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The "Operations" page has been changed by JonathanEllis. The comment on this change is: update repair section for 0.7. http://wiki.apache.org/cassandra/Operations?action=diff&rev1=82&rev2=83 -------------------------------------------------- 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if a low consistency was requested by the reader to minimize latency), and the newest version is sent to any out-of-date replicas.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 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). + 1. Anti-Entropy: when `nodetool repair` is run, Cassandra computes a Merkle tree for each range of 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 o.a.c.concurrent.AE-SERVICE-STAGE, or o.a.c.service.StreamingService. + Like all nodetool operations in 0.7, repair is blocking: it will wait for the repair to finish and then exit. This may take a long time on large data sets. - Repair should be run against one machine at a time. (This limitation will be fixed in 0.7.) + It is safe to run repair against multiple machines at the same time, but to minimize the impact on your application workload it is recommended to wait for it to complete on one node before invoking it against the next. === Frequency of nodetool repair ===
