Constance Eustace created CASSANDRA-14279:
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Summary: Row Tombstones in separate sstables / separate compaction
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Key: CASSANDRA-14279
URL: https://issues.apache.org/jira/browse/CASSANDRA-14279
Project: Cassandra
Issue Type: Improvement
Reporter: Constance Eustace
In my experience if data is not well organized into time windowed sstables,
cassandra has enormous difficulty in actually deleting data if the data has a
"medium term" lifetime. Or for example, you might have an active working set
and be archiving "unused" data to other tables or clusters. Or you may be
purging data. Or you may be migrating/sharding data. Whatever the case, you
want that disk space back.
In STCS and LCS, row tombstones are intermingled with column data and column
tombstones. But a row tombstone represents a big event: large amounts of
"droppable" data from an sstable, or even a shortcut from reading data from
other sstables.
I am wondering that if row tombstones were isolated in their own sstables,
separately compacted and merged, that it might enable compaction to work more
efficiently:
reads can prioritize bloom filter lookups that indicate a row tombstone,
getting the timestamp of the deletion first, then can use that in the data
sstables to filter data or shortcircuit the data if the row data had an overall
"most recent data timestamp".
compaction could be forced to reference all the row tombstone sstables, such
that every time two or more "data" sstables are compacted, they must reference
the row tombstones to purge data.
In LCS, this would be particularly useful in getting data out of the upper
levels without having to wait for data to trickle up the tree. The row
tombstones, being read-only inputs into the data sstable compactions, can be
referenced in each of the LCS levels' parallel compactors.
Based on discussions in the dev list, this would appear to require some sort of
customization to the memtable->sstable flushing process, and perhaps a
different set of bloom filters.
Since the row tombstone sstables are all <rowkey>,<tombstone timestamp>, they
should be comparitively smaller and take less time to compact. They could be
aggressively compacted on a different schedule than "data" sstables.
In addition, it may be easier to repair/synchronize row tombstones across the
cluster if they have already been separated into their own sstables.
Column/range tombstones may also benefit from a similar separation, but my
guess is those are much more numerous and large and fine-grained that they
might as well coexist with the data.
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