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size last snapshot

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Occasional Advisor

size last snapshot


Trying to understand something. I have snapshots on most of my datastores and keep them for a few days/weeks depending on the datastore.

I was looking at a few datastores where I want to clean up some room by keeping less snapshots and I noticed the last snapshot ia allways a lot bigger than any other snaphsot.

Sometimes even by an order of magnitude. On one 2TB datastore the last snapshot is 1.5TB.  The normal daily snapshots are anywhere from 80-300GB.

I have similar numbers on another datastore where the daily snapshot is 50-100 GB but the last snapshot is 500GB.

There must be a logic to this but I cannot find it. Maybe there was a discussion about it before but I cannot find it either.

can someone explain this please?

Bonno Bloksma

Esteemed Contributor

Re: size last snapshot

Snapshot size references the "New" data since the prior snapshot.

You may be asking, “What is new data?”

  • “New Data” = anything changed between snapshots; regardless of whether the snapshot “owns” it.

»Data written to the primary volume

»Existing data which is overwritten

»Data no longer used by the primary volume but retained by the snapshot

  • “New Data” can almost be called “how much do I need to replicate to the DR site to keep the arrays synchronized”…but that doesn’t fit in the GUI.

»It’s still “New Data” even if all data is actually “owned” by the primary volume and not by snapshots.


  • Wait a minute…

»That’s right!  The first snapshot on a volume containing 1.7 GB of data will have 1.7 GB of “new data”… the amount of data written since the prior (non-existent) snapshot.

»Also, the amount we need to replicate for the initial replication (replication seeding).

»If you take a second snapshot and delete the first one. The second snap is now….yay! 1.7 GB because it represents all data prior to itself.