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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7066) Simplify (and unify) cleanup of compaction leftovers
Date Fri, 31 Jul 2015 13:05:05 GMT


Benedict commented on CASSANDRA-7066:

Thanks. Looks like great work as always. 

A few comments (I haven't done a full review):

* The last modified sort order was necessary for the same reason a full checksum of all of
the old files is too heavy handed: restarting in the middle of deleting the files. We want
to finish deleting them, and if we take the max timestamp we can say that will safely stay
the same. We could keep the full checksum and include a count; if the count on disk is lower,
and our max timestamp matches, we can proceed; if the count is the same we require a checksum
match. This may or may not be overkill, but there's on harm in paranoia here.
* It looks like serialization of the sstables now assumes the ancestors won't be present,
but this will break upgrade. We need to check the sstable version, and at least skip the ancestors
if they're present
* The compaction metadata equality is probably not sensibly employed anywhere; we should just
confirm this and remove it IMO since it makes very little sense, especially now that the ancestors
have been removed
* I think you took my comments about panic warnings a little too literally. We should just
use strong language to make it clear to the operator we've detected disk corruption of some
kind, and have taken the most pessimistic course of action
** This warning should be less severe if only the last line is _incomplete_ (and all other
lines are parsed correctly), as this would be expected if we crashed part way through serialization
** We should perhaps not worry at all, and continue if this last situation occurs, in the
event that _all_ files logged are still present on the file system, and our other metadata
all matches for them. In this case we can be very confident we just shutdown in the middle
of writing a record, and can just clean up all of the new files. This should make us robust
both to the scary situation, but also minimise the pessimism, hopefully giving us the best
of both worlds.


> Simplify (and unify) cleanup of compaction leftovers
> ----------------------------------------------------
>                 Key: CASSANDRA-7066
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Assignee: Stefania
>            Priority: Minor
>              Labels: benedict-to-commit, compaction
>             Fix For: 3.0 alpha 1
>         Attachments: 7066.txt
> Currently we manage a list of in-progress compactions in a system table, which we use
to cleanup incomplete compactions when we're done. The problem with this is that 1) it's a
bit clunky (and leaves us in positions where we can unnecessarily cleanup completed files,
or conversely not cleanup files that have been superceded); and 2) it's only used for a regular
compaction - no other compaction types are guarded in the same way, so can result in duplication
if we fail before deleting the replacements.
> I'd like to see each sstable store in its metadata its direct ancestors, and on startup
we simply delete any sstables that occur in the union of all ancestor sets. This way as soon
as we finish writing we're capable of cleaning up any leftovers, so we never get duplication.
It's also much easier to reason about.

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