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From "Kurt Greaves (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-13418) Allow TWCS to ignore overlaps when dropping fully expired sstables
Date Wed, 30 Aug 2017 05:28:00 GMT


Kurt Greaves commented on CASSANDRA-13418:

Yeah more want to document the implications for the user.

Basically my understanding is this:

Case 1 (enabling both: {{unsafe_aggressive_sstable_expiration}} and {{uncheckedTombstoneCompaction}}:
If data is read repaired into an earlier window, we will potentially trigger more compactions
including the overlapping data but only when {{provide_overlapping_tombstones}} is set. If
it is not set we will potentially trigger only single SSTable tombstone compactions which
_may or may not_ provide a benefit. Basically if you enable more options the benefit is that
you potentially get rid of read-repaired expired data faster, at the cost of more compactions.
So you should only really do this if you have a significant amount of read-repaired data and
hitting a lot of tombstones because of it.

Case 2 (enabling only {{unsafe_aggressive_sstable_expiration}}):
If data is read repaired into an earlier window, it will not be removed until the whole SSTable
is expired, which means it will hang around until TTL passes for that table.

If that's not the whole story/correct please elaborate. I think it's worthwhile having it
documented clearly somewhere (docs) why you'd set these options (together or separately).

> Allow TWCS to ignore overlaps when dropping fully expired sstables
> ------------------------------------------------------------------
>                 Key: CASSANDRA-13418
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Compaction
>            Reporter: Corentin Chary
>              Labels: twcs
>         Attachments: twcs-cleanup.png
> explains it well. If you really
want read-repairs you're going to have sstables blocking the expiration of other fully expired
SSTables because they overlap.
> You can set unchecked_tombstone_compaction = true or tombstone_threshold to a very low
value and that will purge the blockers of old data that should already have expired, thus
removing the overlaps and allowing the other SSTables to expire.
> The thing is that this is rather CPU intensive and not optimal. If you have time series,
you might not care if all your data doesn't exactly expire at the right time, or if data re-appears
for some time, as long as it gets deleted as soon as it can. And in this situation I believe
it would be really beneficial to allow users to simply ignore overlapping SSTables when looking
for fully expired ones.
> To the question: why would you need read-repairs ?
> - Full repairs basically take longer than the TTL of the data on my dataset, so this
isn't really effective.
> - Even with a 10% chances of doing a repair, we found out that this would be enough to
greatly reduce entropy of the most used data (and if you have timeseries, you're likely to
have a dashboard doing the same important queries over and over again).
> - LOCAL_QUORUM is too expensive (need >3 replicas), QUORUM is too slow.
> I'll try to come up with a patch demonstrating how this would work, try it on our system
and report the effects.
> cc: [~adejanovski], [~rgerard] as I know you worked on similar issues already.

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