Linux - What directories should I exclude when backing up a server?
/sys are virtual filesystems which reflect the state of the system, and allow you to change several runtime parameters (and sometimes do more dangerous things, like directly writing to the memory or to a device). You should never backup or restore them.
In most modern distributions,
/dev is dynamically created at boot (it is a memory filesystem filled by
udev and friends). There is no point in backing it up, and attempting to restore it is futile. However, if your distribution is configured to use a static
/dev, this does not apply (check
/dev is a
tmpfs it is a memory filesystem).
There are other filesystems you should not back up;
usbfs (usually at
/proc/bus/usb, if mounted at all),
debugfs (supposed to be at
/sys/kernel/debug if mounted at all, but some people put it somewhere else; you probably do not have this one),
devpts (mounted at
tmpfs instances (often found at
/var/lock, and other places; backing them up and restoring them should be harmless but pointless, as their contents are lost on shutdown), and any remote filesystems or magic automounter directories (attempting to backup or restore them could end up in disaster, as you could end up backing up/restoring to a different machine). You should also be careful with
/mnt, as external devices (like a CD you forgot in the drive) could be found there, but you might also have used them on purpose to mount something which should be backed up.
Note that, other than mostly harmless
tmpfs instances, network filesystems/automounters, and removable media, the filesystems you should not back up are all descendents of
/sys. If you have no network filesystems (or automounters), and no removable media, excluding
/proc and rebooting after a restore (to wipe the
tmpfs instances) should be enough.
This really depends on how you are going to restore your system. If you will rebuild then you only need the configuration/data files for your services (eg: /etc, /opt, /var, /home)
If you are after a full system restore, then it you could omit /proc, /boot & /dev. Then you can install the minimum OS from your boot media and then restore your system via your backup.
Of course, the best backup is one that has been tested and verified.
So omit what you don't think you need, try to restore in a VM and verify you can get back your system using this data.
See The Tao Of Backup, chapter 1.
Some of the special files in /proc and /sys confuse rsync. You don't want to back up mounted network filesystems either, usually. Sparse files can also cause problems.
Add -x to limit it to one file system. That avoids all the network file systems and /proc etc. However you then need to run one rsync for each file system you have mounted.
Add -S to handle sparse files sensibly.
/boot, /dev and /proc are quite useless to backup -- though, if you know what you're doing you can backup /boot.
I would also not backup /lib, /media, /mnt, /sbin, /bin, /srv, /sys, or /tmp.
/usr is optional, depending on whether you have anything in /usr worth backup up. If I were you I"d worry most about backing up user's $HOMEs, /var, and /etc (for configuration files).
Again though, this really all depends on the type of backup you want to do. Is this a web server? Is this a personal computer? Is this a shell server with tons of directories under /home?