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Date: Sat, 5 Sep 2015 20:16:22 +0300
From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
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Subject: Re: [Qemu-devel] [PATCH v3 RFC 0/17] block: persistent dirty bitmaps
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To: qemu-devel@nongnu.org
Cc: kwolf@redhat.com, stefanha@redhat.com, pbonzini@redhat.com, den@openvz.org, jsnow@redhat.com

Add feature:

Periodic flushing of the bitmaps

The idea:

Disk is written often.
Bitmap is updated more seldom.
HBitmap previous level is updated even more seldom..

To not store all bitmap levels in file, just save in the image file the 
number of largest consistent level:

flush bitmap: consistent_level = HBITMAP_MAX_LEVEL

change bitmap level X: if consistent_level > X then consistent_level = X 
- 1 (and flush consistent_level to file)

Then, after fail, we can restore the bitmap from last consistent level:

gran = 1 << (level_bits * (HBITMAP_MAX_LEVEL - consistent_level))
bitmap[i] = bitmap[i - i % gran] OR bitmap[i - i % gran + 1] OR ... OR 
bitmap[i - i % gran + (gran - 1)]


to make this scheme independent of HBitmap, it may be better to number 
levels from 0 (0 is largest level), and save level_bits to Image file too.

-- 
Best regards,
Vladimir
* now, @virtuozzo.com instead of @parallels.com. Sorry for this inconvenience.