The Fstab, or file systems table, is a central configuration that defines how file systems (usually on block devices) should be mounted if requested (such as on booting the device or connecting it physically). This way, you don’t have to manually mount your devices when you want to access them. The mounting configuration can consist of static file systems but also swap partitions.
The fstab UCI subsystem is where all the options for all devices and file systems to be mounted are defined, the actual file is located at /etc/config/fstab.
By default this subsystem and its configuration file do not exist, as for the average OpenWrt usecase (network devices) it's not needed.
So if you need to configure this, you must first create it.
Since the tool dealing with mounts is block, all current options can be found in its source code.
You should use the block utility. Install the package block-mount:
opkg update opkg install block-mount
Get a sample fstab UCI subsystem configuration file.
block detect | uci import fstab
Now there is a UCI subsystem, you can use UCI command line to change it or just edit the file itself.
It is possible to set on other devices, but the process is a bit more involved, see Extroot configuration for details.
Also see Mounting Block Devices for technical details of the mounting process and scripts involved.
The configuration file consists of a global section defining defaults, mount sections defining file systems to be mounted and swap sections defining partitions to be activated. Whenever you change your fstab configuration, run this command to mount everything in the new way:
block umount block mount
|anon_swap||boolean||no||0||mount swap devices that don’t have their own config section|
|anon_mount||boolean||no||0||mount block devices that don’t have their own config section|
|auto_swap||boolean||no||1||automatically mount swap devices when they appear|
|auto_mount||boolean||no||1||automatically mount block devices when they appear|
|delay_root||integer||no||0||wait X seconds before trying to mount root devices on boot|
|check_fs||boolean||no||0||run e2fsck on device prior to a mount|
|device||string||no||-||The swap partition’s device node (e.g. sda1)|
|uuid||string||no||-||The swap partition’s UUID|
|label||string||no||-||The swap partition’s label (e.g. mkswap -L label /dev/sdb2)|
|device||string||no||-||The data partition’s device node (e.g. sda1)|
|uuid||string||no||-||The data partition’s UUID|
|target||string||no||-||The data partition’s mount point. Some values have special meanings, see the Extroot section below.|
|options||string||no||-||The data partition's mount options, e.g. noexec,noatime,nodiratim.|
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. This very rough estimate does apply for your embedded device.
Be aware that access time of swap is absymal if compared to real RAM, so having swap may not help much in your specific case.
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. 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.