Error handling: How to "lose" a transaction

[Jeff Mahoney <jeffm@suse.com>]

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Hi Chris -

I'm starting to dig into the fun part of error handling and
btrfs_commit_transaction is a minefield right now.

I've been thinking about how I would go about recovering from a
serious error like an -EIO while writing out or an -ENOMEM in a deep
part of the code that it's prohibitively expensive to recover from.
Mostly I'm looking for the best way to make calling btrfs_std_error()
be functionally equivalent to killing the power on the disk. We
already block off new writers, but that's obviously nowhere near
enough. We could have an open transaction floating around, uncommitted
transactions queued, and then an unrecoverable error hits, forcing us
to shut it all down.

It seems to me that that a similar method of recovery that I wrote for
reiserfs can be used here as well. Am I understanding correctly that
if I go through the motions of committing the transaction *except* for
updating the tree roots, or maybe even doing that but declining to
write the superblocks out, that the transaction essentially doesn't
exist on disk? Including the allocations? The in-memory representation
will not match what's on disk, but that's what happens with every file
system in RO-failure mode. With CoW even for data, data is essentially
frozen in time as well. (I suppose with nodatacow that's not true, but
that's for another day.)

- -Jeff

- -- 
Jeff Mahoney
SUSE Labs
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