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Re: [Qemu-block] [PATCH 2/5] block/qcow2-bitmap: Allow bitmap flushing

From: John Snow
Subject: Re: [Qemu-block] [PATCH 2/5] block/qcow2-bitmap: Allow bitmap flushing
Date: Fri, 8 Mar 2019 17:11:35 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 3/6/19 11:12 AM, Vladimir Sementsov-Ogievskiy wrote:
> 06.03.2019 18:59, John Snow wrote:
>> On 3/6/19 7:58 AM, Eric Blake wrote:
>>> On 3/5/19 5:43 PM, John Snow wrote:
>>>> Usually, we only write out bitmaps when we're about to close out the file,
>>>> so we always remove the bitmaps after to make it easier to determine the
>>>> source of truth for migration purposes.
>>>> However, there may be times we want to flush bitmap data more aggressively,
>>>> like after a truncate event where we need to make the metadata consistent
>>>> again.
>>> Or, as I've mentioned in other threads, after every
>>> bitmap-add/bitmap-delete operation, so that 'qemu-img info -U' can see
>>> which persistent bitmaps exist. But that's one step further than this
>>> series, so I won't insist on it today.
>> Yes, I was keeping that in mind as I wrote this. I think it extends to
>> those cases trivially, so long as Vladimir is OK with what I'm trying to do.
> I still don't see the point of flushing bitmaps, the only thing is optimizing
> qemu-img info --force-share, which should never used except for debugging.
> So, at least, if we'll have it, I'd prefer bitmap flushing to be optional,
> and other code should not rely on it.

I don't have plans to add more uses of it personally, but I do think
that resize represents a genuine case of wanting the feature.

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