Hi,
Update...
Jean-Pierre André wrote:
Gil Barash via ntfs-3g-devel wrote:
Hey,
I have two follow up issues:
[...]
--- 2 --
General question about redo process:
In change_resident_expect (called by redo_update_root_vcn, for
example) we chose not to apply the redo data if the current buffer
state doesn't match the undo data. Note that the operation is
considered successful in this case.
I think the reasoning is a follows :
- the old state was A
- an update is made, the state is now B
- a second update is made, the state is now C
- C is synced to disk, but a failure occurs before
the syncing is recorded in the log.
When restarting, both updates have to be replayed,
and when applying the first one, the state on disk
is not the undo state (which is A), so it is correct
to apply the update (whose result will be overwritten
by the second update).
Avoiding the updates if the undo data does not match
the state on disk probably leads to the same result,
but IMHO it is safer to make sure the final state is
what the redo data says. Other updates which overlap
could be intertwined and they would be processed
incorrectly when not applied to the same state as in
the initial execution.
Well, actually the current code is doing the opposite
(that is applying the update if the current state matches
the undo data).
I have run my tests again after reversing the rule (so
applying the update if the current state does not match
the redo data), and the tests fail when the second
update destroys the attribute (e.g. deleting a file).
In this situation there is nothing the redo data can
be compared against (more exactly the current state is
meaningless and should not be compared to redo data).
There is not enough information to rebuild the intermediate
state, the only possible action is doing nothing, and
some criterion is needed to go this way.
Jean-Pierre
Can you please provide some small explanation or direct me to some
form of documentation as to why it is OK to skip an operation.
I would also be interested to get some form of
documentation...
Regards
Jean-Pierre
I suspect it might cause some kind of "chain-reaction" where future
operations on this "cluster" would also be skipped because they expect
to see the data as it should have been after applying the redo
operation.
Thanks,
Gil
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