[Qemu-devel] [PATCH] Disk image shared and exclusive locks.

[Richard W.M. Jones]

[from the commit message ...]

Allow qemu to acquire shared and exclusive locks on disk images.
This is done by extending the -drive option with an additional,
optional parameter:

  -drive [...],lock=none
  -drive [...],lock=shared
  -drive [...],lock=exclusive

lock=none is the default, and it means that we don't try to acquire
any sort of lock.

lock=shared tries to acquire a shared lock on the disk image.
Multiple instances of qemu may all hold this sort of lock.

lock=exclusive tries to acquire an exclusive lock on the disk
image.  An exclusive lock excludes all other shared and exclusive
locks.

If acquisition of a lock fails, opening the image fails.

The implementation of locks only works for raw POSIX and Win32
files.  However many of the other block types are implemented
in terms of these drivers, so they "inherit" locking too.  Other
drivers are read-only, so don't require locking.  Below we note
only the cases where locking is *not* implemented:

  cloop - directly open()s the file, no locking implemented
  cow - same as cloop
  curl - protocol probably doesn't support locking
  nbd - same as curl

---

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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0001-Disk-image-shared-and-exclusive-locks.patch
Description: Text document