Date: 20 Oct 2005 14:44:05 -0400 From: Lowell Gilbert <freebsd-questions-local@be-well.ilk.org> To: user <user@dhp.com> Cc: freebsd-questions@freebsd.org Subject: Re: FreeBSD UFS2 snapshots, and math ... Message-ID: <44y84ojafe.fsf@be-well.ilk.org> In-Reply-To: <Pine.LNX.4.21.0510201101260.31267-100000@shell.dhp.com> References: <Pine.LNX.4.21.0510201101260.31267-100000@shell.dhp.com>
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user <user@dhp.com> 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.
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