[Merge] lp:~jamesodhunt/ubuntu/vivid/ubuntu-core-upgrader/make-partition-sync-atomic into lp:ubuntu/ubuntu-core-upgrader

[Steve Langasek]

Review: Needs Information

Hi James,

What's the rationale for adding a marker file?  For the normal case, I believe I mentioned that this would be addressed by making sure that the system-image channel information is unpacked last.  If you do that, the upgrade is idempotent; reapplying the delta a second time, since it's only a list of files to add/remove, will still yield the correct result.  So there's no need for either a marker, or a fallback to a full format and copy.

The only case I see that's not handled by this is if there's a hard failure while the partition is mounted read-write, resulting in filesystem corruption and destruction of files *other* than the ones that are part of the delta upgrade. However, this is both a) an unlikely failure scenario on any reasonable filesystem, and b) not the only possible source of corruption of files not being written to by the OS (e.g.: copy errors in the disk firmware when routing around bad physical blocks).  So I think there should be a more general solution to the problem of recovering from disk corruption, and that this isn't something that should be designed into the upgrader per se.
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https://code.launchpad.net/~jamesodhunt/ubuntu/vivid/ubuntu-core-upgrader/make-partition-sync-atomic/+merge/255831
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