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

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH 2/5] block/qcow2-bitmap: Allow bitmap flushing
Date: Wed, 6 Mar 2019 16:12:13 +0000

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.

Best regards,

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