HPE 3PAR StoreServ Storage

Virtual Copies/Snapshots Confusion

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Virtual Copies/Snapshots Confusion

So, I read some stuff today from the HPE 3PAR StoreServ Storage Concepts Guide related to virtual copies/snapshots. I have been under the impressions since our XIV days a snapshot was a picture of the data on the source drive without copying the whole drive such as a clone would be. Let me try to explain myself but I am not sure I can clearly.

In the XIV world we created snapshots of our replicated data at our DR site and used those snapshots as temp disks for DR purposes, then when DR is over we could delete the snapshots. We have done the same thing since 3PAR came into our lives. I am not that is the intended used of the snapshots in the 3PAR world. Below are the two snip-its from the guide.

"Virtual copies are created using copy-on-write techniques. Unlike a physical
copy, which duplicates the entire base volume, a virtual copy records only the changes to the original volume.
This allows an earlier state of the original volume to be recreated by starting with the current state and rolling
back all of the changes that have been made since the virtual copy was created.

Virtual copies are consistent at the virtual volume level, but not at the host system or application level.
In other words, virtual copies only preserve the data that was written on the source virtual volume before the
virtual copy is created. Virtual copies do not preserve the data that is resident within the application or system
buffers and is not flushed to disk before the virtual copy is created."

So, to put it lightly, I am very confused now. It almost sounds like a snapshot is created so any changes that would have been written to the original volume are now written to the snapshot. That doesn't make any sense as we have been using these snapshots to recover servers during our DR exercises for years and have been successful at it. We use the snapshot at DR like it is a real drive, we make changes to it and update it and such.  Unless at this point we are not really looking at the snapshot.  Then after DR is over, we delete the snapshot

It sounds like a promote snapshot will copy all the data you have made to that snapshot back to the original volume, another confusing concept.

Can anyone try to explain to me in some other way, like a "snapshot for dummies" way that I might can get this picture in my head that will make sense.



Sheldon Smith

Re: Virtual Copies/Snapshots Confusion

Yeah, there's a bit of cross-naming confusion.

The 3PAR terminology started using the term "Virtual Copy". In the last few years, the industry tends to use the term "Snapshot", so that is now being used in place of "Virtual Copy".
In the 3PAR realm, Virtual Copy and Snapshot are one and the same. They can be writable or read-only.

A Virtual Volume (which gets tied to a host as a LUN) has pointers to pointers to pointers ... to physical locations. Base volumes and virtual copies aka snapshots are all Virtual Volumes. Any virtual volume can have a LUN assigned so a host can access it.
The LUN's contents will appear consistent to the host.

When a snapshot is taken, only the pointer metadata is duplicated. All of its pointers point to the exact same locations as the parent volume. This is what makes taking a snapshot so fast. Then, like a page fault in memory, when a location is going to be changed the first time after the snapshot occurs, the old data gets updated to the copy CPG before the new stuff (a technical term) is written to the base volume; what was there is saved for the snapshot and various pointers get updated. New data is still written to the original base volume. 
This lets the snapshot appear static as the base volume goes on its merry way.

And finally, yes, a Promote Snapshot will take the state of the virtual copy and overwrite the base volume.
Take a look at the "HPE 3PAR Virtual Copy" technical paper, available in the HPE Information Library.

Note: While I am an HPE Employee, all of my comments (whether noted or not), are my own and are not any official representation of the company

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