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Date: Thu, 24 May 2012 16:19:08 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] Block job commands in QEMU 1.2 [v2,
	including support for replication]
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To: Ori Mamluk <omamluk@zerto.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Luiz Capitulino <lcapitulino@redhat.com>, Eric Blake <eblake@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Il 24/05/2012 16:00, Ori Mamluk ha scritto:
> 
>> The dirty bitmap is managed by these QMP commands:
>>
>> * blockdev-dirty-enable: takes a file name used for the dirty bitmap,
>> and an optional granularity.  Setting the granularity will not be
>> supported in the initial version.
>>
>> * query-block-dirty: returns statistics about the dirty bitmap: right
>> now the granularity, the number of bits that are set, and whether QEMU
>> is using the dirty bitmap or just adding to it.
>>
>> * blockdev-dirty-disable: disable the dirty bitmap.
>>
>
> When do bits get cleared from the bitmap?

drive-mirror clears bits from the bitmap as it processes the writes.

In addition to the persistent dirty bitmap, QEMU keeps an in-flight
bitmap.  The in-flight bitmap does not need to be persistent.


Here is how the bitmaps are handled when doing I/O on the source:
- after writing to the source:
  - clear bit in the volatile in-flight bitmap
  - set bit in the persistent dirty bitmap

- after flushing the source:
  - msync the persistent bitmap to disk


Here is how the bitmaps are handled in the drive-mirror coroutine:
- before reading from the source:
  - set bit in the volatile in-flight bitmap

- after writing to the target:
  - if the dirty count will become zero, flush the target
  - if the bit is still set in the in-flight bitmap, clear bit in the
    persistent dirty bitmap
  - clear bit in the volatile in-flight bitmap

> "using the dirty bitmap or just adding to it" - I'm not sure I
> understand what you mean. what's the difference?

Processing the data and removing from the bitmap (mirroring active), or
just setting dirty bits (mirroring inactive).

Paolo