FreeBSD UFS2 snapshots, and math ...

Lowell Gilbert freebsd-questions-local at
Thu Oct 20 11:44:09 PDT 2005

user <user at> writes:

> I am trying to budget some disk space for filesystems with snapshots
> enabled on them.
> The following is simplified - I am just trying to get my concepts in
> order:
> Let's say I have a filesystem, and on that filesystem I create a snapshot
> every single night, and every night I delete the snapshot from 5 nights
> ago.  This means that at all times, I have four snapshots running on that
> filesystem, one from 1 day ago, one from 2 days ago, one from 3 days ago,
> and one from 4 days ago.
> Let's also assume that the percent change of the filesystem is 5% (every
> day 5% of the blocks in the filesystem are either changed or deleted).
> ----
> Does this mean that if that 5% change is a different 5% every day, that
> the one day ago snapshot will be size 5%_of_filesystem, and that the 2 day
> ago snapshot will be size 10%_of_filesystem, day 3 15% and day 4 20%, for
> a total of 50% of the total filesystem taken up with snapshot data ?

No.  One copy of each version of the file that exists in any
snapshot.  Regardless of how many snapshots it's in.

> Does that sound correct ?  When I say that the 5% change is a different 5%
> every day, what I mean is that it is not the same files/data being altered
> every day, but rather there is 5% of new data changed every day, relative
> to the previous nights snapshot.
> The second question is this:
> If the 5% data changed per day is the _same_ 5% every day (perhaps
> changing the same table in a DB every day, or perhaps changing the same
> block of lines in a text file every day) does that mean that every day
> simply represents 5%_of_filesystem, for a total of 20% of the total
> filesystem in use at all times for snapshot data ?

Whether it's the same data or not doesn't affect how much space you use.

More information about the freebsd-questions mailing list