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What:
Flashybrid is a system to help in setting up and managing hybid
flash/disk/ram based Debian systems which can run most of the time
using only a small flash disk for their root filesystem and do a useful,
but limited task (such as being a router, or a PDA, or a rescue system
on a USB keydrive). The flash can be as small as 32 mb, though 64 to 256
is more comfortable.
When such a system needs to be upgraded or managed, or if you need a
full-fledged Debian system temporarily, a flashybrid system converts
quickly from an embedded system into a larger, normal Debian system.
Flashybrid supports systems with a supplimental hard disk that holds the
parts of Debian that cannot fit on flash. It can also support larger
flash-based systems with no supplimental hard disk. It makes it easy to
mount a flash disk read-only, to avoid excessive writes to the flash, by
storing the writable parts in a ram disk and rsyncing them back to the
flash at shutdown.
Flashybrid needs rsync, and the 2.4 or greater linux kernel with tmpfs
support built in.
Why:
I developed this system for my little OpenBrick computer, which I use as a
wireless access point / dialup firewall most of the time, in fairly extreme
environmental conditions where I do not want a hard disk to be running, but
which I occasionally use for long overnight downloads, or hook up a monitor
and use as a desktop machine, and which I want to be able to manage as a
regular Debian system for easy access to upgrades and security fixes. My
OpenBrick originally had 32 mb of flash, 128 mb of ram, and a 1 gb hard
disk, and now sports 256 mb of flash and 10 gb of disk.
How:
A flashybrid system boots from its flash, and has all the necessary
libraries and programs on the flash to bring up whatever daemons are
needed for it to perform its embedded functions.
Care is taken to keep the flash mounted read-only to prevent excessive
writes to the flash which could wear it out. Directories such as /var/log
which need to be writable are stored on a ram disk, and have their
contents stored back to the flash on reboot.
To turn the embedded system into a full Debian system, a hard disk is
optionally mounted, which contains the bits of the Debian system that
didn't fit on the flash.
Some directories like /usr/bin may be too large to fit on a flash, and yet
your system might need a few files from the directories on the flash.
Flashybrid includes support for such directories; a directory is set up on
the flash containing parts of the directory, and is bind mounted into
place. When the hard disk is mounted, the bind mount is changed to mount
the complete directory into place.
Flashybrid integrates with apt, so when you run apt it makes sure the
full system is available, and after apt runs it updates the
abovementioned directories.
Flashybrid is configured by files in /etc/flashybrid. The fh-full and
fh-embed commands make it easy to transition a system between the two
states. /etc/init.d/flashybrid init script is run at system boot and
shutdown. The fh-update program updates a flashybrid system after apt
is run, or you can run it by hand after making manual changes.
For details on setting up a flashybrid system, see INSTALL.
Who:
Flashybrid was created by Joey Hess <joey@kitenet.net>
When:
I originally wrote flashybrid in 2002, and used it successfully for a year
with a embedded Debian system with only 32 mb of flash. Then I bought a
much larger 256 mb flash, and armed with a year's experience and less
I rewrote flashybrid to make it easier to install, more robust, and finally
released it to the public in fall 2003.
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