[libvirt] A question for external snapshots with flag“VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE”

[Richard W.M. Jones]

[Adding the correct mailing list, removing libvirt-list-owner]

On Fri, Feb 24, 2017 at 05:35:31PM +0800, WangJie (Captain) wrote:
> Hello, I got a question here. When we create consistent active external snapshots with flag “VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE” , the qemuDomainSnapshotFSFreeze will be called firstly to freeze all filesystems in vm, and then create snapshots. For windows vm, freezing filesystems used by vss service. If IO pressure in vm is too big or internal error happened in vss , freezing filesytems will failed , and qemuDomainSnapshotFSFreeze returns 0 which meaning that no filesystems are frozen. In the function qemuDomainSnapshotCreateActiveExternal, libvirt creates external snapshots all the same in such a situation that qemuDomainSnapshotFSFreeze returns 0, but the created snapshots are not consistent snapshots in such a situation . So shouldn't we abandon creating snapshots and goto cleanup in the situation that qemuDomainSnapshotFSFreeze returns 0?
> 
> 
> The code below:
> 
> 
> static int
> qemuDomainSnapshotCreateActiveExternal(virConnectPtr conn,
>                                        virQEMUDriverPtr driver,
>                                        virDomainObjPtr vm,
>                                        virDomainSnapshotObjPtr snap,
>                                        unsigned int flags)
> {
>     virObjectEventPtr event;
>     bool resume = false;
>     int ret = -1;
>     qemuDomainObjPrivatePtr priv = vm->privateData;
>     char *xml = NULL;
>     bool memory = snap->def->memory == VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL;
>     bool memory_unlink = false;
>     bool atomic = !!(flags & VIR_DOMAIN_SNAPSHOT_CREATE_ATOMIC);
>     bool transaction = virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_TRANSACTION);
>     int thaw = 0; /* 1 if freeze succeeded, -1 if freeze failed */
>     bool pmsuspended = false;
>     virQEMUDriverConfigPtr cfg = NULL;
>     int compressed;
>     char *compressedpath = NULL;
> 
>     /* If quiesce was requested, then issue a freeze command, and a
>      * counterpart thaw command when it is actually sent to agent.
>      * The command will fail if the guest is paused or the guest agent
>      * is not running, or is already quiesced.  */
>     if (flags & VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE) {
>         int freeze = qemuDomainSnapshotFSFreeze(driver, vm, NULL, 0);
>         if (freeze < 0) {
>             /* the helper reported the error */
>             if (freeze == -2)
>                 thaw = -1; /* the command is sent but agent failed */
>             goto cleanup;
>         }
>         thaw = 1;
>     }
>     
>     ......
>     ......

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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