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For single-user mode, init(8) can be configured to run a specific script (or directory). This script can provide a featureful or stripped-down single-user mode, at your choice. Different runlevels are supported in the same way. Whatever argument is passed to init(8) at the command line (boot prompt), it is appended to a configurable prefix and together specify the script (or directory) to run. Thus, you can pass in "single", "3", "6" or "multi" and all that is required is the appropiately named script/directory.<br>
<br>
There are two ways in which traditional runlevels can be supported. One is that an appropriate directory is created with symlinks back into /sbin/init.d/. A more elegant solution is to have a script for each runlevel, which would look something like this:<br>
<br>
#! /bin/sh<br>
# /sbin/init.d/runlevel.3<br>
<br>
case "$1" in<br>
start)<br>
need runlevel.2<br>
need portmap<br>
mount -vat nfs<br>
;;<br>
stop)<br>
umount -vat nfs<br>
;;<br>
esac<br>
# End<br>
<br>
Optimisations<br>
One possible optimisation of our scheme is that init(8) reads the /sbin/init.d/ directory in FS order and reads in each file into a dummy buffer, so as to populate the page cache. This will reduce the number of head movements, which I suspect is one of the causes of the slow bootup of RedHat. My handful of boot scripts start up much faster.<br>
Runlevels and rollback<br>
OK, the idea is that init(8)/need(8) keeps track of what boot scripts have been run, forever (well, until we turn the lights off on this kernel and boot again). At any time, you can ask need(8) to run another boot script, with full dependency checking. And the new script that was run is also recorded in the table.<br>
<br>
An orderly shutdown is as simple as rolling back the entire table. The algorithm is trivial: get the last entry in the table and run the appropriate stop script (which removes itself from the table). Repeat the process until the table is empty. All services have been stopped in the reverse order in which they were started.<br>
<br>
Increasing runlevel is easy: just run the desired runlevel script. So going from runlevel 2 to 3 involves running runlevel.3 under the dependency management scheme. Simple.<br>
<br>
Going from runlevel 3 to 2 is slightly more complicated, but not much. Just roll back, stopping each script/service in reverse order. As each is stopped, remove it's entry from the dependency table. Stop at "runlevel.2" (i.e. don't stop "runlevel.2", only the scripts listed after it).<br>
<br>
This scheme works because "runlevel.3" is added to the dependency table after it can pull in new dependencies (because it's added to the list once it completes). So once you've rolled back to "runlevel.2", you know you've stopped all the services "runlevel.3" has started, plus all the services it depended on, but not the services runlevel 2 depended on, or runlevel 2 itself. Magic.<br>
<br>
For switching between runlevels to work, the burden is placed on the runlevel scripts, not init(8), which is important IMO. Earlier I showed a sample implementation for "runlevel.3". It's important because it provides maximum flexibility in the construction of boot scripts and keeps init(8) simple.<br>
Multiple providers and provide(8)<br>
Sometimes, you have multiple service providers for the same generic service. For example, you might have sendmail(8) and qmail(8) installed on your system, and each has a boot script associated with it. Each one provides the "mta" service.<br>
<br>
In this case, you only want one of these script to be started. It might not matter which one is started, or perhaps each script may check some system-specific configuration to determine whether or not it should start the service. Either way, all scripts providing the generic service should be run, but only one should start the service.<br>
<br>
The solution to this is the provide(8) programme. It tells init(8) that the calling programme/script is able to provide the generic service. Init(8) then makes sure that only one provider will actually provide this service. An example script follows:<br>
<br>
#! /bin/sh<br>
# /sbin/init.d/sendmail<br>
<br>
case "$1" in<br>
start)<br>
if [ ! -f /etc/mail/sendmail.cf ]; then exit 2; fi<br>
provide mta || exit 2<br>
need portmap<br>
/usr/sbin/sendmail -bd -q15m<br>
;;<br>
stop)<br>
killall sendmail<br>
;;<br>
esac<br>
# End<br>
<br>
The need(8) implementation<br>
Originally, I had intended to put most of the intelligence into need(8) and have init(8) only maintain the database of scripts. When the time came to start cutting code, I realised that this was not the most effective approach, since there may be no IPC services available, apart from named FIFOs and signals. Supporting parallel full-duplex communications with one or two fixed FIFOs and signals is quite difficult.<br>
<br>
Script starting and stopping, as well as database management, is performed by init(8), and need(8) is a trivial programme which simply writes service requests to the FIFO and waits for a success/failure signal.<br>
<br>
