pohmelfs – the network filesystem of the future !

Hello again fair readers !  Today we’re going to take a look at POHMELFS, which is a network file system that was just recently integrated into the Linux kernel.  This is an excellent exercise for three reasons : we’ll learn about some great new concepts related to network file systems, we get to compile and install our own kernel, and we get to play with a great new platform that could, eventually, become the new standard for network file systems in the *nix world.  Sound good ?  Let’s go !

A network file system is, according to Wikipedia, « any computer file system that supports sharing of files, printers and other resources as persistent storage over a computer network. » It is important to note that there is also a specific protocol called « Network File System », which is exactly what it sounds like, and is highly popular in the *nix world.  For the purposes of this article, unless otherwise stated every time i use « network file system » or « nfs », i’m referring to the concept, not the protocol.

Building on the idea of an nfs, there are other types of network-aware file systems which fall into similar categories, such as « distributed file systems », « parallel file systems », « distributed parallel file systems », and so forth.  These terms are largely non-standard, and conventional wisdom tends to put all of these sub-categories into the one big nfs family.  POHMELFS, for example, is described by its creator as a « distributed parallel internet filesystem » – in fact, the name itself is an acronym for « Parallel Optimized Host Message Exchange Layered File System », which is, mercifully, pronounced simply as « poh-mel ».

The two interesting portions of the name are « parallel » and «optmized »…


One of the interesting aspects of POHMELFS is that a given chunk of data can (and in many cases, should) reside in more than one distinct storage unit (different physical servers, for example).  The client can be made aware of this, which means that accesses to the data can happen in a « parallel » fashion, which has two major advantages :

  • Seamless real-time replication of the data during write operations
  • Faster overall data accesses during read operations

Imagine a scenario whereby you have two file servers : serverA and serverB, and any given number of clients (clientA, B, C, etc..).  In a classic non-parallel (and non-replicating) file system scenario, data would not be identical between the two file servers.  Clients would need to know which server had the data they wanted ahead of time, and if one of the servers went down, it would take all of its unique data with it.  Furthermore, if all of the clients wanted to read data from serverB at once, everybody would get bogged down overall, since there are a finite amount of resources available for everybody to use.

The same scenario with a parallel file system is much better indeed !  Firstly, data would be identical on the two file servers, meaning that if one were to go down, there would be no direct loss of data availability (already a huge improvement).  Secondly, and this is also huge, clients could spread their requests for data between the two servers, thus reducing the direct load on any one given server, and resulting in better overall resource usage (translation : better performance).


The author claims that POHMELFS is designed from the ground-up with performance in mind.  This design principle has resulted in some amazing benchmarks which, frankly stated, make more or less every other network file system look slow in comparison ; some types of data access processes are merely rapid, whereas others are revolutionary.  Of course, speed isn’t everything, and as you might expect, POHMELFS isn’t quite as feature rich (yet?) as many of the other nfs options on the market.

take it for a test drive

At the top of the article i mentioned that POHMELFS was recently introduced into the Linux kernel.  What i really meant was that as of kernel version 2.6.30, POHMELFS is located in the « staging » tree of the overall source collection.  This is already a little bit of a warning – code in the staging area is generally considered usable, but not ready to be merged into the kernel proper.  If this doesn’t sound like your bag, it’s time to bail out now, but for those of you in the mood for adventure, the staging tree is a great place to find interesting new features and functionalities.

In the test scenario we’re about to run through, there are two servers : « host_75 » and « host _166 ».  Both of these machines are very basic installations of Fedora 8, which from a server perspective is not very different from any other Fedora prior or since – therefore, unless otherwise noted, these operations should be identical on any other Fedora box.

kernel configuration

Long ago, in a faraway land where dragons battled wizards for supremacy over ancient battlements, the process of properly configuring, compiling, and installing a new Linux kernel was arcane knowledge – the stuff of legends.  In our modern, enlightened age, installing a new kernel on your machine is (relatively) simple !  Heck, you even get a menu these days…

The first step in compiling a new kernel is to make sure that your system has the necessary tools with which to work.  On a standard Fedora system, the fastest way is simply to install all of the development tools as a single mass operation.  This is overkill, really, but it’s simple, and since we’re just testing things out anyways, fast and simple are our primary criteria :

[root@host_75 ~]# yum groupinstall 'Development Tools'

The kernel menu i mentioned a few moments ago is going to require an additional package which is not part of the development tools group : « ncurses-devel ».

