Snapshot & Cloning

[Vpro]

Hi

I am new to HP-EVA and would like to understand the Snapshot mechanism in EVA.

Also does EVA have support for cloning? Any pointers on the above 2 topics wld be great...

Regards

[Greybeard]

Hi, there are two types of snapshot. Fully allocated, this starts out the same size as the original and as the original is updated it's original data is placed in the snapshot kind of like an instant backup of any changed data. Demand allocated, this works in the same way but no need to allocate the whole capacity of the original as it will grow over time, better for a big LUN that only has changes to a few files.
There is an option called CA (continuous access) that replicates data between two EVAs that may be what you're after.

[Uwe Zessin]

A snapshot is always bound to the original virtual disk. It is a view that contains unmodified data from the original volume and saved data that has been replaced on the original volume.

A snapclone starts as a fully-allocated snapshot, but the EVA automatically copies all data from the source volume, even if some blocks have not modified by the host. If the copy is done, the snapclone is converted into a new 'original' virtual disk and all links to the former source virtual disk are cut.

There is also a 'mirror-clone'. Think of it as two virtual disks that are kept identical during normal operation. It is possible to suspend mirroring. The host keeps writing on one of them and the other can be presented to a different host.
They are still linked through a bitmap that tracks which blocks have been modified on either side.
Later, it is possible to 'synchronize' them back. The bitmap helps to optimize this and only resolve the differences.
Finally, you can remove the link and end up with two independent virtual disks.

All these features are covered by a Business Copy (BC) license.


The data replication feature (CA) works a bit like a mirror-clone, except that the second virtual disk is on a different EVA. There is no bitmap, but a full journal if the replication is suspended and some other features from a mirror-clone (e.g. rollbacks) are nor available.

[Vpro]

Hi

Is there any limitation on number of snapshots per volume.....if yes is there a way to come around it??

[Uwe Zessin]

Yes, there are limits, but they depend on the EVA model and the firmware revision the controllers are running. I have not tested it myself, but as far as I know those are HARD limits.