The Microsoft Exchange Replication Service on the passive node is responsible for copying the 1MB logs
as they are created on the active node. Because the passive node is always at least one log file behind (the
active log) Exchange implemented Lost Log Resiliency (LLR). LLR delays the active node from
committing the log files as they are closed. This allows an administrator to mount the passive node
without all of the log files, and not cause divergence. Divergence is when the active and passive nodes
have different copies of a database. Once the nodes have diverged, the only remedy is to reseed the
database. When a failover occurs and one or more log files are missing, this is called a lossy failover. If all
the logs are able to be copied to the passive node, this is called a lossless failover.
During a planned (administrative) failover, all of the logs are copied to the passive node. This means that
a planned failover is always lossless.
The transport dumpster feature helps minimize loss of mail during a lossy failover. The transport
dumpster holds on to a limited amount of mail. After a lossy failover, the mailbox server then requests
redelivery of messages to minimize any data loss from the missing log files. The mail server just deletes
any messages that it already has committed.
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