Re: [Qemu-devel] [PATCH] block: Document BDRV_REQ_WRITE_UNCHANGED support

[Eric Blake]

On 05/02/2018 09:03 AM, Max Reitz wrote:
> Add BDRV_REQ_WRITE_UNCHANGED to the list of flags honored during pwrite
> and pwrite_zeroes, and also add a note on when you absolutely need to
> support it.
>
> Signed-off-by: Max Reitz <address@hidden>
> ---

Thanks, that helps.

Reviewed-by: Eric Blake <address@hidden>

> I did not include a note on how this might be useful to protocol
> drivers, because BDRV_REQ_WRITE_UNCHANGED requests will usually not end
> up on the protocol level anyway.
> (I suppose it would be possible in theory if a protocol driver reports a
>   certain range as unallocated and the user installs a copy-on-read
>   driver on top, but I think every protocol driver currently reports
>   everything as allocated.)

It doesn't help that I still haven't revisited an audit of whether all 
the protocol drivers have a sane status return to begin with (fallout 
from Kevin's review of my byte-based block status that landed in 2.12). 
But that doesn't hold up this patch.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org