[Cassandra Wiki] Update of "Durability" by TylerHobbs

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The "Durability" page has been changed by TylerHobbs:
http://wiki.apache.org/cassandra/Durability?action=diff&rev1=4&rev2=5

Comment:
Update commitlog options to match current cassandra.yaml

  
  Instead, like other modern systems, Cassandra provides durability by appending writes to a commitlog first.  This means that only the commitlog needs to be fsync'd, which, if the commitlog is on its own volume, obviates the need for seeking since the commitlog is append-only.  Implementation details are in ArchitectureCommitLog.
  
- Cassandra's example configuration shows !CommitLogSync set to periodic, meaning that we sync the commitlog every CommitLogSyncPeriodInMS ms, so you can potentially lose up to that much data in a crash.  This is decently performant even with the commitlog shared with data directories.  You can also select "batch" mode, where Cassandra will guarantee that it syncs before acknowledging writes, i.e., fully durable mode, in batches of CommitLogSyncBatchWindowInMS.  To use this mode we strongly recommend splitting your commitlog onto a separate device, as described above.
+ Cassandra's default configuration sets the `commitlog_sync` mode to `periodic`, causing the commitlog to be synced every `commitlog_sync_period_in_ms` milliseconds, so you can potentially lose up to that much data if all replicas crash within that window of time. This default behavior is decently performant even when the commitlog shares a disk with data directories.  You can also select `batch` mode, where Cassandra will guarantee that it syncs before acknowledging writes.  To avoid syncing after every write, Cassandra groups the mutations into batches and syncs every `commitlog_batch_window_in_ms`.  When using this mode, we strongly recommend putting your commitlog on a separate, dedicated device, as described above.
  
- Users familiar with PostgreSQL may note that CommitLogSyncPeriodInMS and CommitLogSyncBatchWindowInMS correspond to the [[http://www.postgresql.org/docs/8.4/static/runtime-config-wal.html|PostgreSQL settings]] of wal_writer_delay and commit_delay, respectively.
+ Users familiar with PostgreSQL may note that `commitlog_sync_period_in_ms` and `commitlog_batch_window_in_ms` correspond to the [[http://www.postgresql.org/docs/8.4/static/runtime-config-wal.html|PostgreSQL settings]] of wal_writer_delay and commit_delay, respectively.