Re: [Libguestfs] [PATCH v3 0/6] launch: direct: Disable qemu locking when opening drives readonly.

On Wed, Sep 13, 2017 at 11:25:32AM +0300, Roman Kagan wrote: qemu 2.10 implements mandatory file locking, so if two instances of qemu open the same file for writing you'll get an error: $ qemu-system-x86_64 -hda backing & # works $ qemu-system-x86_64 -hda backing qemu-system-x86_64: Failed to get "write" lock Is another process using the image? So nothing wrong with that and it's an important mechanism to prevent image corruption. (Where two qemu instances actually need shared write access you can use ‘share-rw=on’). However the exclusive lock prevents libguestfs from *reading* from an open disk image, eg. to do virt-df to monitor space or virt-tail to monitor log files. Such disk images are opened using the guestfs_add_drive_opts readonly=true flag, which is implemented by placing a qcow2 overlay on top of the disk image, the purpose of the overlay being to take any writes and protect the underlying disk image from being modified. This kind of access is safe[1]. However qemu mandatory locking prevents it by trying to acquire a lock on the backing file (because in general terms qemu could write to the backing file eg if you committed a snapshot, although libguestfs never does this). So in the case where readonly=true, this patch series uses file.locking=off to turn off locking on the backing file. It doesn't affect the readonly=false case. An exclusive lock is acquired in this case, so you'll now not be able to corrupt a disk which is in use, which is an advance. We also have some tests (in tests/qemu) to ensure that qemu never does any funny stuff like modifying a qcow2 file when it is opened readonly. Rich. [1] Safe from the point of view that it won't ever modify the disk image. It's not guaranteed that libguestfs won't see strange corruption, so users of this have to be prepared to retry operations if they see errors. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into KVM guests. http://libguestfs.org/virt-v2v