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

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH] Discard old bitmap directories in QCOW2 image
Date: Wed, 27 Feb 2019 14:00:10 +0100
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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.


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