phaq

For my company’s hard disk-based backup system I needed the ability to automount disk drives by their device name into a standard directory structure.

One possible approach would be to add some lines like these to fstab:

/dev/sda1       /mnt/sda1       ext3    defaults,noauto 0       0

This may be good enough in some cases, though it wasn’t sufficient for me, when there were dozens of device nodes which could get mounted eventually.

So I basically wanted something that would allow me to just access a directory, while the underlying disk was mounted automatically, then having it unmounted automatically if not in use, but still being dynamic in it’s nature so it would auto-adjust.

Now there’s a simple trick using an AutoFS feature called “executable maps”, which would allow me to achive this all.

The idea is, that all devices (let’s say /dev/sda1, /dev/sda2, /dev/sdb1, /dev/sdc1 as an example) will get mounted to /mnt/disks/[devicename].

First make sure, that AutoFS is installed. On Debian for example, it is installed like this:

apt-get install autofs

Then create a file called /etc/auto.disks with the following lines therein:

#!/bin/bash

# $1 is passed-over from automount
# key refers to the mount point we are looking for
key="$1"

# default mount options
opts="-fstype=ext3,rw"

# if a block device exists at /dev/[key]
# pass it back to automount
[ -b /dev/${key} ] && { echo "$opts \"; echo -e "t:/dev/${key}"; }

Don’t forget to chmod 755 /etc/auto.disks.

This script will create an automounter map dynamically as soon as it passed
a device node. It it finds it (e.g. while looking up /dev/sda1, which exists), it’ll
return the map to automount, which will cause the device node to be mounted.

In my case, the script didn’t need to be very sophisticated as I only have ext3-formatted disks, but it’s easy to script it for automatic file system recognition.

Btw, the script can be tested like this to see if it’s actually working:

satyr:~# bash /etc/auto.disks sda1
-fstype=ext3,rw
        :/dev/sda1
satyr:~# bash /etc/auto.disks sdx1

The first command returns the map for an existing device node /dev/sda1, while the second command returns nothing as /dev/sdx1 doesn’t exist on the system.

Now set AutoFS to use the executable map for /mnt/disks directory. Add this line to /etc/auto.master:

/mnt/disks  /etc/auto.disks --timeout=360

This will cause AutoFS to examine the executable map on all requested sub directories beneath /mnt/disks. So if you’re going to access /mnt/disks/sda1, /mnt/disks/sda2, /mnt/disks/sdb1, /mnt/disks/sdc1, the block devices corresponding to the directories are mounted automatically — as long as the devices exist of course.

The timeout value designates after how much time (of inactivity) an automounted file system expires and get’s unmounted.

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