[root@host_75 ~]# yum install ncurses-devel

Next up, we need to download and unpack the kernel source package.  Your best bet is from kernel.org, which is reliable, rapid, and most importantly, secure :

[root@host_75 ~]# cd /usr/src
[root@host_75 src]# wget http://eu.kernel.org/pub/linux/kernel/v2.6/linux-2.6.30.tar.bz2
[root@host_75 src]# tar -xvjf linux-2.6.30.tar.bz2
[root@host_75 src]# ln -s linux-2.6.30 linux

Finally, we can take a look at the configuration menu.  You’ll notice that we issue the command « make », which is a mechanism for compiling source code used across the *nix world.  We’ll see it again a little later on, but for now, understand that all we’re doing here is «making » the configuration menu, not the kernel itself.

[root@host_75 src]# cd linux
[root@host_75 linux]# make menuconfig

Now there are a lot (a LOT) of possible options for configuring a kernel, and if you’ve got the time and the inclination, going through each one and reading the description can be very enlightening.  That said, what we’re interested in is POHMELFS, and in order to enable it, we’re going to have to explicitly tell the configuration that we’re interested in staging-level code.

First, enable « Staging Drivers » :

Device Drivers ---> [*] Staging Drivers

Then disable « Exclude Staging drivers from being built ».  It’s set up this way in order to prevent somebody from accidentally building anything from staging by accident :

Device Drivers ---> Staging Drivers ---> [ ] Exclude Staging drivers from being built

Next, enable POHMELFS as a module (if you’d like a refresher on modules, just check out any post on this blog with the « modules » tag) :

Device Drivers ---> Staging Drivers ---> <M> POHMELFS filesystem support

And, optionally, support for encryption :

Device Drivers ---> Staging Drivers ---> <M> POHMELFS filesystem support > [*] POHMELFS crypto support

Finally, you may wish to add a « local version string » – this is an identifier that you can customise to help you keep track of each kernel build.

General Setup ---> (-pohmelfs_test) Local version

Now we save and exit !

build & install the kernel

From here, all that’s left is to let it build – depending on your hardware, this can take a while.  Patience, grasshopper.

[root@host_75 linux]# make && make modules && make modules_install && make install

There are four distinct commands which, assuming the previous one is successful, will execute consecutively – that’s what the double-ampersand (&&) does.

  • make : Builds the actual kernel (this is the part that takes forever)
  • make modules : Builds the modules (everything enabled as <M>, such as POHMELFS)
  • make modules_install : Copies the modules to their proper positions
  • make install : Creates the initrd (which i discussed in a previous post), sets up the bootloader (which we’ll take a look at), and so forth

Once the process is done, which is to say that all four items executed successfully, the last thing we need to check before we reboot is the bootloader – in this case, « GRUB ».  The « make install » will add an entry for our new kernel into the GRUB configuration.  This is fairly automatic, but i like to check it, just to be safe.  The new entry should look something like this :

title Fedora (2.6.30-pohmelfs_test)
 root (hd0,0)
 kernel /vmlinuz-2.6.30-pohmelfs_test ro root=/dev/sda1
 initrd /initrd-2.6.30-pohmelfs_test.img

From here, we reboot, and when the GRUB menu appears, select our new POHMELFS item instead of the default entry.

userspace tools

The actual POHMELFS executables – the « userspace tools », so called since they are used by the user, not by the system, are not included with the kernel.  This is normal.  Even though our kernel now supports POHMELFS, our system doesn’t actually have any of the software which will interface with the kernel module yet.  This has to be downloaded, configured, and compiled in the same fashion as the kernel ; however, whereas the kernel was easily downloaded via HTTP, the POHMELFS source is only available via « GIT ».

GIT, in a nutshell, is a platform for managing source code (like « CVS », « Subversion », or « VSS », just to name a few).  For our purposes today, we’re only going to use one of its many features : copying the source code from the official POHMELFS site so that we can compile it ourselves.  If you don’t already have GIT installed, now is the time !  Don’t delay, act today !

