[gentoo-user] Re: Making sense of reported ZFS disk usage

[Remy Blank]

Jeremi Piotrowski wrote on 2015-10-31 15:25:
> USEDSNAP refers to _data_ that is not in pool/data but in the snapshots.
> The value for USED is _data_ that is only present in *this one* snapshot,
> and not in any other snapshots or in pool/data. _data_ that is shared
> between atleast two snapshots is not shown as USED because removing one of
> the snapshots would not free it (it is still referenced by another
> snapshot).

Indeed, this makes sense. The man page says about "used":

  As the file  system  changes, space  that  was  previously shared
becomes unique to the snapshot, and counted in the snapshot's space  used.

So "used" means "used exclusively by this snapshot". And the missing
9.1G is data shared by two or more snapshots. I hadn't understood that
before. Thank you.

I guess I was looking for something like "how much data is shared
between this snapshot and the next one", but since there's no link
between snapshots (only between the dataset and the snapshots), ZFS
can't provide it in "zfs list".

But maybe there's a way to get this information? How can I find the
amount of data shared between two snapshots of the same dataset?

-- Remy