Re: [Qemu-devel] [PATCH 1/1] protocol: Add NBD_CMD_FLAG_FAST_ZERO

From: Wouter Verhelst <w@uter.be>
To: Eric Blake <eblake@redhat.com>
Cc: libguestfs@redhat.com, qemu-devel@nongnu.org,
	qemu-block@nongnu.org, nbd@other.debian.org
Subject: Re: [Qemu-devel] [PATCH 1/1] protocol: Add NBD_CMD_FLAG_FAST_ZERO
Date: Fri, 23 Aug 2019 20:48:34 +0200	[thread overview]
Message-ID: <20190823184834.brhsfbc4sdq5xuij@grep.be> (raw)
In-Reply-To: <20190823143426.26838-2-eblake@redhat.com>

On Fri, Aug 23, 2019 at 09:34:26AM -0500, Eric Blake wrote:
> +- bit 4, `NBD_CMD_FLAG_FAST_ZERO`; valid during
> +  `NBD_CMD_WRITE_ZEROES`. If set, but the server cannot perform the
> +  write zeroes any faster than it would for an equivalent
> +  `NBD_CMD_WRITE`,

One way of fulfilling the letter of this requirement but not its spirit
could be to have background writes; that is, the server makes a note
that the zeroed region should contain zeroes, makes an error-free reply
to the client, and then starts updating things in the background (with
proper layering so that an NBD_CMD_READ would see zeroes).

This could negatively impact performance after that command to the
effect that syncing the device would be slower rather than faster, if
not done right.

Do we want to keep that in consideration?

-- 
<Lo-lan-do> Home is where you have to wash the dishes.
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