[HDFS-7501] TransactionsSinceLastCheckpoint can be negative on SBNs

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.5.0
• Fix Version/s: 2.8.0, 3.0.0-alpha1
• Component/s: namenode
• Hadoop Flags: Reviewed
• Release Note: Fixed a bug where the StandbyNameNode's TransactionsSinceLastCheckpoint metric may slide into a negative number after every subsequent checkpoint.
• Assignee: Gautam Gopalakrishnan
• Reporter: Harsh J
• Created: 09/Dec/14 22:52
• Updated: 28/Dec/16 09:37
• Resolved: 28/Mar/15 19:18

Description

The metric TransactionsSinceLastCheckpoint is derived as FSEditLog.txid minus NNStorage.mostRecentCheckpointTxId.

In Standby mode, the former does not increment beyond the loaded or last-when-active value, but the latter does change due to checkpoints done regularly in this mode. Thereby, the SBN will eventually end up showing negative values for TransactionsSinceLastCheckpoint.

This is not an issue as the metric only makes sense to be monitored on the Active NameNode, but we should perhaps just show the value 0 by detecting if the NN is in SBN form, as allowing a negative number is confusing to view within a chart that tracks it.

Issue Links

• is duplicated by: HDFS-11276 The "TransactionsSinceLastCheckpoint" metrics is wrong in Standby NameNode