Re: [libvirt] [RFC] Image Fleecing for Libvirt (BZ 955734, 905125)

[replying with useful information from another off-list email] On 07/15/2013 03:04 PM, Richard W.M. Jones wrote: > On Mon, Jul 15, 2013 at 05:57:12PM +0800, Fam Zheng wrote: >> Hi all, >> >> QEMU-KVM BZ 955734, and libvirt BZ 905125 are about feature "Read-only >> point-in-time throwaway snapshot". The development is ongoing on >> upstream, which implements the core functionality by QMP command >> drive-backup. I want to demonstrate the HMP/QMP commands here for image >> fleecing tasks (again) and make sure this interface looks ready and >> satisfying from Libvirt point of view. >> On 07/15/2013 06:24 AM, Paolo Bonzini wrote:> Il 15/07/2013 11:57, Fam Zheng ha scritto: >> Hi all, >> >> QEMU-KVM BZ 955734, and libvirt BZ 905125 are about feature "Read-only >> point-in-time throwaway snapshot". The development is ongoing on >> upstream, which implements the core functionality by QMP command >> drive-backup. I want to demonstrate the HMP/QMP commands here for image >> fleecing tasks (again) and make sure this interface looks ready and >> satisfying from Libvirt point of view. > > And since we are at it, here is a possible libvirt API to expose this > functionality (cut-and-paste from an old email). If needed, VDSM can > provide a similar API and proxy the libvirt API. > > Would something like this work? > > int virDomainBlockPeekStart (virDomainPtr dom, > const char ** disks, > unsigned int flags); > > Make it possible to use virDomainBlockPeek on the given disks > with the new VIR_DOMAIN_BLOCK_PEEK_IMAGE flag. > > It is okay to create multiple "snapshot groups", i.e. to invoke > the function multiple times with VIR_DOMAIN_BLOCK_PEEK_SNAPSHOT. > It is however not okay to specify the same disk multiple times > unless all of them are _without_ VIR_DOMAIN_BLOCK_PEEK_SNAPSHOT. > > flags: > VIR_DOMAIN_BLOCK_PEEK_SNAPSHOT > Make an atomic point-in-time snapshot of all the disks included > in the list of strings "disks", and expose the snapshot via > virDomainBlockPeek > > Note: if the virtual machine is running, this will use > nbd-server-start/add/end. If the virtual machine is paused, > this will use qemu-nbd. Libvirt should be able to switch > transparently from one method to the other. > > int virDomainBlockPeekStop (virDomainPtr dom); > > Stop communication with qemu-nbd or the hypervisor. > > > VIR_DOMAIN_BLOCK_PEEK_IMAGE > > A new flag for virDomainBlockPeek. If specified, > virDomainBlockPeek will access the disk image, not the "raw" > file (i.e. it will read data as seen by the guest). This > is only valid if virDomainBlockPeekStart has been called before > for this disk. > > > Because libvirt would use a local (Unix) socket to communicate with QEMU > and pass the file descriptor, there is no need to authenticate the NBD > connection. There is no need for ticketing, though if necessary we can > make QEMU only accept connections from libvirtd's pid. libvirt and VDSM > already do authentication and/or encryption. > > Paolo > >

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