Using fully mirrored system disks on Linux

March 24, 2009

I'm on record as building systems that use mirrored system disks with plain /boot and swap partitions (that is, non mirrored, just duplicated), mostly through ancient caution.

You know what? I was wrong. Totally wrong. I've now built a system with a fully mirrored system disk and not only does it work, it works better than my old way. I will henceforth now mirror /boot and swap, in addition to all of the regular filesystems, on all future systems that I build with mirrored system disks.

With /boot, mirroring keeps things in sync automatically, and it means that the system will come up without manual intervention when there's only one disk. With a /boot and a /boot2, not only do you have to keep them in sync by hand (in practice we don't), but if a disk fails the system will pause in boot because one of your filesystems isn't there and you have to fix it by hand.

With swap, you probably don't need that much swap, it works, and while init scripts seem to be much more tolerant of missing swap areas than missing filesystems I don't see any reason to take chances if I don't have to. And 'mirror everything' is a very simple rule to keep straight.

(I don't have any idea of the performance tradeoffs of mirrored swap versus non-mirrored swap, but my view these days is that if you are worried about the performance characteristics of your swap space, something horrible has gone wrong to start with.)

Written on 24 March 2009.
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Last modified: Tue Mar 24 23:48:40 2009
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