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

[Max Reitz]

On 27.02.19 16:15, Andrey Shinkevich wrote:
> 
> 
> On 27/02/2019 16:25, Max Reitz wrote:
>> On 27.02.19 14:16, Denis Lunev wrote:
>>> 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.
>>
>> Well, it's a trade-off.  For small structures like the bitmap directory,
>> I don't see the point in forcefully unmapping them.  We don't do it for
>> the old L1 table either when that has to be resized.
>>
>> Discarding a whole bitmap (like in clear_bitmap_table()) is something
>> else, though.  So I was too quick to dismiss the whole patch; parts of
>> it do make sense.
>>
>> For bitmap tables...  I don't know exactly.  I don't think a bitmap
>> table is ever going to be larger than the L1 table, so it should
>> probably be handled the same way -- which is to keep QCOW2_DISCARD_OTHER.
>>
>> Max
>>
> Yes, Max. All what you wrote about do make the sense.
> The reason behind the proposition to discard the obsolete bitmap
> directories in the image is that it makes debugging of the iotests
> easier for a developer. Otherwise, lots of irrelevant symbolic
> information in the image forces to track which one is valid.
> Particularly, I locate the byte that designates the bitmap flags to see
> if the flag 'in-use' is not set. And I do that for every single bitmap
> directory in the image.
> Again, cleaning up the obsolete bitmap directories in the image serves
> for the convenience of debugging. What do you think about it?

I don't think convenience of debugging is a good reason to implement
something that is probably worse behavior when not debugging.  Of
course, if the changed behavior is neither worse nor better, then we
might as well go for it if it eases debugging.  But I think it is a bit
worse.

Also, if you want to debug images you have created yourself (or at least
are used by VMs you have control over), you can simply set
pass-discard-other=on.

Max

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