Re: [Qemu-devel] [PATCH 13/14] raw-posix: Implement image locking

[Richard W.M. Jones]

On Mon, Oct 31, 2016 at 05:01:44PM -0500, Eric Blake wrote:
> On 10/31/2016 10:38 AM, Fam Zheng wrote:
> > This implements open flag sensible image locking for local file
> > and host device protocol.
> > 
> > virtlockd in libvirt locks the first byte, so we start looking at the
> > file bytes from 1.
> 
> What happens if we try to use a raw file with less than 3 bytes? There's
> not much to be locked in that case (especially if we round down to
> sector sizes - the file is effectively empty) - but it's probably a
> corner case you have to be aware of.

It would be nice if qemu could handle zero-length or even < 512 byte
drives, but we (libguestfs) gave up that battle a long time ago after
encountering several bugs in this area.  These days when you try to
add an empty file[1] through the libguestfs API, the implementation
replaces it with a 4096 byte raw-format file of zeroes.

More here:
https://github.com/libguestfs/libguestfs/blob/master/src/drives.c#L395

Rich.

[1] It has the name "/dev/null" for historical reasons, but is not
that actual device for the reasons given above.

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