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

[Paolo Bonzini]

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.

Paolo