Re: [Qemu-devel] [PATCH] docs: clarify that qcow2 file size is not always a cluster multiple

[Paolo Bonzini]

Il 27/05/2014 18:00, Benoît Canet ha scritto:
> > Are you worried that the host file system will lay out data poorly
> > because the file looks like this?
> >
> > | header (1C) | refcounts (2C) | L1 (512B) | hole | Next cluster |
> >
> > B = bytes
> > C = clusters
> >
> > My guess is the next cluster will be aligned to a reasonable boundary on
> > the physical disk.
>
> I have some kind of doubt. Does anyone knows a filesystem guru ?

Not a guru, but indeed there is a risk that the layout will be worse 
than necessary.

I think holes are ignored unless they are big enough, but 64K-512 is 
probably enough to create one.  Indeed here I get this:

    $ qemu-img create -f qcow2 foo.qcow2 10G
    $ qemu-io -c 'write 0 512' foo.qcow2
    $ qemu-img map -f raw foo.qcow2 10G
    Offset          Length          Mapped to       File
    0               0x31000         0               foo.qcow2
    0x40000         0x20000         0x40000         foo.qcow2

I don't know if this has any practical impact, but if the fix is easy...

Paolo