Different snapshot sizes.

David Gilbert dgilbert at dclg.ca
Tue Nov 16 15:22:33 PST 2004

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.


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

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