[Libguestfs] [nbdkit PATCH 1/3] server: Add internal support for NBDKIT_FLAG_FAST_ZERO

[Eric Blake]

On 8/23/19 9:40 AM, Eric Blake wrote:
> Qemu was able to demonstrate that knowing whether a zero operation is
> fast is useful when copying from one image to another: there is a
> choice between bulk pre-zeroing and then revisiting the data sections
> (fewer transactions, but depends on the zeroing to be fast),
> vs. visiting every portion of the disk only once (more transactions,
> but no time lost to duplicated I/O due to slow zeroes).  As such, the
> NBD protocol is adding an extension to allow clients to request fast
> failure when zero is not efficient, from servers that advertise
> support for the new flag.
> 
> In nbdkit, start by plumbing a new .can_fast_zero through the backends
> (although it stays 0 in this patch, later patches will actually expose
> it to plugins and filters to override).  Advertise the flag to the
> client when the plugin provides a .can_fast_zero, and wire in passing
> the flag down to .zero in the plugin.  In turn, if the flag is set and
> the implementation .zero fails with ENOTSUP/EOPNOTSUPP, we no longer
> attempt the .pwrite fallback.

Based on our release plans,...


> +++ b/docs/nbdkit-protocol.pod
> @@ -173,6 +173,17 @@ This protocol extension allows a client to inform the server about
>  intent to access a portion of the export, to allow the server an
>  opportunity to cache things appropriately.
> 
> +=item C<NBD_CMD_FLAG_FAST_ZERO>
> +
> +Supported in nbdkit E<ge> 1.13.9.

I'm changing this to nbkdit E<ge> 1.15.0 (ie. the first non-stable
release after 1.14.0), with no plans to backport the feature to stable-1.14.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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