[root@host_75 ~]# yum install git

Depending on your existing installation, this may cause a fairly large number of new packages to be downloaded and installed, so don’t worry if the list looks huge.  With that out of the way, we can download the source – this is known as « cloning » a « project » in GIT terminology :

[root@host_75 ~]# git clone http://www.ioremap.net/git/pohmelfs-server.git

Preparation of the source and so forth for POHMELFS was, not too long ago, a bit of a pain in the yoohoo.  Now, the author has graciously included a tool which will take most of the pain out of the process – though there are still a few things to look out for :

[root@host_75 ~]# cd pohmelfs-server
[root@host_75 pohmelfs-server]# ./autogen.sh

This will take a little while, and will output a handful of lines as it goes along.  The next step is to is a standard « ./configure », which, if you’ve never compiled anything before, is just about the most standard possible way to pre-configure source for compilation in the *nix world.  Normally, ./configure accepts a variety of options (take a look at « ./configure –help » for a taste), but for now, we’re only interested in one :

[root@host_75 pohmelfs-server]# ./configure --with-kdir-path=/usr/src/linux/drivers/staging/pohmelfs

The supplied option tells ./configure where the POHMELFS kernel code is – in specific, where the « netfs.h » file is located.  This is important later on, so take note.  This process output tonnes of lines as it checks for the capabilities and desires of your environment, and customises the configuration as best it can for your machine.  Once it’s done, we can go ahead and « make » :

[root@host_75 pohmelfs-server]# make

As of this writing, the above make may fail.  As it turns out, certain elements in the POHMELFS source, as they stand now, expect OpenSSL to be installed on the system.  This is true even if you were clever and provided « –disable-openssl » to ./configure above (good thinking, though !).  We’ve got two options here : either modify the POHMELFS source in order to remove the references to things which do not exist, or simply install OpenSSL and be done with it.  If you’ve already got OpenSSL on your system, then no worries, you probably didn’t even see this problem.

OpenSSL, briefly, is an open-source implementation of a series of encryption protocols and cryptographic algorithms which, among other things, allow for « secure » websites (i.e. via HTTPS), and other such things.  As such, it’s a pretty standard thing to have on a machine (especially a server), and since it’s so easy to install in our test scenario, we’ll just go ahead and do that now :

[root@host_75 pohmelfs-server]# yum install openssl openssl-devel

Back to POHMELFS, it’s time to reconfigure.  We’ll enable openssl now, since we’ve got it anyways…

[root@host_75 pohmelfs-server]# ./configure --with-kdir-path=/usr/src/linux/drivers/staging/pohmelfs --enable-openssl
[root@host_75 pohmelfs-server]# make

As of this writing, the above make will fail (again, possibly).  In this instance, a required file can’t be located : netfs.h .  Remember that one from above ?  Of course you do.  As it turns out, even though we explicitly specified the path to find this file, certain elements in the source (possibly auto-generated) expect it to be elsewhere.  As with OpenSSL, we have two options : alter the code ourselves, or just satisfy the requirement as painlessly as possible.  Well, you already know how we roll in these parts :

[root@host_75 pohmelfs-server]# mkdir /usr/src/linux/drivers/staging/pohmelfs/fs
[root@host_75 pohmelfs-server]# ln -s /usr/src/linux/drivers/staging/pohmelfs /usr/src/linux/drivers/staging/pohmelfs/fs/pohmelfs

What we’ve done here is create a link that points from where the source wants netfs.h to be, to where it actually is.  Hey, it’s staging-level code, remember ?  No worries – this was an easy one anyways.  With that out of the way, we can make away !

[root@host_75 pohmelfs-server]# make
[root@host_75 pohmelfs-server]# make install

The make install will put the necessary binaries in the appropriate places on the system.  In particular :

[root@host_75 ~]# which fserver cfg

and again !

That’s one server down, one to go.  But, wait, that was a lot of work, and even more waiting, wasn’t it ?  Doing it again would suck ; luckily, there are some shortcuts we can take.

If the hardware of both machines is more or less the same, there’s a better-than-average chance that the same kernel you’ve already compiled will work on the other server – you can just port it over.  Now, there are very particular, very clean ways to go about this, and to those that like their test environments clean and tidy, i salute you ; we, however, know better.  Let’s just pack up the source, copy it over, and deploy it all at once :

[root@host_75 ~] cd /usr/src/
[root@host_75 src] tar -cvzf src.tar.gz linux
[root@host_75 src] scp src.tar.gz root@host_166:/usr/src/

From here on in you’ll want to keep an eye on the hostname being used – we’re dealing with two machines now…

[root@host_166 ~] cd /usr/src/
[root@host_166 src] tar -xvzf src.tar.gz
[root@host_166 src] cd linux
[root@host_166 src] make modules_install && make install

Nice !  Reboot the second machine now, and don’t forget to choose the new kernel from the GRUB menu.

