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

From: Eric Blake
Subject: Re: [Qemu-devel] [Nbd] [PATCH v2 1/1] NBD proto: add WRITE_ZEROES extension
Date: Fri, 1 Apr 2016 14:26:54 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.1

On 04/01/2016 02:37 AM, Wouter Verhelst wrote:
> Hi,
> Thanks, applied.
> On Thu, Mar 31, 2016 at 04:02:05PM +0300, Denis V. Lunev wrote:
>> From: Pavel Borzenkov <address@hidden>
>> 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.

>> +
>> +- bit 1, `NBD_CMD_MAY_TRIM`; defined by the experimental `WRITE_ZEROES`
>> +  extension; see below.

Hmm, we had an unfinished conversation about whether the default sense
of this bit should be reversed. I'll propose a followup patch, now that
the original has been merged.

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

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