Understand Replication Lag | Teradata Data Mover - 17.11 - Understanding Replication Lag - Teradata Data Mover

Teradata® Data Mover User Guide

Product
Teradata Data Mover
Release Number
17.11
Release Date
October 2021
Content Type
User Guide
Publication ID
B035-4101-091K
Language
English (United States)
A slowing in the data replication progress between the active repository and standby repository is called lag. If the slowing exceeds the user-specified lag threshold, TVI alert 4604002 is triggered.

The property sync.data.lagging.threshold specifies the threshold in the sync.properties file.

The Postgres transaction process is called write-ahead logging (WAL).

The sync.log reports the following when the synchronization service is slow:
  • sending_lag: how many WALs have been generated, but not yet sent to the standby servers.
  • receiving_lag: WALs in the network that have been sent but not yet written.
  • write_lag: WALs that have been written but not moved to the permanent storage. If Postgres crashes, these changes are lost.
  • replaying_lag: WALs that has been moved to the permanent storage but not yet replayed.
The following are possible reasons for system slowing based on the messages in the report:
  • sending_lag:
    • Active repository performance issue (such as a heavy load)
    • Low throughput of the network between the active and the standby repositories
    • The standby server being offline over a long period of time prior to its starting
  • receving_lag:
    • Low throughput of the network between the active and the standby repositories
    • Standby repository performance issue (such as a heavy load)
  • write_lag and replay_lag
    • Standby performance issues such as over-utilized storage, stuck recovery process, or a heavy load on standby