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Various simplications of this algorithm are possible in special cases,
but I believe that in the most general case, all of the snapshots are
needed. If the list of intentions all appear in one file block, so
that the entire list is either present or not after a crash, then it
is possible to dispense with the intentions bit. If all changes will
be in a single file, as well as the intentions list, and the system
will guarantee that all of the contents of a file after a crash did
exist simultaneously at some time before the crash, then all snapshots
except step xv) can be removed.
It should be noted that the logical state represented by the file(s)
changes at step x), but if a crash occurs before step
xi) is completed, the representation may return to the old
logical state.
Finally, snapshots require a significant amount of real time. Disk
operations must be started, and completed. Some of the vital
information (e.g., user accounts) in CAL TSS was maintained using a
simplified version of this algorithm, and this contributed to our
system overheads.
Next: COMMAND PROCESSOR
Up: A CONSISTENCY PROBLEM FOR
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Paul McJones
1998-06-22