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I have a volume that is 125GB in space. Each snapshot I have is 125GB, exactly. I thought snapshots only grabbed data that changed, using its pointers. If not what the difference between a snapshot and a clone? Why are all my snapshots the exact same size as the original volume?

PS4000 if it is pertinent.

EDIT: I believe this is the answer to my question:

"Note: Snapshot reserve, local replication reserve, and replica reserve for a volume are all based on the current volume reserve, not the reported volume size."

http://psonlinehelp.equallogic.com/V3.3/understanding_thin_provisioning.htm

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You are right about the storage requirements for snapshots vs clones, with clones requiring the full size of the volume vs snapshots which start out zero sized and will grow as changes are made to the underlying disk (up to the size of the snapshot reserve).

Since you are using the default snapshot reserve of 100%, each snapshot will appear to be occupying the same size as the volume it was created from. Within Group Manager, under Volumes->(volume name)->Status->Volume and Snapshot Space, you should see how much of the snapshot space is actually In-use vs Free. If this is a new snapshot, it may not be possible to see the In-Use field, until some data has changed on the parent volume.

Alternatively via a command line, you can run:

show volume volume-name

and look for the values of Snap-Reserve percentage and Snap-Reserve-Avail, to calculate the actual size that the snapshots are using.

Sources:

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  • Why would the reserve make the snapshots, the individual snapshots, all the same size?
    – johnny
    Dec 19, 2014 at 14:39
  • An individual snapshot can grow to the same size as the underlying volume, but the in-use space of a new snapshot will be zero. You can reduce the size that snapshots can grow to by lowering the snapshot reserve of the volume - this will free up space in the group for other purposes. As for why the snapshot reserve is the same size as the volume by default, here's a section from the document I linked: "This is to ensure that a single snapshot can be held even if 100% of the base volume is modified"
    – mvermaes
    Dec 19, 2014 at 15:47
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Highlight the base volume in Group Manager.

In the right-pane, you can see the Volume Space and Snapshot Space sections.

The Volume Space section will tell you the reported size of the base volume, how much is free, how much is in-use, whether the volume is thin or thick, and the volume reserve.

The Snapshot Space section will tell you the snapshot reserve, number of snapshots, how much snapshot space is free, and how much snapshot space is in-use.

A snapshot is a point-in-time copy of a volume. There is a parent-child relationship between a base-volume and it's snapshots. Snapshots are space-efficient and only store the deltas between itself and the parent/source volume.

A clone is an independent/full copy of a volume. There is no parent-child relationship between a volume and it's clone. Clones are not space-efficient and store the entire contents of the original volume. If a volume is 100 GB in size, and you clone it, the clone will be 100 GB in size.

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  • That is my confusion. How is the snapshot saving me space if it is the exact size of the volume? It's the same size as a clone would be.
    – johnny
    Dec 18, 2014 at 21:21

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