FreeBSD UFS2 snapshots, and math ...

Andrew P. infofarmer at gmail.com
Thu Oct 20 12:34:40 PDT 2005


On 10/20/05, user <user at dhp.com> wrote:
>
> Hello,
>
> On 20 Oct 2005, Lowell Gilbert wrote:
>
> > user <user at dhp.com> writes:
> >
> > > 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.
>
>
> That doesn't make much sense to me ... if the snapshot keeps track of
> changed_data_since_snapshot_was_taken, then ...
>
> Well, think of it this way - let's say I have a 1G filesystem, which is
> filled with a single 500M text file.  Now let's say I snapshot that FS.
> At this point, the snapshot takes up 0 bytes.  Now let's say the next day
> I alter 10% (50M) of that single 500M file - now the snapshot takes up
> that exact same amount of space, namely, 50M.
>
> Now I create a second snapshot, which immediately yakes up 0 bytes.  The
> next day, I change a totally different 50M of my text file ... so now, the
> first snapshot needs to keep track of yesterdays 50M of changes/deletions
> as well as todays, because todays operates on totally different disk
> blocks.  So now 2-day-ago snapshot is size 100M, and the snapshot from one
> day ago is now 50M.
>
> I think my interpretation is correct ... can you look over my and your
> conclusions again ?
>
>
> > > 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.
>
>
> Yeah ... see, I think it does matter, for the reasons above ... if, as in
> this second example, I am changing the same blocks on disk every day, the
> snapshot just needs to keep track of them once, namely "this is what they
> were during the snapshot, and you can change those same blocks all you
> want, I just need to keep track of what they were when you took the
> snapshot.."
>
> comments ?
>
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>

What makes you so reassured, I wonder.

Imagine that each data block is marked with labels
on change. It doesn't matter how many labels there
are, there will be only one data block saved.


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