The /etc/config/fstab configuration file defines static file systems and swap partitions that should be activated at boot. Note that the disks these filesystems and swap partitions reside on must be available when the system is booting. Else you will have to mount them manually. Also, the modules for the particular file systems must be installed and loaded into the kernel. The paket names are kmod-fs-ext3, kmod-fs-ext4, kmod-fs-vfat, etc. Bear in mind that the file system ntfs is still closed source, thus kmod-fs-ntfs will only grant you read access. To gain read-write access to this file system, you need to install the paket ntfs-3g. Then type ntfs-3g /dev/sda1 /mnt/dir -o rw. Be aware that this performs very poorly.

The block-mount and block-hotplug packages must be installed to use this configuration file!

There is a bug: vim /etc/init.d/fstab and put a # in front of the line echo '# WARNING: this is an auto generated file, please use uci to set defined filesystems' > /etc/fstab. This creates the file /etc/fstab and thus prevents creating a symlink to /tmp/fstab. In order for uci to work, there need to be symlink!

The configuration file can consist of sections defining file systems to be mounted and swap partitions to be activated.

There is one global section named automount which defines the hotplug automounting behaviour.

This example is included by default:

config global automount
    option from_fstab 1
    option anon_mount 1

The automount section contains these settings:

There is one global section named autoswap which defines the hotplug auto swap behaviour.

This example is included by default:

config global autoswap
    option from_fstab 1
    option anon_swap 0

The autoswap section contains these settings:

Each mount section defines a filesystem to be mounted at boot. Filesystems must be formatted before they can be used.

This example is included by default:

config mount
        option target   /home
        option device   /dev/sda1
        option fstype   ext3
        option options  rw,sync
        option enabled  0
        option enabled_fsck 0

The mount section contains these settings:

Each swap section defines a swap partition to be activated at boot.
Swap partitions must be formatted before they can be used and the swap-utils package needs to be installed!

This example is included by default:

config swap
        option device   /dev/sda2
        option enabled  0

The swap section contains these settings:

If you ask people or search the net, you will find as a general rule of thumb double RAM for machines with 512MiB of RAM or less than, and same amount as RAM for machines with more. But this very rough estimate does apply for your embedded device! Be aware that there are exactly two differences between RAM and SWAP, that matter: the access time and the price. A CUPS spooling server will run just fine, when only SWAP is available, whereas some applications may perform very poorly when their data it stored on the SWAP rather then being kept in the "real" RAM. The decision which data is kept in the RAM and which is stored on the SWAP is made by the system. As explained here Debian Forum (german) since Kernel 2.6 you can define the swapiness of your system:

$ sysctl -w vm.swappiness=60
$ echo 60 > /proc/sys/vm/swappiness

In contrast to other operating systems, Linux makes ample use of memory, so that your system runs smoother and more efficiently. If memory is then needed by an application, the system will unload stuff again, and make memory available. For OpenWrt, let us just say to use as much SWAP-Space as needed by your applications running. If this should not perform well, because of the poor access time, it would not help to decrease the amount of SWAP, but only to run fewer services at the same time or increase the amount of RAM with a soldering iron

OpenWrt vanilla will run just fine, with no SWAP at all. But after you installed a couple of applications, you could try to mount a SWAP-Partition and see what is does for you. Do not worry, you can not brake anything by doing that. The manual commands are swapon /dev/sdaX to mount respectivly swapoff /dev/sdaX to unmount. Type in free to see the usage of memory:

# free
              total         used         free       shared      buffers
  Mem:        29484        28540          944            0         1116
 Swap:       524280         2336       521944
Total:       553764        30876       522888

In this example there is 32MiB of RAM and 512MiB of SWAP. SWAP is a SWAP-formated 512MiB-Partition on a large USB-Harddisk. In this example, only 2MiB are being used! And you do not see here, that actually half the RAM is being wasted for a TMPFS-Drive. Use df to see that:

#df  
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/root                 1280      1280         0 100% /rom
tmpfs                    14744       116     14628   1% /tmp       <====up to 14MiB from the RAM //could// be wasted here. tmpfs-partitions grow and shrink with usage!
tmpfs                      512         0       512   0% /dev
/dev/mtdblock3            5440      4988       452  92% /overlay
mini_fo:/overlay          1280      1280         0 100% /
/dev/sda2              2709204     86276   2485284   3% /mnt/sda2
/dev/sda3              7224600    147320   6710196   2% /mnt/sda3

In this example, it would probably perform better, to mount the /tmp-directory on the USB-Harddisk and not use TMPFS at all, rather than using a 512MiB-Swap Partition. But you can also see, that overlay has only 8% space left to install programs on. So in this example it would be even better to use the exroot option. See rootfsonexternalstorage for that.