Re: [PATCH for-4.2 1/4] Revert "qcow2: skip writing zero buffers to empty COW areas"

[Kevin Wolf]

Am 01.11.2019 um 15:01 hat Max Reitz geschrieben:
> On 01.11.19 13:40, Eric Blake wrote:
> > On 11/1/19 11:00 AM, Max Reitz wrote:
> >> This reverts commit c8bb23cbdbe32f5c326365e0a82e1b0e68cdcd8a.
> >>
> >> This commit causes fundamental performance problems on XFS (because
> >> fallocate() stalls the AIO pipeline), and as such it is not clear that
> >> we should unconditionally enable this behavior.
> >>
> >> We expect subclusters to alleviate the performance penalty of small
> >> writes to newly allocated clusters, so when we get them, the originally
> >> intended performance gain may actually no longer be significant.
> >>
> >> If we want to reintroduce something similar to c8bb23cbdbe, it will
> >> require extensive benchmarking on various systems with subclusters
> >> enabled.
> >>
> >> Cc: address@hidden
> >> Signed-off-by: Max Reitz <address@hidden>
> >> ---
> > 
> >> +++ b/qapi/block-core.json
> >> @@ -3304,8 +3304,6 @@
> >>   #
> >>   # @cor_write: a write due to copy-on-read (since 2.11)
> >>   #
> >> -# @cluster_alloc_space: an allocation of file space for a cluster
> >> (since 4.1)
> >> -#
> >>   # @none: triggers once at creation of the blkdebug node (since 4.1)
> > 
> > Deleting released qapi is not backwards-compatible.
> 
> Right. :-/
> 
> I’ll just not make changes to the QAPI schema.  It doesn’t hurt to leave
> this in.

Why would it be incompatible to drop an enum value that is only ever
used in output and that QEMU doesn't generate?

Kevin

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