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

[Benoît Canet]

The Tuesday 27 May 2014 à 17:29:16 (+0200), Paolo Bonzini wrote :
> 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

That seems to be 4k aligned. It's better than I though.

Best regards

Benoît

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