Solaris - 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

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