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

[Max Reitz]

On 01.11.19 16:42, Kevin Wolf wrote:
> 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?

This isn’t output at all, it’s input for blockdev-add for blkdebug nodes.

Max

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