FreeBSD UFS2 snapshots, and math ...
user at dhp.com
Wed Oct 19 14:23:09 PDT 2005
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
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 ?
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 ?
Finally, are there any snapshot diag tools at all ? Like, something that
reports snapshot sizes, percent of disk used for snapshots, and maybe even
a way for me to actually calculate what the percent change for time period
X is for a particular filsystem >?
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