Likewise, we don’t need to install GIT on the second machine – we’ll just do like we did with the kernel :

[root@host_75 ~]# tar -cvzf poh.tar.gz --exclude=.git pohmelfs-server/
[root@host_75 ~]# scp poh.tar.gz root@host_166:~/

Notice the « –exclude » line in the tar command ?  This is to prevent the GIT-specific stuff (which is substantial) from being archived, as it is not useful where we’re going.

[root@host_166 ~]# tar -xvzf poh.tar.gz
[root@host_166 ~]# cd pohmelfs-server/
[root@host_166 pohmelfs-server]# make install

testing time

The first thing we need to do is load the POHMELFS module, which was generated way back when we built the kernel :

[root@host_75 ~]# modprobe pohmelfs
[root@host_75 pohmelfs-server]# lsmod
Module                  Size  Used by
pohmelfs               59284  0

You will likely have a lot more items in this list – but one of them must be « pohmelfs ».  Before we start the server daemon, we’ll have to decide which directory we want to « export », which is to say which one we’d like to share on the network.  For now, let’s pick « /tmp », since it’s simple (and it’s the daemon default).  Let’s put a file in there so that we can check to see if our share is properly working :

[root@host_75 ~]# touch /tmp/POHTEST.TXT

Next, we start the server daemon.  For our first test, we’ll just launch the binary without any options – by default, the daemon will launch in a local console, export /tmp (as noted above), and bind the process to port 1025.  Eventually you may wish to change some or all of these defaults, but for now, we’ll keep it simple :

[root@host_75 ~]# fserver
Server is now listening at

The most basic test possible at this point is to telnet from the second machine to the first, just to see if we can connect on the port :

[root@host_166 ~]# telnet host_75 1025
Connected to
Escape character is '^]'.
telnet> quit
Connection closed.

This will create some output on the server console :

Accepted client
fserver_recv_data: size: 40, err: -104: Success [0].
Dropped thread 1 for client
Disconnected client, operations served: 0.
Dropping worker: 3086465936.

Looks good !  Let’s try a proper connection with the POHMELFS userspace tool : « cfg ».  The options we’ll pass to it are the most basic possible set :

  • « -A add » : Action is to add a new connection
  • « -a <address> » : Connect to this server
  • « -p <port> » : Connect on this port
[root@host_166 ~]# cfg -A add -a -p 1025
Timed out polling for ack
main: err: -1.

Uh oh !  What happened ?  The error message tells us that the client « timed out » (or waited too long) to receive an acknowledgement of the connection from the server.  But why ?  The answer, though simple, is not immediately obvious : we forgot to load the POHMELFS module on the second machine.  No worries, go ahead and do that now, and we’ll try again :

[root@host_166 ~]# modprobe pohmelfs
[root@host_166 ~]# cfg -A add -a -p 1025
[root@host_166 ~]#

In a stroke of user-friendliness to last the ages, a successful cfg execution will produce no output.  No news is good news, i suppose…

Alright, now that we’ve got the server up, and we’ve prepared the client for a connection, we’ll need to pick a spot to « mount » the remote share, then initiate the mount itself :

[root@host_166 ~]# mkdir pohtest
[root@host_166 ~]# mount -t pohmel -o idx=1 none pohtest/
mount: unknown filesystem type 'pohmel'

Curses, foiled again !  Well, i was, at least – your mileage may vary on this one.  If you get this error instead of a successful mount, the problem is very likely that the « pohmel » file system type isn’t declared in all the proper places.  Check « proc » and the « filesystems » etc config :

[root@host_166 ~]# cat /proc/filesystems | grep poh
nodev    pohmel
[root@host_166 ~]# cat /etc/filesystems | grep poh
[root@host_166 ~]#

Ah ha !  Let’s rectify that little oversight and try again :

[root@host_166 ~]# echo "nodev pohmel" >> /etc/filesystems
[root@host_166 ~]# mount -t pohmel -o idx=1 none pohtest/

No output ?  Great success !  Let’s verify :

