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Typical mail server, let's say 20GB of mails, partition size 100GB. I'm transferring to a LUN on shared storage P2000 and designing the whole thing. What it is typical snapshot size in this situation ? Requirement is to have 2,3 days of incremental backup. Considering the fact, that the space is not for free in this world, the snapshot size should be only 20GB or do I have to add another 100GB in case I would do the full backup ?

So the question is if the snapshot size should be (partition_size)+(size of 2,3 days of incremental backups) or only the size of the second part?

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The snapshot space is a "copy-on-write" area - everything that needs to be changed on the disk is first copied to the CoW before it gets overwritten (or the "clean" space from the snapshot is re-referenced for this purpose depending on the snapshotting algorithm and its implementation). You only would need to dedicate the space to the snapshot which is likely to be needed for your changes.

Bear in mind though that most algorithms operate with a certain allocation unit size (depends on the implementation and sometimes on the configuration), so any change, no matter how small, would require at least one allocation unit of the CoW area.

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