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Re: [Qemu-devel] [Nbd] [PATCH] doc: Propose NBD_FLAG_INIT_ZEROES extensi


From: Alex Bligh
Subject: Re: [Qemu-devel] [Nbd] [PATCH] doc: Propose NBD_FLAG_INIT_ZEROES extension
Date: Tue, 6 Dec 2016 08:46:43 +0000

> On 5 Dec 2016, at 23:42, Eric Blake <address@hidden> wrote:
> 
> While not directly related to NBD_CMD_WRITE_ZEROES, the qemu
> team discovered that it is useful if a server can advertise
> whether an export is in a known-all-zeroes state at the time
> the client connects.

I think this is good to go, and ...

> Signed-off-by: Eric Blake <address@hidden>
> ---
> doc/proto.md | 5 +++++
> 1 file changed, 5 insertions(+)
> 
> This replaces the following qemu patch attempt:
> https://lists.gnu.org/archive/html/qemu-devel/2016-12/msg00357.html
> which tried to add NBD_CMD_HAS_ZERO_INIT with poor semantics. The
> semantics in this proposal should be much better.
> 
> Patch is to the merge of the master branch and the
> extension-write-zeroes branch.  By the way, qemu 2.8 is due
> to be released "real soon now", and implements NBD_CMD_WRITE_ZEROES,
> so maybe it is time to consider promoting the extension-write-zeroes
> branch into master.

I would support this.

In fact the patch is sufficiently simple I think I'd merge this
into extension-write-zeroes then merge that into master.

Wouter?

Alex

> diff --git a/doc/proto.md b/doc/proto.md
> index afe71fc..7e4ec7f 100644
> --- a/doc/proto.md
> +++ b/doc/proto.md
> @@ -697,6 +697,11 @@ The field has the following format:
>   the export.
> - bit 9, `NBD_FLAG_SEND_BLOCK_STATUS`: defined by the experimental
>   `BLOCK_STATUS` 
> [extension](https://github.com/NetworkBlockDevice/nbd/blob/extension-blockstatus/doc/proto.md).
> +- bit 10, `NBD_FLAG_INIT_ZEROES`: Indicates that the server guarantees
> +  that at the time transmission phase begins, all offsets within the
> +  export read as zero bytes.  Clients MAY use this information to
> +  avoid writing to sections of the export that should still read as
> +  zero after the client is done writing.
> 
> Clients SHOULD ignore unknown flags.
> 
> -- 
> 2.9.3
> 
> 
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-- 
Alex Bligh







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