[root@host_166 ~]# df | grep poh
none                 154590376  10007348 144583028   7% /root/pohtest

The server console also confirms the connection :

fserver_root_capabilities: avail: 148053020672, used: 10247524352, export: 0, inodes: 39911424, flags: 2.
Accepted client

And, finally, we can see our test file :

[root@host_166 ~]# cd pohtest
[root@host_166 pohtest]# ls -l
total 0
-rw-r--r-- 1 root root 0 2009-06-16 15:39 POHTEST.TXT

Closing the connection cleanly is as simple as umounting :

[root@host_166 ~]# umount pohtest

This is confirmed on the server console :

Dropped thread 1 for client
Disconnected client, operations served: 1.
Dropping worker: 3086400400.

that’s a wrap, for now

Now i know what you’re thinking : where’s the parallel storage ?  Where’s the real-time mirroring and all that fun stuff ?  It’s coming.  For now, we’re just getting our feet wet with technology.  As time and testing permits, i’ll post more about POHMELFS – so stay tuned !

UPDATE : Check out the next instalment in the series ! http://www.dark.ca/2009/07/06/pohmelfs-pt-2-return-of-pohmelfs/

Last but not least, be sure to check out « fserver -h » for such useful features as « fork to background » and « logfile » – both of which, i guaruntee, you’ll want to look into if you intend to play around anymore with POHMELFS.

Finally, remember that while the code is very mature for staging, it’s still considered highly experimental.  Good luck, and happy hacking !

initrd, modules, and tools

An explanation of initrd, how modules live within it, and an example of how it can be modified on a Fedora-based system. Contains supplementary information about gzip, tar, and cpio.

Today i thought i’d discuss initrd, how modules live within it, and an give a generic example of how it can be modified on a Fedora-based system.  Along the way, i’ll provide supplementary information about gzip, tar, and cpio.  Let’s begin !


The term « initrd » is short for « initial ramdisk ».  It refers to a file of the same name used by Linux systems during the boot process in order to load certain modules (drivers, and the like) prior to the full system being brought online.  An example usage of initrd would be to load a module for a RAID card, so that the actual filesystem – and all of the delicious data contained within – can be accessed properly.  As with most everything, the related Wikipedia entry has more general information on the subject.

There are (perhaps unfortunately) a number of ways in which initrds can be constructed and implemented.  Generally speaking, the actual file is a compressed archive, which may contain further archives, filesystems, configuration files, and so forth.  Over the years, different distributions have handled the initrd in different ways, such that one distro’s method of constructing an initrd may be totally incompatible with that of another.  In fact, even within a single distribution, different releases may alter the internals of the initrd – Fedora being a recent popular example of this.


In the earlier days of Fedora, the initrd was a compressed gzip archive that contained a « squashfs » file, which was itself a compressed filesystem.  If you wanted to modify the contents of the initrd, all you had to do was un-gzip the initial file, then mount the squashed filesystem within. If some or all of these terms are fuzzy for you, don’t despair – i’ll explain everything as we go along.  The important thing you need to know now is that in Fedora 8, the nature of the initrd changed significantly.

In order to work with the current Fedora initrd, you need to be familiar with a few terms, their related technologies, and their usage…


A « ramdisk » is, as the name implies, a sort of filesystem which has been loaded into RAM.  Although it does not physically exist, it appears for all intents and purposes to be a normal disk, which has the advantage of being seamless to the user (and to the system).  Ramdisks are handy because, compared to physical disks, they are fast.  Within the context of the initrd, however, speed isn’t a concern – it’s the versatility of the ramdisk that is key.  As in the module example i mentioned initially, the various bits and peices of hardware in a given computer often require special drivers to make them function properly.  These drivers can be built directly into the kernel, but often, for reasons of ease of use and cross-compatibility, it’s easier to put them into the initrd, and then load them via the initial ramdisk that gets built when the machine boots.

modules, in general

The basic idea behind a « loadable kernel module » (or just module for short) is, again, to add support for hardware, special filesystems, and the like, which the kernel doesn’t natively understand.  Modules can be loaded at any time, but for the most part, they are initialised at boot time, so that the kernel can interact with things like RAID, audio, or video cards, non-Linux filesystems (NTFS, for example), or even more esoteric items like robotic controllers and (the dreaded) software modems.

As a general rule, modules themselves are single files which are referenced via one or more configuration files.  We’ll take a closer look at these ideas below.

gzip (and tar, too !)

