[Code-review] [PATCH] Maintain a metadata copy outside the index.

[Benjamin M. Schwartz]

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Martin Langhoff wrote:
| NOTE NOTE NOTE: the backup process does not make too many promises
| about the internal consistency of the data being backed up. Downstream
| users of the data will have to fend for themselves - spot inconsistent
| data. On the upside, if backups have been running, there'll be several
| versions to pick from. We are no different from Amanda, for example,
| in that regard.
|
| (Solutions? No practical ones: Ask activity writers to aim for atomic
| updates, lock-the-whole-system-on-backup, use a snapshot-capable FS.)

I agree that it's impractical to hack atomicity into the current system,
but any future datastore should _definitely_ provide strong
atomicity/checkpoint guarantees.  Designing this in should not be
difficult, and even if it is, it is worth doing.

- --Ben
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