 
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.


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