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Chapter 27: Database Mirroring
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Monitoring Using System Monitor
Apart from using the Database Mirroring Monitor, you can also use System Monitor (previ-
ously referred to as Performance Monitor) to monitor database mirroring performance. The
SQLServer:Database Mirroring object contains the database mirroring performance counters.
Following are some of the key counters:Key principal server counters:■ Log Bytes Sent/sec: Indicates the rate at which the principal transfers transaction
log records to the mirror.
■ Log Send Queue KB: Indicates the total number of kilobytes of the transaction log
that has not been sent to the mirror yet.
■ Transaction Delay: Indicates the delay in milliseconds spent in waiting for the
commit acknowledgment from the mirror. This counter is useful to see if database
mirroring is impacting performance on the principal server.■ (^) Log Compressed Bytes Sent/sec: Indicates the number of compressed bytes of the
transaction log sent in the last second. To fi nd the factor by which the transac-
tion log stream has been compressed, also referred to as the log compression ratio,
divide the Log Bytes Sent/sec by Log Compressed Bytes Sent/sec.
■ (^) Log Bytes Sent from Cache/sec: Indicates how much of the transaction log bytes
are sent from the principal to the mirror and is read from the principal’s in-memory
transaction log cache.
Key mirror server counters:
■ (^) Redo Bytes/sec: Indicates the rate at which log bytes are rolled forward on the
mirror database.
■ (^) Redo Queue KB: Indicates the total number of kilobytes of the transaction log
that has not been rolled forward to the mirror database yet. To estimate the time it
takes the mirror to redo the log, divide Redo Queue KB by Redo Bytes/sec.
■ Log Bytes Received/sec: Indicates the rate at which the log bytes are received
from the principal. To estimate the time it takes the mirror to catch up with the
principal, divide Log Send Queue KB by Log Bytes Received/sec.
■ (^) Log Compressed Bytes Received/sec: Indicates the number of compressed bytes of
the transaction log received in the last second.
■ (^) Log Bytes Redone from Cache/sec: Indicates the number of redone transaction log
bytes that were read from the mirror’s in-memory transaction log cache.
For more information on all the performance counters available for database mirroring refer to SQL
Server, Database Mirroring Object at http://msdn.microsoft.com/en-us/library/
ms189931.aspx.
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