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Re: [Qemu-devel] [PATCH v3 2/8] block: add basic backup support to block

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v3 2/8] block: add basic backup support to block driver
Date: Tue, 21 May 2013 18:03:01 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6

Il 21/05/2013 17:54, Dietmar Maurer ha scritto:
>>>> Hmm, right.  But do we need the bitmap at all?  We can just use
>>>>> bdrv_is_allocated like bdrv_co_do_readv does.
>>> If a write occur, we read and backup that cluster immediately (out of
>>> order). So I am quite sure we need the bitmap.
>> This is the same as how copy-on-read happens during image streaming, and it
>> doesn't use a bitmap.
> But a read does not modify the content.

Copy-on-read modifies the topmost image even without changing the
content, just like copy-before-write modifies the backup image.

But writes to the backup copy are serialized anyway via
wait_for_overlapping_requests, so there is no problem here.


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