[Cassandra Wiki] Update of "FAQ" by JeremiahJordan

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The "FAQ" page has been changed by JeremiahJordan:
https://wiki.apache.org/cassandra/FAQ?action=diff&rev1=159&rev2=160

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  == I delete data from Cassandra, but disk usage stays the same. What gives? ==
- Data you write to Cassandra gets persisted to SSTables. Since SSTables are immutable, the data can't actually be removed when you perform a delete, instead, a marker (also called a "tombstone") is written to indicate the value's new status. Never fear though, on the first compaction that occurs after `gc_grace_seconds` (configurable per-column family as part of the schema) have passed, the data will be expunged completely and the corresponding disk space recovered. See DistributedDeletes for more detail.
+ Data you write to Cassandra gets persisted to SSTables. Since SSTables are immutable, the data can't actually be removed when you perform a delete, instead, a marker (also called a "tombstone") is written to indicate the value's new status. Never fear though, on the first compaction that occurs between the data and the tombstone, the data will be expunged completely and the corresponding disk space recovered. See DistributedDeletes for more detail.
  
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