[openstack-dev] [Discussion] Different types of snapshots
Kashyap Chamarthy
kchamart at redhat.com
Thu Jun 13 04:19:54 UTC 2013
On 06/12/2013 09:02 PM, Duncan Thomas wrote:
> On 12 June 2013 13:27, Kashyap Chamarthy <kchamart at redhat.com> wrote:
>>>> A point of contention for a number of folks is that currently the base
>>> implementation is such that the snapshot is dependent on the source volume,
>>> but not all back-ends require this.
>>>> To be fair, each snapshot-type has its use-case.
>>>> Isn't the whole point of snapshot is a point in time to quickly revert to a known sane
>> state, so depending on source is okay, I believe.
>>>> Maybe what you mean is a "clone".
>> You're walking into the middle / end of a long discussion here,
Maybe, I'm just trying to understand. It was my poor wording.
> but a
> clone is different to a snapshot in that a snapshot is immutable. This
> is a very useful property for certain usecases (e.g. golden images)
Sure, I myself do a lot of clones/snapshots while testing and understand the difference.
And understand the use-case of golden image.
Just to note, in QEMU, you can merge snapshot chains (i.e. r/w or mutable).
e.g.
.------------. .------------. .------------. .------------.
| | | | | | | |
| RootBase |<---| Snap-1 |<---| Snap-2 |<---| Active |
| | | | | | | |
'------------' '------------' '------------' '------------'
/ |
/ |
/ commit data |
/ |
/ |
/ commit data |
v |
.------------. <---------------------' .------------.
| | | |
| RootBase | | Active |
| |<---------------------------------------| |
'------------' Backing File '------------'
You're merging top-level images -- [Snap-1] and [Snap-2] into a lower-level base image --
[RootBase] which will now be the backing file of the current [Active] image.
But the above demonstrates what can be done with qcow2 files.
--
/kashyap
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