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

[Eric Blake]

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.  However, given that 
the only known user of this interface is debug testing via iotests, I'm 
not too concerned that we would be impacting any external users.


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