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From Michael Theroux <>
Subject TTL, Tombstones, and gc_grace
Date Thu, 25 Jul 2013 13:24:56 GMT

Quick question on Cassandra, TTLs, tombstones, and GC grace.  If we have a column family whose
only mechanism of deleting columns is utilizing TTLs, is repair really necessary to make tombstones
consistent, and therefore would it be safe to set the gc grace period of the column family
to a very low value?

I ask because of this blog post based on Cassandra .7:

"The first time the expired column is compacted, it is transformed into a tombstone. This
transformation frees some disk space: the size of the value of the expired column. From that
moment on, the column is a normal tombstone and follows the tombstone rules: it will be totally
removed by compaction (including minor ones in most cases since Cassandra 0.6.6) after GCGraceSeconds."

Since tombstones are not written using a replicated write, but instead written during compaction,
theoretically, it shouldn't be possible to lose a tombstone?  Or is this blog post inaccurate
for later versions of cassandra?  We are using cassandra 1.1.11.


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