Dirsync
Dirsync is a script I wrote to synchronise directories when a system boots up or shuts down.
Background
To provide some basic network functionality (DNS, logging stats, a webserver, etc..) I used to run Debian on a Linksys NSLU2 for many years. The NSLU2 is a small device, fanless with an internal Disk-on-chip (flash) for storing the original Operating System (with Debian it's running off a 1GB USB flash drive). Around June 2010 I upgraded to a more powerful alternative, the Sheeva Plug. Small, fanless, internal flash and an SDCard slot (from which I'm running the O/S). I leave these machines running 24/7 and I want them to be quiet and consume as little power as possible. I therefore use an SDCard to place the O/S on. Since SDCards fail much faster than spinning HDD's during normal operation I make sure the system is optimised to write as little to the disk as possible. Most logging is turned off, I make extensive use of RAM filesystems (ramfs) and for additional (non-O/S) data storage I use a 2.5" USB HDD (which spins down and powers off after so many minutes of inactivity).
For logging some system statistics I use rrdtool and to prevent rrdtool from constantly writing to the SDCard flash I put the datbase file (.rrd) in a RAM filesystem.
My main reason for writing this script is because I didn't want to lose my .rrd files every time the system shuts down.
This script will sync from one directory to another (saving the .rrd from /dev/shm/rrd to /var/lib/rrd) when the system shuts down and copies it back (restoring the database back to /dev/shm/rrd) upon system boot up.
The script also has an 'update' feature which updates to the backup directory (to /var/lib/rrd in this example) to make sure we don't lose too much information in case the server shuts down without going through the normal shutdown procedure (e.g. in case of a power outage). I run the 'update' feature only once a week, which is the trade off between losing database information and losing the SDCard due to extensive write cycles.
Latest version: File:Dirsync-1.0.tar.gz
http://www.globalscaletechnologies.com/p-22-sheevaplug-dev-kit-us.aspx
http://en.wikipedia.org/wiki/SheevaPlug