Gzip is, along with « tar », arguably the most ubiquitous archival format pairing in the Linux world.  Generally speaking, a gzip archive is a single file which has been compressed using the gzip binary.  Commonly, gzip is used to compress a « tar » archive, which normally does not use compression, but which can contain any number of files in a single archive.  Together, they are used to create « .tar.gz » or « .tgz » files, which are more or less the standard way to create compressed archives in Linux ; together, they’re like « zip » or « rar » (or « arj », or « boo ») in the Windows world.  (Wikipedia)

Within the context of the Fedora initrd, however, tar is not used – from the pair, only gzip is important.


It is a rare and glorious thing when ancient technology is updated and used constantly from day one.  Take the lightbulb, for example.  Or fire.  Or « cpio » .  It’s basically like tar (mentioned above), in that it can be used to take any number of given files, and create a single archival file containing said files.  Like tar, compression isn’t part of the deal, and gzip can be used to compress a cpio archive as normal.

While cpio has a long and storied history (it was first specified way back in 1977), over the years, it has increasingly replaced by tar in many implementations.  Some time ago, Red Hat decided to use cpio (instead of tar) for their ultra-popular packaging format « RPM ».  This sparked a renewed interest in cpio in many communities, including, as you may have already guessed, the Fedora team – it’s now employed regularly in the initrd (among other things).  (Wikipedia)

Ok, lets go !

The first thing we’ll need to do is copy the initrd to somewhere handy, like « /tmp », so that we can play with a copy .  Never play with the original – it should be preserved somewhere in case our modifications cause the file to become unusable.  The initrd will be named « initrd.img » – but what is it, exactly ?  Let’s find out :

$ cp <original_initrd.img> /tmp/
$ cd /tmp
$ file initrd.img
initrd.img: gzip compressed data, from Unix, last modified: Fri Nov  2 15:54:12 2007, max compression

Ah, it’s our old friend, gzip.  Since we know that gzip is a file compression tool, we know that initrd.img contains a file – let’s extract it :

$ gzip -d initrd.img
gzip: initrd.img: unknown suffix -- ignored

Uh oh ! By default, gzip doesn’t handle files with suffixes it doesn’t understand (i.e. « .gz » ).  The easiest way to deal with this is simply to add the appropriate suffix, then try again :

$ mv initrd.img initrd.img.gz
$ gzip -d initrd.img.gz

This gives us a single file (as we expected) : initrd.img .  Though it has the same name as the file we started with, it’s not the same data at all.  This initrd.img is larger than the original, and most importantly, it’s a different type of file :

$ file initrd.img
initrd.img: ASCII cpio archive (SVR4 with no CRC)

Ah ha ! It’s a cpio archive – the first of two we’ll encounter during this process.  Note the trailing information, that it’s an « SVR4 » archive, as this is important for the next step : unarchiving.

Unpacking the initial cpio archive

As i mentioned above, cpio has a long and storied history, which is reflected in the myriad of arguments and options that can be provided during normal usage.  The command we’ll be using at this stage will be :

$ cpio -i -d -H newc -F <file> --no-absolute-filenames

The arguments are, in order :

  • -i : « copy-in » is more or less like saying « extract » in the parlance of cpio.
  • -d : « make directories » allows cpio to re-create the directory structure in the archive, instead of just unpacking everything to the same place.
  • -H newc : « format type » defines the particular format used by this cpio archive.  Over the course of cpio’s history, numerous formats have been used for various reasons : « newc » is an SVR4-based format that allows for large files and filesystems.  Remember the archive type from above ?  This is why we needed to make note of it.
  • -F <file> : « file filename » simply indicates the file we want to work with.
  • –no-absolute-filenames : as the name suggests, this forces cpio to unpack the contents relative to where we executed the command.  Basically speaking, it prevents the possibility of cpio trying to write to our root filesystem, which could have disastrous results.

For more information about cpio, simply consult the GNU cpio manual.

