[Tux3] Tux3 will always recover on mount
phillips at phunq.net
Tue Jul 22 14:38:23 PDT 2008
I'm just writing down points as they occur to me now. One small detail
of Tux3 that should have quite a bit impact on robustness: Tux3 will
always "initiate recovery" on mount, which is to say that it will read
its forward logs and apply them to the memory image of btrees etc, to
return to just the state it was in when it stopped. It will not roll
the logs up, for example, and write out "perfect" copies of the btrees.
This will not only save time on shutdown, but it will give constant
exercise to the recovery logic, which all too often is the last part of
a filesystem to be perfected, if it ever is perfected.
Addendum to the last note: that is a 512 times advantage for changing
one block of a file. If many or all blocks of a file are changed (a
common case) then recursive copy-on-write gets closer to the space
efficiency of versioned pointers, but it never quite gets there.
In the case of indexed directories it is very common for a single block
to be changed, and in that case the metadata efficiency advantage of
versioned pointers will be large.
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