Comment # 30 on bug 832501 from Marco M.

> 
> If I edit /etc/systemd/system.conf, uncomment
>  #DefaultTimeoutStartSec=90s
> 
> and change it to 300s, and run "mkinitrd" then again the boot succeeds, but
> without the need to set rd.retry=80
> 

This workaround has worked fine also for me. I did those two tests:

1) degraded raid1 device with root on it plus one data missing filesystem (a
simple partition with a file system on it, no raid involved): root filesystem
is mounted and the emergency shell appears.

2) degraded raid1 device with root on it, no other missing filesystem (there
was a missing filesystem, but i marked it with the "nofail" tag in /etc/fstab):
the system boots correctly in runlevel 5 ( in the systemd equivalent runlevel
5...)