Re: [Qemu-devel] [RFC 1/1] nbd (specification): add NBD_CMD_WRITE_ZEROES command

From: Eric Blake <eblake@redhat.com>
To: Roman Kagan <rkagan@virtuozzo.com>,
	"Denis V. Lunev" <den@openvz.org>,
	nbd-general@lists.sourceforge.net, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [RFC 1/1] nbd (specification): add NBD_CMD_WRITE_ZEROES command
Date: Thu, 18 Feb 2016 09:35:47 -0700	[thread overview]
Message-ID: <56C5F2E3.1090102@redhat.com> (raw)
In-Reply-To: <20160218091857.GA12337@rkaganb.sw.ru>

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On 02/18/2016 02:18 AM, Roman Kagan wrote:
> On Wed, Feb 17, 2016 at 01:58:47PM -0700, Eric Blake wrote:
>> On 02/17/2016 11:10 AM, Denis V. Lunev wrote:
>>> @@ -446,6 +448,11 @@ The following request types exist:
>>>      about the contents of the export affected by this command, until
>>>      overwriting it again with `NBD_CMD_WRITE`.
>>>  
>>> +* `NBD_CMD_WRITE_ZEROES` (6)
>>> +
>>> +    A request to write zeroes. The command is functional equivalent of
>>> +    the NBD_WRITE_COMMAND but without payload sent through the channel.
>>
>> This lets us push holes during writes. Do we have the converse
>> operation, that is, an easy way to query if a block of data will read as
>> all zeroes, and therefore the client can bypass reading that portion of
>> the disk (in other words, an equivalent to lseek(SEEK_HOLE/SEEK_DATA))?
> 
> The spec doesn't have anything like that.
> 
> OTOH, unlike the write case, where you have all the information and just
> choose whether to send normal write or zero write, the extra round-trip
> of a separate SEEK_HOLE/SEEK_DATA request may lead to actually degrading
> the overall throughput.
> 
> Rather it may be a better idea to add something like sparse read where
> the server would, instead of sending the full length of data in the
> response payload, send a smarter variable-length package with a
> scatter-gather list or a bitmap of used blocks in the beginning, and let
> the client decode it and fill the gaps with zeros.

Sure, that would work too, and sounds nicer.  Either way, the point is
that we should strongly consider improving the NBD protocol to allow
more efficient handling of sparse files, in both the push and in the
pull direction.  Qemu already has a desire to use both directions of
improvements, but there are more programs, both clients and servers,
outside of qemu, that could benefit from such protocol improvements.

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

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