- Physical space refers to how much physical blocks are being used.
- Logical space refers to how theoretical space may be used without taking into account things like deduplication or compression.
If the space savings based on deduplication and compression is high, this can appear to inflate the logical space.
In that case, if deduplication and compression were not enabled the used space could exceed the capacity of the volume.
- A block in ONTAP is 4k in size. Let's say I have a ~4k file that I have 20 copies of and the 4k block is identical for each one.
- If I enable deduplication it removes the duplicate blocks and creates pointers to the shared block.
- In theory I would then only be using ~4k physical space where as the logical space would be about ~80k.
- If I disabled deduplication and undid it, the used space would grow from ~4k to around ~80k.
- When you create a snapshot of a volume it creates a set of pointers to all of the used blocks in the active file system.
- The snapshot doesn't own the blocks until the block is no longer used by the active file system.
- The oldest snapshot with references to the block then owns it.
- The block isn't freed until the last snapshot with references to it is deleted.
- This is how you can have snapshots taking up logical space but not physical space.