You can avoid the not-fast problem by rsyncing /usr/bin and /usr/lib (and parts of /usr/share?) into battery-backed/flash RAM, so most of the time very few writes would be needed. (Obviously rsync for this purpose is a kludge; some sort of replicated write at the kernel level seems preferable. Perhaps dm mirroring could do this, but it'd be forced to replicate all of /usr into flash, whether we care about it or not...)
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