How are Dell Equallogic Snapshots different from a Clone as it relates to disk space consumption

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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

Best Answer

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:

http://en.community.dell.com/techcenter/storage/w/wiki/3619.equallogic-snapshots-and-clones-best-practices-and-sizing-guidelines-by-sis

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Best Answer

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