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The "Operations" page has been changed by JonathanEllis. The comment on this change is: add notes on repair. http://wiki.apache.org/cassandra/Operations?action=diff&rev1=55&rev2=56 -------------------------------------------------- Cassandra repairs data in two ways: 1. Read Repair: every time a read is performed, Cassandra compares the versions at each replica (in the background, if a low consistency was requested by the reader to minimize latency), and the newest version is sent to any out-of-date replicas. - 1. Anti-Entropy: when `nodetool repair` is run, Cassandra performs a major compaction, computes a Merkle Tree of the data on that node, and compares it with the versions on other replicas, to catch any out of sync data that hasn't been read recently. This is intended to be run infrequently (e.g., weekly) since major compaction is relatively expensive. + 1. Anti-Entropy: when `nodetool repair` is run, Cassandra computes a Merkle tree of the data on that node, and compares it with the versions on other replicas, to catch any out of sync data that hasn't been read recently. This is intended to be run infrequently (e.g., weekly) since computing the Merkle tree is relatively expensive in disk i/o and CPU, since it scans ALL the data on the machine (but it is is very network efficient). + + Running `nodetool repair`: + Like all nodetool operations, repair is non-blocking; it sends the command to the given node, but does not wait for the repair to actually finish. You can tell that repair is finished when (a) there are no active or pending tasks in the CompactionManager, and after that when (b) there are no active or pending tasks on AE-SERVICE-STAGE. + + Repair should be run against one machine at a time. (This limitation will be fixed in 0.7.) === Handling failure === If a node goes down and comes back up, the ordinary repair mechanisms will be adequate to deal with any inconsistent data. Remember though that if a node is down longer than your configured GCGraceSeconds (default: 10 days), it could have missed remove operations permanently. Unless your application performs no removes, you should wipe its data directory, re-bootstrap it, and removetoken its old entry in ghe ring (see below).