First we must make a directory in which to unpack the archive, then we can proceed :

$ mkdir /tmp/initrd
$ cd /tmp/initrd
$ cpio -i -d -H newc -F ../initrd.img --no-absolute-filenames
16442 blocks

The number of « blocks » refers to the amount of data that was unpacked – it is a measurement, like bytes.  This value may be different for you, as not all initrd’s are created equal.

the goods

A simple « ls » reveals the contents of the initrd.  Astute readers will notice that it looks very familiar – this is not a coincidence – it looks like the root of a normal filesystem :

$ ls -l
total 36
lrwxrwxrwx 1 dan dan    4 2009-06-10 16:37 bin -> sbin
drwxrwxr-x 2 dan dan 4096 2009-06-10 16:37 dev
drwxrwxr-x 3 dan dan 4096 2009-06-10 16:37 etc
lrwxrwxrwx 1 dan dan   10 2009-06-10 16:37 init -> /sbin/init
drwxrwxr-x 2 dan dan 4096 2009-06-10 16:37 modules
drwxrwxr-x 2 dan dan 4096 2009-06-10 16:37 proc
drwxrwxr-x 2 dan dan 4096 2009-06-10 16:37 sbin
drwxrwxr-x 2 dan dan 4096 2009-06-10 16:37 selinux
drwxrwxr-x 2 dan dan 4096 2009-06-10 16:37 sys
drwxrwxr-x 2 dan dan 4096 2009-06-10 16:37 tmp
drwxrwxr-x 6 dan dan 4096 2009-06-10 16:37 var

The directory we’re interested in is « modules » :

$ cd modules
$ ls -l
total 5128
-rw-r--r-- 1 dan dan   12904 2009-06-10 16:37 module-info
-rw-r--r-- 1 dan dan  137235 2009-06-10 16:37 modules.alias
-rw-r--r-- 1 dan dan 4971479 2009-06-10 16:37 modules.cgz
-rw-r--r-- 1 dan dan   37966 2009-06-10 16:37 modules.dep
-rw-r--r-- 1 dan dan   60204 2009-06-10 16:37 pci.ids

Aah, delicious modules, at last ! But wait – what’s that « .cgz » file ?

$ file modules.cgz
modules.cgz: gzip compressed data, from Unix, last modified: Fri Nov  2 15:54:07 2007, max compression

Another gzip – but the suffix gives us another hint.  Just as a .tgz is a gzip’d tar archive, a .cgz indicates a gzip’d cpio archive.  Of course, as before, we’ll need to rename this file in order to work with it :

$ cp modules.cgz /tmp/modules.gz
$ cd /tmp
$ gzip -d modules.gz
$ file modules
modules: ASCII cpio archive (SVR4 with CRC)

As before, we must create a working directory into which we’ll unpack the cpio archive.  Be sure to take note of the cpio arguments – there’s been a change :

$ mkdir /tmp/work
$ cd /tmp/work
$ cpio -i -d -H crc -F ../modules --no-absolute-filenames
25396 blocks

How did i know to use a different format ? Look again at the file output for modules above, then compare it to the first cpio archive we dealt with.  The difference is subtle, but critical – the cpio manual (linked above) lists them all, just in case.  Ok, let’s take a look at what we’ve got.

$ tree -d
    `-- i586

Here, the string « » refers to the kernel version that the initrd is built for, and the « i586 » refers to the architecture.  Yours may be different (i randomly selected one that was hanging around on my test system), and that’s OK, because the principles are still the same.  For now, you’ll note that the i586 directory is full of « .ko » files – these are the module files themselves.

adding the new module

The module that you want to add will be a .ko file as well.  You may have been supplied with this file directly, or perhaps you compiled it from source (which is outside of the scope of this document for today).  Regardless, as you may have guessed, you’ll want to put it along with all the other modules :

$ cp /tmp/new_module/network_card.ko /tmp/work/

With that in place, we’ll re-pack cpio archive, so that it can be inserted back into the initrd.  Through the magic of re-direction, we’ll accomplish this all in one swift command :

$ cd /tmp/work
$ find | cpio -o -H crc | gzip > /tmp/modules.cgz
25590 blocks

Whoah ! Let’s break that down :

  • « find » was used to create a list of all the files with their relative directories.  Run « find <dir> » by itself to get a better idea of what this means.
  • That list was then piped to cpio, where « -o » indicated that a new archive should be created (« copy-out » in the cpio parlance).
  • That was then directly passed to gzip, which ultimately outputted the gzip’d cpio file « modules.cgz » .  Simple !

