Apr 032014

What is this about?

How to set up a Raspbian system as a read-only system in a couple of easy steps.


This penguin has been doing a lot of reading as of lately.

Why is this of interest to me?

The filesystem on the Raspberry Pi’s SD card, as every filesystem on every computer, can be corrupted if you disconnect the power suply.

Once you make the filesystem read-only (all changes go to a RAM-Disk), you will be able to disconnect the power supply whenever you want.

An additional bonus is less wear on the SD card – sometimes you do NOT want the logfiles which a standard Linux / Raspbian will write to anyway. Our solution will write those to a RAM-Disk and discard them on reboot / power cycling.

Last but not least, you can set up a known state to boot into, experiment, and if things go wrong, simply reboot.

How-To make Raspbian read-only

all commands to be run as root (sudo su) if not specified otherwise.

This how-to is based on another how-to I found on the Internet, in German. Thanks guys!

Disable swap

dphys-swapfile swapoff
dphys-swapfile uninstall
update-rc.d dphys-swapfile disable

Install UnionFS

UnionFS is an overlaying filesystem (you can mount from several different locations into the same folder). In case of collisions, UnionFS uses priorities for the filesystems. It is frequently used to create RAM-Disk Overlays for read-only systems, for instance also with Live CDs.

aptitude install unionfs-fuse

Create mount script

nano /usr/local/bin/mount_unionfs

Add the following content to this file:

 ROOT_MOUNT=$(awk '$2=="/" {print substr($4,1,2)}' < /etc/fstab)
 if [ $ROOT_MOUNT = "rw" ]
   /bin/mount --bind ${DIR}_org ${DIR}
   /bin/mount -t tmpfs ramdisk ${DIR}_rw
   /usr/bin/unionfs-fuse -o cow,allow_other,suid,dev,nonempty ${DIR}_rw=RW:${DIR}_org=RO ${DIR}

make it executable:

 chmod +x /usr/local/bin/mount_unionfs

Update /etc/fstab:

nano /etc/fstab

Modify the file to read (adjust partitions for NOOBS!) as follows:

proc            /proc           proc    defaults          0       0
/dev/mmcblk0p1  /boot           vfat    ro                0       2
/dev/mmcblk0p2  /               ext4    ro,noatime        0       1
mount_unionfs   /etc            fuse    defaults          0       0
mount_unionfs   /var            fuse    defaults          0       0
none            /tmp            tmpfs   defaults          0       0

Prepare the directories

cp -al /etc /etc_org
mv /var /var_org
mkdir /etc_rw
mkdir /var /var_rw

At this point your Raspberry Pi should be read-only already. You can verify this with


This command should show something like the following:

/dev/root on / type ext4 (ro,noatime,data=ordered)
devtmpfs on /dev type devtmpfs (rw,relatime,size=215824k,nr_inodes=53956,mode=755)
tmpfs on /run type tmpfs (rw,nosuid,noexec,relatime,size=44820k,mode=755)
tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /run/shm type tmpfs (rw,nosuid,nodev,noexec,relatime,size=89620k)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620)
/dev/mmcblk0p1 on /boot type vfat (ro,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=ascii,shortname=mixed,errors=remount-ro)
ramdisk on /etc_rw type tmpfs (rw,relatime)
unionfs-fuse on /etc type fuse.unionfs-fuse (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other)
fusectl on /sys/fs/fuse/connections type fusectl (rw,relatime)
ramdisk on /var_rw type tmpfs (rw,relatime)
unionfs-fuse on /var type fuse.unionfs-fuse (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other)
none on /tmp type tmpfs (rw,relatime)

This indicates to you that the filesystems are mounted read-only and unionfs is active.

Clean up the Log directory

mount -o remount,rw /
for f in $(find . -name \*log); do > $f; done
cd /var_org/log
rm -f *.gz


Another reboot and you’re done!


How-To Install additional Packages after “read-only-modding”

You’ve got to remount the root filesystem as readable. For instance, as root, you can do:

mount -o remount,rw /
aptitude update
aptitude install stress
Dec 072012

As has frequently been written elsewhere, the main source of problems for the Raspberry Pi systems is the power supply. I would even go as far as saying: before debugging anything else, measure the voltage!

On the Raspberry PI two test points are located. TP1 and TP2. With a voltmeter you can measure the voltage between them. According to the Raspberry Pi handbook by Eben Umpton (page 49) it should be between 4,8 V and 5 V.

I am testing several different power supply units (mostly USB chargers …), and will be offering only know good units in the shop. By buying a known good power unit you can avoid the following

Problems / symptoms of low voltage / weak power supplies

  • corrupted SDHC cards (even quality cards will get corrupted / dismounted – eventually the filesystem will not be able to restore itself)
  • frequent USB connects and disconnects
  • some distributions, i.e. Raspbmc / OpenELEC not working correctly / not recognizing mouse and keyboard, while others (Raspbian Wheezy) will work just fine …
  • keyboard (on a powered hub!) working, while mouse (on the same hub) does NOT work

Causes of low voltage

  • weak power supply
  • power supply requires negotiation for more than 500 mA, which the Raspberry PI does not do (Raspberry Pi handbook)
  • greedy USB peripherals attached to the pi, for example illuminated keyboard
  • using rechargeable batteries which are not freshly charged (I see 4,44 V with batteries which are giving 5 + V without load)

Even if your power supply states that it will provide up to 1 A, this will not necassarily be with 4,8 – 5 V – many supplies I’ve tried show this behaviour.

According to Gert van Loo, whom I met at the fair, the Raspberry Pi will take at maximum 1 – 1,5 A, depending on the ambient temperature, before the fuses start kicking in. As such, there will be no point in trying to attach a 4 A powersupply, for instance, in order to provide power to the USB ports.

Known good power supplies

The following adapters or compatible, thoroughly tested, models will be available in our shop:

goobay Model 42438 car adapter (12 V -> 5 V)

Provides between 4,94 V and 5,10 V in our tests. It has a micro USB plug.

D-Parts Mobilphon & Zubehör Gmbh (DP-LGS) Model AC0122-051000 (230 V -> 5 V)

Provides 4,93 V in idle mode. It has a micro USB plug.

Known “bad” power supplies

The power drawn from the Raspi in our tests has been measured at roughly 500 mA – well below the 1 A level specified by the vendors of the following adapters:

LogiLink USB Travel Charger Combo Kit PA0008A v2.0

This (white) mains power supply is quite unstable and NOT suitable to power the Raspberry Pi. It will provide about 4,3 – 4,6 V under load / in idle mode, at about 0,5 A of current being drawn. We’ve tested two supplies of this series. Another downside: the USB plug will not have a good, secure fit in it’s outlet.

We’ve also seen huge oscillations in the voltage with it – ranging from about 3,9 V to 5,1 V.

All in all, the Raspi will display the known symptoms (see above).

The car adapter (which converts 12 V to 5 V) also runs at 4,74 V with the Raspberry Pi in idle moad (no load).

The package states that it will provide 1 A. Maybe. But probably not at 5 V.


FRIWO FW7713 (230 V -> 5 V)

This adapter performs better than the LogiLink one, still voltage levels drop below the magical 4,8 V level. Under load we have measured 4,68 V / 4,62 V for instance. (With a nominal 460 mA load from the Raspi).

The USB plug has a more secure fit than in the Logilink counterpart (see above).

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