Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command

[Paolo Bonzini]

Il 21/05/2013 09:31, Stefan Hajnoczi ha scritto:
> On Mon, May 20, 2013 at 09:23:43AM +0200, Paolo Bonzini wrote:
>> Il 20/05/2013 08:24, Stefan Hajnoczi ha scritto:
>>>>> You only need to fdatasync() before every guest flush, no?
>>> No, you need to set the dirty bit before issuing the write on the
>>> host.  Otherwise the image data may be modified without setting the
>>> appropriate dirty bit.  That would allow data modifications to go
>>> undetected!
>>
>> But data modifications can go undetected until the guest flush returns,
>> can't they?
> 
> You are thinking about it from the guest perspective - if a flush has
> not completed yet then there is no guarantee that the write has reached
> disk.
> 
> But from a host perspective the dirty bitmap should be conservative so
> that the backup application can always restore a bit-for-bit identical
> copy of the disk image.  It would be weird if writes can sneak in
> unnoticed.

True, but that would happen only in case the host crashes.  Even for a
QEMU crash the changes would be safe, I think.  They would be written
back when the persistent dirty bitmap's mmap() area is unmapped, during
process exit.

Paolo