Re: [Qemu-devel] [PATCH v2 1/1] NBD proto: add WRITE_ZEROES extension

[Eric Blake <eblake@redhat.com>]

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On 03/31/2016 07:02 AM, Denis V. Lunev wrote:
> From: Pavel Borzenkov <pborzenkov@virtuozzo.com>
> 
> There exist some cases when a client knows that the data it is going to
> write is all zeroes. Such cases include mirroring or backing up a device
> implemented by a sparse file.
> 
> With current NBD command set, the client has to issue NBD_CMD_WRITE
> command with zeroed payload and transfer these zero bytes through the
> wire. The server has to write the data onto disk, effectively denying
> the sparseness.
> 
> To remedy this, the patch adds WRITE_ZEROES extension with one new
> NBD_CMD_WRITE_ZEROES command.
> 

> +++ b/doc/proto.md
> @@ -261,6 +261,8 @@ immediately after the handshake flags field in oldstyle negotiation:
>    schedule I/O accesses as for a rotational medium
>  - bit 5, `NBD_FLAG_SEND_TRIM`; should be set to 1 if the server supports
>    `NBD_CMD_TRIM` commands
> +- bit 6, `NBD_FLAG_SEND_WRITE_ZEROES`; should be set to 1 if the server
> +  supports `NBD_CMD_WRITE_ZEROES` commands

Hmm, we've picked overlapping bits between your proposal and mine for
`NBD_FLAG_SEND_DF`. Obviously, whoever goes in second gets bit 7.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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