Now we can copy this shiny new modules.cgz into the initrd tree :

$ cp /tmp/modules.cgz /tmp/initrd/modules/
cp: overwrite `/tmp/initrd/modules/modules.cgz'? y

At this point, there’s a very good chance that you’ll need to modify a handful more items related to modules – in particular, the other files in the modules directory :

$ cd /tmp/initrd/modules
$ ls -l
total 5228
-rw-r--r-- 1 dan dan   12904 2009-06-10 16:37 module-info
-rw-r--r-- 1 dan dan  137235 2009-06-10 16:37 modules.alias
-rw-r--r-- 1 dan dan 5074769 2009-06-10 17:35 modules.cgz
-rw-r--r-- 1 dan dan   37966 2009-06-10 16:37 modules.dep
-rw-r--r-- 1 dan dan   60204 2009-06-10 16:37 pci.ids

There’s our new modules.cgz, as you can see from the timestamp difference, along with the other items from before.  Those other items are all text files which contain various bits and pieces of information related to the modules contained in modules.cgz .  As this is a generic sort of post, i can’t go into specifics about how each of these files would be modified for any actual module ; that said, a general overview of each of these items is most certainly in order :

  • « module-info » : This file contains a an alphabetical list of each of the modules, with two lines per module that describe the type of device the module affects, and a verbose (i.e. human-readable) description of the module.  It is the most simplistic of the four, and the one most easily read by a human.  Example :
        "Intel(R) PRO/1000 Network Driver"

In this case, the module is named « e1000» , it affects Ethernet ( « eth » ) devices, and is an Intel gigabit NIC driver.

  • « modules.alias » : Under normal usage, hardware will expose certain information to the kernel that helps the kernel to properly identify and assign said hardware.  This is done via a sort of identification string, which contains (in machine-readable format) such things as the bus used by the hardware, the vendor and device id’s, and other such sundry items.  Normally, the system takes care of populating this file on its own, but in the case of the initrd, this doesn’t happen.  As a result, it is highly likely that you’ll need to insert the identification string for your hardware manually ; hopefully, your module came with some documentation to help with this.
    • If your module didn’t come with the appropriate documentation, you could always try putting the hardware into an already-installed system, and then poking around for it manually.  More information is available from the ArchLinux wiki.
    • This file, since it’s normally generated automatically, and used behind the scenes, is the most daunting of the four.  Once you get a handle on how the string is built, however, it stops being scary, and starts being merely irritating.  I’m not going to lie – this file is tough.  Google is your friend on this one !
    • Example :
alias pci:v00008086d0000108Bsv*sd*bc*sc*i* e1000
  • « modules.dep » : This lists the « dependencies » for each module – effectively, it’s a list of what modules the other modules depend on.  Sometimes modules are designed to operate completely independently (such as the above-noted e1000 module), while other times they build on each other, with each module providing certain functionalities used by the next.  This file builds those relationships.  Example :
msdos: fat
  • « pci.ids » : Finally, can you guess what’s listed in this file ?  If you guessed « PCI IDs », you’re right !  This file is in the same genre as modules.alias, in that it contains a list of hardware identifiers that are used by the kernel in order to properly assign drivers and so forth.  Again, as with modules.alias, this data is not necessarily obvious, and your hardware documentation should list the appropriate identifiers.
    • In the event that your documentation doesn’t have this information, there are two excellent sites that contain information on known PCI IDs : the PCI ID Repository, and the PCI Vendor and Device List.  Not exactly bedtime reading, i know, but they’re both life-savers !
    • Example :
101e  American Megatrends Inc.
        1960  MegaRAID
        9010  MegaRAID 428 Ultra RAID Controller
        9060  MegaRAID 434 Ultra GT RAID Controller

Once these files have been properly updated, the only step that remains is re-packaging everything into a proper initrd.img !

new initrd.img

Here in the final step we’ll leverage the power of re-direction and create our initrd.img in one fell swoop :

$ cd /tmp/initrd
$ find . | cpio -o -H newc | gzip > /tmp/initrd.img
$ file /tmp/initrd.img
/tmp/initrd.img: gzip compressed data, from Unix, last modified: Wed Jun 10 19:39:51 2009

And there you have it, a shiny new initrd.img, ready for use in your PXE bootstrap, disk image, or wherever else such things are needed.

Aren’t sure what a PXE bootstrap is ?  Not sure why you’d want to use a disk image ?  Stay tuned, fair reader, all will be revealed as we delve deeping into the mysterious land of the Linux System Administrator…