The 15/03/13, Eric Blake wrote: > On 03/15/2013 06:17 AM, Nicolas Sebrecht wrote: <...> > > Here is where we are in the workflow (step C) for what we are talking about: > > > > Step 1: create external snapshot for all VM disks (includes VM state). > > Step 2: do the backups manually while the VM is still running (original disks and memory state). > > During this step, the qcow2 files created in step 1 are getting larger > proportional to the amount of changes done in the guest; obviously, the > faster you can complete it, the smaller the deltas will be, and the > faster your later merge steps will be. Since later merge steps have to > be done while the guest is halted, it's good to keep small size in mind. > More on this thought below... > > > Step 3: save and halt the vm state once backups are finished. Step3: virsh save && virsh destroy. > > Step 4: merge the snapshots (qcow2 disk wrappers) back to their backing file. > > This step is done with raw qemu-img commands at the moment, and takes > time proportional to the size of the qcow2 data. > > > Step 5: start the VM. Step 5: virsh restore. > ...As mentioned above, the time taken in step 2 can affect how big the > delta is, and therefore how long step 4 lasts (while the guest is > offline). If your original disk is huge, and copying it to your backup > takes a long amount of time, it may pay to do an iterative approach: > > start with raw image: > raw.img > > create the external snapshot at the point you care about > raw.img <- snap1.qcow2 > > transfer raw.img and vmstate file to backup storage, taking as long as > needed (gigabytes of data, so on the order of minutes, during which the > qcow2 files can build up to megabytes in size) > raw.img <- snap1.qcow2 > > create another external snapshot, but this time with --disk-only > --no-metadata (we don't plan on reverting to this point in time) > raw.img <- snap1.qcow2 <- snap2.qcow2 > > use 'virsh blockcommit dom vda --base /path/to/raw --top /path/to/snap1 > --wait --verbose'; this takes time for megabytes of storage, but not > gigabytes, so it is faster than the time to copy raw.img, which means > snap2.qcow2 will hold less delta data than snap1.qcow2 > raw.img <- snap2.qcow2 > > now stop the guest, commit snap2.qcow2 into raw.img, and restart the guest > > By doing an iteration, you've reduced the size of the file that has to > be committed while the guest is offline; and may be able to achieve a > noticeable reduction in guest downtime. Looks like a very good optimization for recent enough libvirt. Thanks! -- Nicolas Sebrecht _______________________________________________ libvirt-users mailing list libvirt-users@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvirt-users
