Booting a Linux system without a root mirror
May 13, 2009
Suppose that you have a system with a mirrored root filesystem and,
for some reason, you need to boot it without
the mirror being available. At one level, this is very easy; all you
need to do is to find the partition of one of the mirrors and specify
it as the filesystem root device on the kernel command line with
This works because while software RAID embeds its own metadata in the partitions, it puts the metadata at the end of the partition, not the start. So if you look at the partition without RAID, what you see is a normal filesystem (that happens to not be using all of the available space).
However, doing this comes with a huge warning. If you ever write to a filesystem accessed this way, you will have desynchronized the two sides of your mirror behind Linux's back; they will have different data when software RAID counts on them being exactly the same. What happens when the mirror is next reassembled ranges from unpredictable to explosive.
Therefor, you need to also boot your system in what I will call 'utterly
single user mode', by supplying '
Booted this way, the system will then come up with
If you actually need to write to the root filesystem, well, I think you get to break out a rescue CD of some sort and figure out how to assemble your mirror.
(There might be tricks you can play with
Written on 13 May 2009.
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