[svlug] Inits (was: On the process of picking up systemd)

Akkana Peck akkana at shallowsky.com
Sat Jan 17 10:04:46 PST 2015


Rick Moen writes:
> Steve Litt wrote:
> 
> > You have me at a disadvantage because I don't think I've ever seen a
> > system boot indeterminately, or at least not in the last 10 years. I've
> > never seen eth0 and eth1 switch, and the machine I've been doing all
> > this testing on has both.
> 
> Anyway, truth to tell, I'm likewise at a disadvantage in attempting to
> intelligently discuss this problem, being both late to the party and having
> through either luck or careful avoidance of problem scenarios not seen the
> referenced problems.

Things may have changed -- this is based on problems I used to hit
five or so years ago -- but here are two cases that used to cause
indeterminacy:

1. Boot with two or more USB disks plugged in. Then add /etc/fstab
entries that try to mount partitions on those disks using /dev/sdb1,
/dev/sdc2 etc. rather than using SSIDs or labels. Sometimes it'll
work, sometimes you'll get something entirely wrong.

2. Multiple network cards. For instance, try booting a laptop that
has built-in wi-fi but also a USB wi-fi dongle; or try plugging in
two different USB wi-fi dongles. But -- here's the kicker -- first
remove the udev entries that most modern distros add as workarounds
to get around the indeterminacy. For instance, on Debian, remove
both of these files:
/etc/udev/rules.d/70-persistent-net.rules
/lib/udev/rules.d/75-persistent-net-generator.rules 

Those persistent net rules are merely workarounds, and cause other
problems. If you add a new network card, your old connections may
stop working because they're all set up to use wlan0 and now your
network interface is named wlan1.

Now, a new init system isn't the only, or necessarily the best, fix
for these problems; but at least they're relatively common situations
(which I've hit myself, and had to figure out how to fix) where an
ordinary user can get race conditions at boot time. I'm sure there
are other, thornier problems where a dependency-based init really is
the best fix.

        ...Akkana



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