Monthly Archives: January 2013

Live backup with external disk snapshots and libvirt’s blockpull

There’s been some interesting work going on in blocklayer for libvirt/qemu/kvm. I have a few pending posts related to that, thought I’ll start with a post about live backup with external disk snapshots & using libvirt’s blockpull. External snapshots were previously discussed here.

[1] List the block device associated with a guest.

# virsh domblklist f17-base
Target     Source
vda        /export/vmimages/f17-base.qcow2


[2] Create external disk-only snapshot (while the guest is running).

# virsh snapshot-create-as --domain f17-base \
  snap1 snap1-desc --disk-only \
  --diskspec vda,snapshot=external,file=/export/vmimages/sn1-of-f17-base.qcow2 \              
  -- atomic \
 Domain snapshot snap1 created

[3] Now, list the block device associated(use cmd from step-1, above)
with the guest, to ensure it reflects the new overlay image as
the current block device in use.

# virsh domblklist f17-base
Target     Source
vda        /export/vmimages/sn1-of-f17-base.qcow2


[4] Here, backup the original disk image /export/vmimages/f17-base.qcow2
using rsync or any other preferred method, while the guest is now
running ( &amp writing all its new changes into sn1-of-f17-base.qcow2).

[5] Now, let’s say, there are changes accumulated over time into
sn1-of-f17-base.qcow2(current active layer). We can now merge
the contents from original disk image(f17-base) into the current disk using ‘blockpull’.

Two possible here:

(a) If we had the below chain:

 [base] <-- [sn1] # the current active layer

We can make sn1-of-f17-base.qcow2 a standalone image by
pulling/merging contents from the orignial disk image (f17-base.qcow2). Resulting in sn1-of-f17-base becoming a self-contained disk image without any backing file.

# virsh blockpull --domain f17-base --path \
  /var/lib/libvirt/images/sn1-of-f17-base.qcow2 --verbose --wait

The resultant chain will be:


which is an active, self-contained image with all the data from [base] pulled into it.

(b) If we have more snapshots:

Say, we start with this chain,

  [base] <-- [sn1] <-- [sn2] <-- [active],

it can be shortened to:

  [base] <-- [sn1] <-- [active]

by doing:

  # virsh blockpull --domain f17-base --path \
    /var/lib/libvirt/images/active.qcow2 \
    --base /var/lib/libvirt/sn1.qcow2 --verbose --wait

The above command pulls the data from [sn2] into [active], and changes the backing file of [active] to [sn1].

As an optional next step, we can again perform 'blockpull' operation,
with the above shortened chain, resulting in [active] becoming a standalone image:

# virsh blockpull --domain f17-base --path \
  /var/lib/libvirt/images/active.qcow2 --verbose --wait

The above command pulls data from [sn1] and [base]
into [active] and makes it a self-contained image without any backing file,
resulting in the chain with a single image: [active]

NOTE: 'blockpull' needs to be done for every disk image that was
snapshotted (by optionally supplying –base. If a –base option
is not supplied, the current disk image will pull data from its
entire backing chain, & makes it standalone without any backing

[6] Once the 'blockpull' operation above is complete, we can clean-up
the tracking of snapshots(metadata) by libvirt to reflect the new reality:

# virsh snapshot-delete $dom $name --metadata

There are couple of other ways doing guest live backups using Libvirt’s newer capabilities like ‘blockcopy’ mechanism. An unpolished example is here (This test was was done by building libvirt & qemu from git, a couple of months ago. I haven’t yet gotten to testing newest qemu/libvirt)



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