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Discussion

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 up previous contents
Next: COMMAND PROCESSOR Up: A CONSISTENCY PROBLEM FOR Previous: A solution
Paul McJones
1998-06-22