Re: [Qemu-block] [PATCH] Discard old bitmap directories in QCOW2 image

[Denis Lunev]

On 2/27/19 4:00 PM, Max Reitz wrote:
> On 18.02.19 16:36, Vladimir Sementsov-Ogievskiy wrote:
>> 12.02.2019 15:35, Andrey Shinkevich wrote:
>>> Clean QCOW2 image from bitmap obsolete directory when a new one
>>> is allocated and stored. It slows down the image growth a little bit.
>>> The flag QCOW2_DISCARD_ALWAYS allows a call to raw_co_pdiscard()
>>> that does the actual cleaning of the image on disk.
>>> With the flag QCOW2_DISCARD_OTHER, a reference count of the cluster
>>> is updated only.
>>>
>>> Signed-off-by: Andrey Shinkevich <address@hidden>
>> Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>>
>> side question: should not we change discard_passthrough[QCOW2_DISCARD_OTHER] 
>> to
>> true or at least flags&BDRV_O_UNMAP by default? What is the reason of not 
>> discarding
>> things in qcow2-cluster?
> As far as I remember the reason is that whenever you clean up something
> its cluster is probably going to be reused rather soon.  So cleaning up
> takes longer, repopulating that cluster takes longer, and you save only
> rather little space.
>
> This is also why I don't know whether this patch makes much sense.
>
> Max
>
This depands upon the amount of actually free clusters in the image.
If there a lot of them at the moment, this one could be refilled
any time later on.

Den