> dhruba borthakur - 29/Mar/08 11:03 PM
> I can make the configurable to be per file, but maybe it makes more sense to make it applicable to the
> entire system. The reason being that datanodes do not know much about the name of the HDFS file
> that a block belongs to. To make this configurable "per file" would need lots of protocol change.
I don't think it needs to be per file. Aside from our redo log, other files are written and then immediately
closed and re-opened for read.
> If a client dies while writing to the last block of that file, that block is not yet part of the blocksmap in the
> namenode. (A block gets inserted in the blocksmap when a complete block is received by the datanode
> and it sends a blockReceived message to the namenode). If the lease for this file on the namenode
> expires before the block report from the datanode arrives, then the namenode will erroneously think
> that no datanodes have a copy of that block. As part of lease recovery, the namenode will delete the
> last block of the file because it has no entry in the blocksMap. To prevent this from occuring, the block
> report periodicity should be set to 30 minutes.
I think this is ok, but let me give a scenario to verify that my understanding is correct.
We open our redo log and flush it either every N seconds or after M records have been written.
If the process writing the log crashes, we will notice much sooner than the file lease timeout.
At that point another process should be able to open the file for read, and all flushed data
will be visible, unflushed data will not. Since the amount of unflushed data should be small
the amount of data lost should be minimal. Once the redo log has been read and processed,
the file will be deleted by the process reading the file.
If this is how this patch works, +1.