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It absolutely doesn't need any daemons and it will work with any guest OS (or without OS for that matter). Google for "virsh active blockcommit".
I feel like there is whole lot of misunderstanding in this thread...
First of all i am not arguing about how hosts are evil or something. Sure it is possible to do A LOT from host node side, and sure in practice it will (almost)never be done.
The reason why this thread interests me is - someone mentioned that it is basically possible to write to FS which is not designed to be "clustered" from 2 different operating systems unaware of each other without breaking things. Something i always thought (and still think) to be impossible. In resulting discussion nothing specific was said about how it is supposed to be done, like either it is not possible or it is some arcane secret which is not supposed to be revealed. I also do not see how any snapshot-related technologies are relevant here, because no matter how fancy they are in the end it all comes down to the same issue with fs/data consistency.
In this case either some "server" running inside guest or guest shutdown seem to be the only options to avoid this situation.
If i am mistaken though, and it is indeed possible, i would be very grateful if someone could explain how it is supposed to work (or point towards such explanation) instead of just stating "it is easy".
Is it two different operating systems or is it one operating system?
In case of KVM (or any virtualization for that matter, containers are entirely different thing) it is 2 different operating systems, since guest is running full unmodified kernel and in some cases (old windows? probably the case for initial topic of discussion...) may be completely unaware that it is running inside of VM.
So that KVM instance talks directly to the disk hardware independent of the host?
It does not, but the host os, which handles hardware is unaware of what guest os is doing, unless it has some way of communicating with it, like already mentioned guestfsd.
Sure host OS can physically modify data, but it will be destructive, the same way as mounting disk on 2 different computers over iscsi will.
Also scenarios where guest os communicates directly with storage are not that unimaginable, for example working with san storage through fc adapter attached to vm.