Because the dependency table for init(8)-started processes is kept in init(8), it makes partial rollbacks (switching between runlevels) easier to implement. One nice thing that we can rely on is that init(8) never dies, and doesn't crash (if it does, we're rooted anyway). So keeping the table in VM is quite safe. And it's small anyway, so we don't have to worry about memory limitations.<br>
<br>
Note that need(8) and provide(8) are actually symbolic links to the initctl(8) programme.<br>
Wrapup<br>
I think that covers it. We've addressed the issues of ordering and modularity with a scheme that is simple but should give the full flexibility required.<br>
<br>
There are other issues we still have to deal with (such as personalities (a scheme I came up with in my old boot scripts for configuration control)), but that's orthogonal to the above issues.<br>
History<br>
Some people have queried how this project came to be, so I've jotted down this rambling account. If this interests you, read on, otherwise pass onto the next section.<br>
<br>
For years, Patrick and I have been bitching and moaning about the way Unix systems boot. "One day" we were going to figure out a better way. That day came on Saturday, 29th January 2000. After an afternoon checking out the new VW Beetle, we settled back and started plotting and arguing.<br>
<br>
One significant issue where we disagreed was in how dependency data were to be gathered. Patrick suggested parsing the scripts, which I didn't like. While it would have made some things easier, this approach would either be too limiting (it would not support conditional dependencies), or it would require writing a full parser. Writing a full parser would yield something almost as complex as bash. In addition, it would not support scripts in other shell languages, or binaries. In the end, I convinced Patrick a parser was not practical.<br>
<br>
That night, I sent off an email to Larry (with whom I'd sparred before, and he had previously expressed discontent with the way booting worked). Soon, he pointed out David had done something similar for his distribution (Mastodon linux). David had done something using a support script, which required a writable root FS. I considered this a significant limitation. Also, he hadn't gone far enough in my opinion, as he still had some large script orchestrating the boot process. Patrick and I both agreed that there should be no "special" or master script.<br>
<br>
After (too much) email discussion, I decided that Patrick and I were still on the right track, and went forth and coded. This resulted in the basic need(8) implementation and a set of sample boot scripts using this scheme. Parallel booting even worked.<br>
<br>
In March, Wichert Akkerman (Debian project leader) was in town for the linux conference and Expo, which was our chance to sell the idea to the Debian project. Wichert liked the idea (in fact he'd once tried something similar, but never got it fully working), but wanted the addition of the provide(8) feature. This required more work, and thus the project stalled, since on my return from Sydney I had a large backlog of work to deal with.<br>
<br>
Finally, in October, sitting on a plane headed for ALS, I had the time to get back to boot scripts, and started writing the provide(8) implementation. Shortly after my return from this trip I had finished this work, and finally had emptied out the ToDo list. Joy of joys!<br>
<br>
As a side note, I find it interesting that other people have considered a similar approach (I've talked to people at conferences and had resonses like "oh, yeah, I've thought about doing that"). It seems this idea was ripe for the plucking. Hopefully this will translate into community acceptance.<br>
Status<br>
All the described changes to simpleinit(8) have been made, and we're already using it for our init(8). I have finished the need(8) and provide(8) implementation and the basic rollback mechanism. shutdown(8) makes use of rollback support to give ordered shutdown.<br>
<br>
All these changes have been incorporated into util-linux-2.10q.<br>
Things to do:<br>
<br>
* I've considered keeping a full dependency history inside simpleinit(8) (right now it only keeps track of the currently depended-on service for each script). This would allow any service to be stopped and all services which depend on it to be stopped (dependent services would be stopped first, of course). This would be more flexible than either runlevels or rollback. In addition, a stopped service could still be recorded in the database and thus restarted with all the services that depended on it also being restarted. I have not yet determined whether these features would yield sufficient benefit to justify the implementation effort.<br>
<br>
A set of sample boot scripts are available as a gzipped tarfile. I consider these "production quality". They have been in use on workstations and servers for years, and perform very well. They are fast and reliable. If there are missing features (such as configurations or daemons that are not supported), please let me know and I'll update them.<br>
Go back to my: Home Page or linux Page.<br>
<br>
Richard Gooch (rgooch@atnf.csiro.au)<br>
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