[ovirt-users] Scheduling daily Snapshot

[Nir Soffer]

On Wed, Dec 6, 2017 at 6:02 PM Jason Lelievre <jlelievre at folksvfx.com>
wrote:

> Hello,
>
> What is the best way to set up a daily live snapshot for all VM, and have
> the possibility to recover, for example, a specific VM to a specific day?
>

Each snapshot you create make reads and writes slower, as qemu has to
lookup data through the entire chain.

When we take a snapshot, we create a new file (or block device) and make
the new file the active layer of the chain.

For example, assuming you have a base image in raw format:

image-1.raw (top)

After taking a snapshot, you have:

image-1.raw <- image-2.qcow2 (top)

Now when qemu need to read data from the image, it will try to get the data
from
the top layer (image-2), if it does not exists it will try the backing file
(image-1).
Same when writing data, if qemu need to write small amount of data, it may
need
to get entire sector from a another layer in the chain and copy it to the
top layer.

So after a month of backups, will have 30 layer in the chain. I don't know
how
much slower it will be, I never measured this.

For backups, I think you like to have something like a daily backup for the
last week,
a weekly backup for the last month, and so on.

For daily backup of the last week you can take a snapshot every day, and
delete
the 7th snapshot every day. Then backup the snapshot bellow the active
layer.
You can download the snapshot using the SDK, see the download_snapshot
example (requires 4.2). All this can be done while the vm is running.

On your backup, you will have a snapshot per day. You can keep all snapshot
or merge old snapshot using qemu-img commit.

It is also should be possible to reconstruct an image on storage from your
backup
by uploading the the snapshots back to storage - but it is tricky to get
right.

I think the best option for users is to get some 3rd party solution based
on
the oVirt SDK.

Adding qemu-block mailing list, they can probably add more info on how much
you pay for large chain of images, and have more ideas about backing up
qcow2 chains.

I use a Hyperconverged Infrastructure with 3 nodes, gluster storage.
>

Maybe a solution based on gluster or the layers below (e.g. lvm snapshots)
will
be easier to use and more efficient. You can ask in the gluster mailing
list.

Adding Sahina to help with the gluster side.

Nir

>
>

>
> Thank you,
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