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Re: [Qemu-devel] [Nbd] [PATCH] doc: Flip bit sense for allowing trim dur

From: Alex Bligh
Subject: Re: [Qemu-devel] [Nbd] [PATCH] doc: Flip bit sense for allowing trim during WRITE_ZEROES
Date: Mon, 4 Apr 2016 16:15:49 +0100

On 4 Apr 2016, at 14:56, Eric Blake <address@hidden> wrote:

> Question - qemu already has the notion of an nbd server that can be
> configured to explicitly check for large repetitions of zero during
> NBD_CMD_WRITE, and create holes in the corresponding file as a result
> (basically because qemu is compensating for NBD_CMD_WRITE_ZEROES not
> being part of the protocol yet, so that the destination file can still
> be sparse even though lots of zeroes were sent over the wire). Should we
> also allow servers to accept NBD_CMD_FLAG_NO_HOLE during NBD_CMD_WRITE
> as an explicit request to NOT perform the magic of searching for large
> runs of zeroes, but to just write everything as written?

Seems a good idea, but I'd make it a SHOULD rather than a MUST. IE
it's saying "please don't make a hole" but really this is up to the
nbd server. This is true in any case, as the underlying filing system
could create a hole.

Alex Bligh

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