March 16, 2016

Database Mirroring and index maintenance in AWS

Database Mirroring and index maintenance in AWS

Database Mirroring, the database has to be in the full recovery mode, so all index rebuilds are fully logged. It all comes down to the amount of transaction log generated and whether this causes a problem.

MS SQL Server Standard edition in AWS allows  synchronous mirroring only.In synchronous mirroring, the additional log being generated could overload the network link between the principal and mirror. This will slow down the hardening of regular transactions in the mirror’s transaction log, and thus led to a decrease in transaction throughput on the principal.

Basically, the way mirroring works is by shipping all the transaction log from the principal to the mirror, and then replaying the log records on the mirror database. Network bandwidth can be a bottleneck for mirrored databases where the rate of transaction log generation is high enough to saturate the network. On synchronously mirrored databases (where a transaction cannot commit on the principal until all the transaction log for it has been hardened in the log of the mirror database), this can even lead to workload throughput degradation.

The index creation time for a clustered and a nonclustered index for various mirroring safety levels. Creating an index on a large table takes longer with safety FULL compared to safety OFF. The performance impact of synchronous mirroring is more pronounced if you create a clustered index than if you create a nonclustered index. This is because creating a clustered index generates much more transaction log compared to that generated by a nonclustered index.

Performance under database mirroring is highly dependent upon the network performance and the log I/O performance. If either the network performance or the log I/O performance is a bottleneck, the performance of database mirroring for the workload can degrade significantly with the safety FULL operation.

The network becomes a bottleneck for mirroring and it slows down both the maintenance and reduces the workload throughput.

Solution: log stream compression

SQL Server 2008 introduces a new feature called "Database Mirroring Log Compression". With SQL Server 2008, the outgoing log stream from the principal to the mirror is compressed, thereby minimizing the network bandwidth used by database mirroring. In a network constrained for bandwidth, compressing the log stream helps improve the application performance and throughput.

Log compression compression rates of at least 12.5% are achieved.

The downside of this is that compression comes with a cost as extra CPU must be used. The obvious CPU load increases comes from having to compress the transaction log on the principal and then decompress it again on the mirror before hardening it in the log. The non-obvious increase will come because the principal and mirror should end up being able to process more transactions/second, which means more CPU is needed.

This extra CPU could be a problem for some systems that are already pegged in terms of CPU load, and so they may actually see a drop in performance when log stream compression is enabled. Now, it’s on by default when you upgrade to SQL Server 2008 but there is a way to turn it off.

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