Using fssnap in Solaris 8 update 4/01 & beyond


What Is fssnap?
In the 04/01 maintenance release of Solaris 8, Sun kindly introduced
the ability to produce file-system snapshots of UFS file-systems.
This allows you to take consistent back-ups of active file-systems.

How Does It Work?
When you create a snapshot of a file-system you conceptually create
two file-systems that are initially identical. These are
conventionally referred to as the front and back file-systems. The
front file-system is the "real" file system and it continues to look
and behave in the same manner (almost - see below), as prior to the
snapshot being taken (i.e. people can still read and write to the
file-system as normal). However the back file-system is a read-only
copy of the front-file system taken at the time of the snapshot
creation (i.e. it as an exact snapshot of the file-system which does
not change). The effect of the back-file system is created by having
a backing file and every time a write is applied to the front-file
system a copy of the existing (i.e. unchanged) data is placed in the
backing file. You can then mount the back-end file system using
special devices and back-up the file system. When reading from this
file-system you are using a virtual file-system. Each time you read
from the back file-system (i.e. the frozen copy), the file-system
code checks if the block had changed in the front file-system. If it
has, it returns the copy from the backing file (i.e. the unchanged
copy). This means that you can keep a static copy of a file-system
for the duration of a back-up, as long as the file-system in which
the backing file is located, is large enough to hold all the data
that changes in the front file-system during the backup.

What Are The Drawbacks?
There are a couple of negative effects that should be noted. Due to
the overhead of the copy to the backing file during a write to the
front file-system, write performance is slightly reduced during the
snapshot operation. It should also be noted that if the front
file-system is quite active (in terms of writes) then obviously the
size of the backing file will grow very quickly. There are no adverse
side effects (aside from filling a file-system) if the file-system
containing the backing file runs out of space, usually the backing
file will automatically be deleted and whatever commands are running
against the mounted snapshot will fail.

Where Can I Get The Details?
An administrators guide is available from Sun at:
http://docs.sun.com/ab2/coll.736.2/S8ADMINSUPP/@Ab2PageView/2977?DwebQuery=fssnap&oqt=fssnap&Ab2Lang=C&Ab2Enc=iso-8859-1.



-by Ken Robson