Qcow2 snapshots and Gluster Geo-replication

Gluster introduced sharding feature to store large files(which can grow beyond a single brick) or to support running Virtual machines in Gluster Volumes. Read more about sharding here and here.

How to backup VM images?

Backing up VM images is not easy, rsync will consume more CPU to calculate the checksum to sync only incremental changes.

Geo-replication is aware of Gluster Sharding feature and taking the advantage of syncing small sharded files instead of big qcow2 image files. But is the data consistent? In this blog we will understand how to backup VM images to DR site consistently.

Read here to know more about Geo-replication support for sharding.

Setup:

VMs hosted in Gluster Volume(Master Volume) and Geo-replicated to another Gluster Volume(Slave/Backup Volume). Sharding enabled in both the Gluster Volumes.

Using Internal Qcow2 snapshot:

Before starting Geo-replication every day, take qcow2 snapshot of all the disks in Master Volume. Geo-rep will sync the data including the created snapshots.

virsh snapshot-create-as --domain <DOMAIN> <SNAP_NAME>

For example,

virsh snapshot-create-as --domain fedora22 sn1

But while taking internal snapshot, guest is paused :(

# virsh list
 Id    Name                           State
----------------------------------------------------
 3     fedora22                       paused

Note: If Guest has more RAM and actively modifying state, then more time to take Snapshot.

Run Geo-replication using the scheduler script, which will Set the checkpoint and automatically stops Geo-replication once checkpoint is reached.(This utility will be available with glusterfs-3.7.9 release.)

Run /usr/share/glusterfs/scripts/schedule_georep.py --help for more details about the script.(/usr/local/share/ in case of source install)

Psudeo code:

pool_dir=<PATH_OF_MASTER_VOL_MOUNT>
images=$(ls ${pool_dir})
For each images, take qcow2 snapshot
Run schedule_georep script

Once the scheduler script completes, check the qemu-img info in Slave and confirm that Geo-rep synced everything from master Volume including Snapshots created.

qemu-img info /mnt/gv2/f22.qcow2

Example Output:

image: /mnt/gv2/f22.qcow2
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 2.8G
cluster_size: 65536
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
2         sn2                    693M 2016-02-23 18:40:10   01:37:34.881
3         sn3                    693M 2016-02-23 18:47:06   01:44:15.950
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

Using External Qcow2 snapshot

Once we take external snapshot, qcow2 image will become read only base image and snapshot file will become overlay(Read more about backing chain and overlay here).

New changes will be recorded in the overlay instead of base image, Since base image is frozen Geo-rep will sync the consistent image to Slave. Start Geo-replication and wait for scheduler script to end.

When multiple external snapshots taken, it is very difficult to maintain the backing chain and reverting to a snapshot is not easy when external snapshot is used. Once Geo-rep Scheduler script is complete, blockcommit the image in Master side to prevent growing backing chain.

Delete the external snapshot once the blockcommit returns success.

pool_dir=<PATH_OF_MASTER_VOL_MOUNT>

# Take External snapshot
virsh snapshot-create-as --domain <DOMAIN> <SNAPNAME>  \
    --diskspec vda,file=${pool_dir}/snaps/<SNAPNAME>.qcow2 \
    --disk-only --atomic --no-metadata

# Run Geo-replication
/usr/share/glusterfs/scripts/schedule_georep.py \
    <MASTERVOL> <SLAVE> <SLAVEVOL>

# Blockcommit
virsh blockcommit <DOMAIN> vda --active --verbose --pivot

# Remove the external Snapshot file
rm ${pool_dir}/snaps/<SNAPNAME>.qcow2

With this method, Slave will always have consitent base image.

Ref: http://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit

Conclusion

We should use qcow2 external snapshot if Live backup is required. External snapshot file will be deleted once blockcommit is done in Master side.

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