The /etc/config/fstab configuration file defines static file systems and swap partitions that should be activated at boot. Please read the Notes before proceeding.

The package block-mount is required:

1. Install

   opkg update
   opkg install block-mount

2. enable (make it start automatically on every boot up)

   /etc/init.d/fstab enable

3. and start (right now)

   /etc/init.d/fstab start

Whenever you change your configuration, restart this to make it take effect:

/etc/init.d/fstab restart

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:

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:

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. mount section can not be renamed by user (such as "storage" instead of "mount").

This example is included by default:

The mount section contains these settings:

Name	Type	Required	Default	Description
device	string	yes or uuid or label	(none)	Device (partition) to mount the filesystem from. If uuid or label are specified, they are used, rather than device.
uuid	string	yes or device or label	(none)	UUID of device (partition) to mount the filesystem from, as shown by blkid. If uuid is present in the mount section, it is used in preference to label or device.
label	string	yes or device or uuid	(none)	LABEL of device (partition) to mount the filesystem from, as shown by blkid. If uuid is present in the mount section, it takes precedence. label takes precedence over device.
enabled	boolean	no	1	Whether to mount this filesystem automatically at boot.
fstype	string	no	auto	Type of the filesystem (i.e. ext3). remote file systems
options	string	no	rw	Mount options for this filesystem.
target	string	yes	(none)	Target directory to mount the filesystem onto. After trunk r25787 specifying '/overlay' here means this filesystem will be mounted as the overlay-based rootfs, while after trunk r26109 specifying '/' here means the filesystem will be mounted as regular rootfs (that is not overlay-based) (see extroot). Neither '/' nor '/overlay' aplies to Backfire.
enabled_fsck	boolean	no	0	Whether to automatically check for/repair errors before mounting the filesystem.
is_rootfs	boolean	no	0	»Please read the particular article extroot for this functionality!« deprecated after trunk r25787, still needed in Backfire. Before r25787 it is required for block-extroot mounts

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!
Swap partitions must be first defined using the mkswap /dev/device command!

This example is included by default:

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:

root@OpenWrt:~# sysctl -w vm.swappiness=60
root@OpenWrt:~# 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:

root@OpenWrt:~# 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:

root@OpenWrt:~# 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 of RAM may be used here, the//
tmpfs                      512         0       512   0% /dev         //tmpfs partitions grow and shrink with usage!//
/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 extroot for that.

If you want to store large amount of data in your /tmp folder you may hit size limit of tmpfs partitions, which is by default 50 % of your RAM. Because your RAM size is limited and probably better utilized by applications, you want the files in /tmp to be moved to swap as soon as applications need more RAM for themself.

1. We will remove the limit of 50 % of RAM. You can set size parameter to the size of your swap.
   • temporarily:

     mount -t tmpfs -o remount,rw,nosuid,nodev,noatime,size=256M tmpfs /tmp

   • permanently in /etc/config/fstab:

     config 'mount'
       option 'target' '/tmp'
       option 'device' 'tmpfs'
       option 'fstype' 'tmpfs'
       option 'options' 'remount,rw,nosuid,nodev,noatime,size=256M'
       option 'enabled_fsck' '0'
       option 'enabled' '1'
     

2. And we will set /tmp files to be more likely swapped than applications in RAM.
   • temporarily:

     echo 5 > /proc/sys/vm/swappiness

   • permanently:

     sysctl -w vm.swappiness=5

You can revert the changes by:

mount -t tmpfs -o remount,rw,nosuid,nodev,noatime,size=50% tmpfs /tmp

echo 60 > /proc/sys/vm/swappiness

sysctl -w vm.swappiness=60

I installed the packages, enabled the fstab, and changed my fstab config file. When I restart, the drive does NOT show up. I can issue an /etc/init.d/fstab start and I get an error that it can't find a file (/tmp/fstab) but the drive mounts. I took the advice from this posting https://forum.openwrt.org/viewtopic.php?id=27210 and put "sleep 15" in the start function of the /etc/init.d/fstab file. (I'm using a USB stick) That fixed the problem.

https://forum.openwrt.org/viewtopic.php?id=28311

Symptoms: You can mount your partition with mount -t ext4 /dev/sda1 /mnt -o rw,sync, but fstab won't mount it automatically on reboot. You are using Backfire or older version.

Solution: Despite you have installed block-mount try to install block-hotplug separately:

opkg update
opkg install block-hotplug

Note: This may or may not apply also to block-extroot. This problem should not occur in Attitude Adjustment.

• remove mount, umount provided by BusyBox

rm /bin/mount
rm /bin/umount

• install mount-utils

opkg update
opkg install mount-utils

• find UUID or Label

blkid /dev/sda1

• mount a disk

mount LABEL=xxx /mnt
mount UUID=xxx /mnt