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

From: Eric Blake <eblake@redhat.com>
To: "Denis V. Lunev" <den@openvz.org>
Cc: 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: Wed, 17 Feb 2016 13:58:47 -0700	[thread overview]
Message-ID: <56C4DF07.9020806@redhat.com> (raw)
In-Reply-To: <1455732653-3106-1-git-send-email-den@openvz.org>

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On 02/17/2016 11:10 AM, Denis V. Lunev wrote:
> This patch proposes a new command to reduce the amount of data passed
> through the wire when it is known that the data is all zeroes. This
> functionality is generally useful for mirroring or backup operations.
> 
> Currently available NBD_CMD_TRIM command can not be used as the
> specification explicitely says that "a client MUST NOT make any

s/explicitely/explicitly/

> assumptions about the contents of the export affected by this
> [NBD_CMD_TRIM] command, until overwriting it again with `NBD_CMD_WRITE`"
> 
> Particular use case could be the following:
> 
> QEMU project uses own implementation of NBD server to transfer data
> in between different instances of QEMU. Typically we tranfer VM virtual

s/tranfer/transfer/

> disks over this channel. VM virtual disks are sparse and thus the
> efficiency of backup and mirroring operations could be improved a lot.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> ---
>  doc/proto.md | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/doc/proto.md b/doc/proto.md
> index 43065b7..c94751a 100644
> --- a/doc/proto.md
> +++ b/doc/proto.md
> @@ -241,6 +241,8 @@ immediately after the global 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
>  
>  ##### Client flags
>  
> @@ -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))?

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

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