Hi Hongbin, The WAL class is used internally to the region server. Typically an HBase write operation will first call WAL.append() with the data, then later, after releasing locks, call WAL.sync() to ensure that the data for that write has been synced to be durable before returning to the client (unless the client instructed it to skip that step).
Dave On Mon, Feb 16, 2015 at 5:21 AM, hongbin ma <mahong...@apache.org> wrote: > hi, all > > It seems WAL.append() in hbase, the javadoc says: > > * * Append a set of edits to the WAL. The WAL is not flushed/sync'd after > this transaction* > * * completes BUT on return this edit must have its region edit/sequence > id assigned* > * * else it messes up our unification of mvcc and sequenceid. On return > <code>key</code> will* > * * have the region edit/sequence id filled in.* > > This is really confusing me, if it does does flushed every update is > instantly to disk, how can hbase ensures the WAL contains everything that > was appended? What is hbase's philosophy in this? Does it allow some degree > of data loss? > > > thanks > hongbin >
