freebsd stable - Nov 2004 - Different snapshot sizes.

I'm somewhat confused.  When I take snapshots, offtimes, they seem to
be different sizes.

I have a system that has two 120 Gig disks that are vinum (now gvinum)
raid1'd.  On that I have a root, swap, usr, var and 'u' partitions.
All vinum created.

As one (of several) backup policies, we take three filesystem
snapshots per day, and progressively delete them such that the oldest
snapshot we keep is about 30 days old.

But the snapshots are often different sizes.  This strikes me as odd,
as I understood the "format" of a snapshot was to be the same as the
"format" of the partition itself --- so they should all have the same
size as the physical partition.

An example:

Making snapshot /usr/snap/20041116_180000 done.
total 193248
-r--r-----  1 root  operator  6442451256 Nov 16 17:32 20041116_094346
-r--r-----  1 root  operator  6442451256 Nov 16 18:00 20041116_120001
-r--r-----  1 root  operator  6442451264 Nov 16 18:00 20041116_180000

... the script above calls mksnap_ffs and then ls -l's the directory.
Note that the last snapshot is 8 bytes larger.  Very odd.  It's not
divisible by 512, but then neither are the other two.

At any rate, this all wouldn't worry me if the server didn't crash
with some regularity ... which it does.  In fact, the /var partition
gets the widest variety of snapshot sizes ... so I've disabled
snapshots there alltogether.

Dave.

-- 
===========================================================================|David
Gilbert, Independent Contractor.       | Two things can only be     |
|Mail:       dave@daveg.ca                    |  equal if and only if they |
|http://daveg.ca                              |   are precisely opposite.  |
=========================================================GLO================

David Gilbert wrote:
 > and progressively delete them such that the oldest
 > snapshot we keep is about 30 days old.

Off topic comment: consider using ports/sysutils/fileprune for this task.

Diomidis Spinellis

On Tue, Nov 16, 2004 at 06:22:29PM -0500, David Gilbert
wrote:
> But the snapshots are often different sizes.  This strikes me as odd,
> as I understood the "format" of a snapshot was to be the same as
the
> "format" of the partition itself --- so they should all have the
same
> size as the physical partition.
(To my surprise also) snapshot files seem to contain more data that one
might expect. There's a comment in the code that says `Write out the
list of allocated blocks to the end of the snapshot' (pointed out by
folks on my local BSD mailing list. Full thread at [0]). Changes in the
`list of allocated blocks' might account for the differences you're
seeing.


[0] http://lists.redbrick.dcu.ie/pipermail/bugi/2004-August/thread.html#579

ATB,
james