Re: [Qemu-devel] [Libguestfs] cross-project patches: Add NBD Fast Zero support

[Richard W.M. Jones]

On Fri, Aug 23, 2019 at 09:30:36AM -0500, Eric Blake wrote:
> I've run several tests to demonstrate why this is useful, as well as
> prove that because I have multiple interoperable projects, it is worth
> including in the NBD standard.  The original proposal was here:
> https://lists.debian.org/nbd/2019/03/msg00004.html
> where I stated:
> 
> > I will not push this without both:
> > - a positive review (for example, we may decide that burning another
> > NBD_FLAG_* is undesirable, and that we should instead have some sort
> > of NBD_OPT_ handshake for determining when the server supports
> > NBD_CMF_FLAG_FAST_ZERO)
> > - a reference client and server implementation (probably both via qemu,
> > since it was qemu that raised the problem in the first place)

Is the plan to wait until NBD_CMF_FLAG_FAST_ZERO gets into the NBD
protocol doc before doing the rest?  Also I would like to release both
libnbd 1.0 and nbdkit 1.14 before we introduce any large new features.
Both should be released this week, in fact maybe even today or
tomorrow.

[...]
> First, I had to create a scenario where falling back to writes is
> noticeably slower than performing a zero operation, and where
> pre-zeroing also shows an effect.  My choice: let's test 'qemu-img
> convert' on an image that is half-sparse (every other megabyte is a
> hole) to an in-memory nbd destination.  Then I use a series of nbdkit
> filters to force the destination to behave in various manners:
>  log logfile=>(sed ...|uniq -c) (track how many normal/fast zero
> requests the client makes)
>  nozero $params (fine-tune how zero requests behave - the parameters
> zeromode and fastzeromode are the real drivers of my various tests)
>  blocksize maxdata=256k (allows large zero requests, but forces large
> writes into smaller chunks, to magnify the effects of write delays and
> allow testing to provide obvious results with a smaller image)
>  delay delay-write=20ms delay-zero=5ms (also to magnify the effects on a
> smaller image, with writes penalized more than zeroing)
>  stats statsfile=/dev/stderr (to track overall time and a decent summary
> of how much I/O occurred).
>  noextents (forces the entire image to report that it is allocated,
> which eliminates any testing variability based on whether qemu-img uses
> that to bypass a zeroing operation [1])

I can't help thinking that a sh plugin might have been simpler ...

> I hope you enjoyed reading this far, and agree with my interpretation of
> the numbers about why this feature is useful!

Yes it seems reasonable.

The only thought I had is whether the qemu block layer does or should
combine requests in flight so that a write-zero (offset) followed by a
write-data (same offset) would erase the earlier request.  In some
circumstances that might provide a performance improvement without
needing any changes to protocols.

> - NBD should have a way to advertise (probably via NBD_INFO_ during
> NBD_OPT_GO) if the initial image is known to begin life with all zeroes
> (if that is the case, qemu-img can skip the extents calls and
> pre-zeroing pass altogether)

Yes, I really think we should do this one as well.

Rich.

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