3.3.2. Monitoring Compression at Runtime

The current version of the InnoDB Plugin provides only a limited means to monitor the performance of compression at runtime. Overall application performance, CPU and I/O utilization and the size of disk files are the best indicators of how effective compression is for your application.

The InnoDB Plugin does include some Information Schema tables (see Example 6.1, “Using the Compression Information Schema Tables”) that reflect the internal use of memory and the rates of compression used overall. The INNODB_CMP tables report information about compression activity for each compressed page size (KEY_BLOCK_SIZE) in use. The information in these tables is system-wide, and includes summary data across all compressed tables in your database. You can use this data to help decide whether or not to compress a table by examining these tables when no other compressed tables are being accessed.

The key statistics to consider are the number of, and amount of time spent performing, compression and uncompression operations. Since InnoDB must split B-tree nodes when they are too full to contain the compressed data following a modification, you should also compare the number of successful compression operations with the number of such operations overall. Based on the information in the INNODB_CMP tables and overall application performance and hardware resource utilization, you may decide to make changes in your hardware configuration, adjust the size of the InnoDB buffer pool, choose a different page size, or select a different set of tables to compress.

If the amount of CPU time required for compressing and uncompressing is high, changing to faster CPUs, or those with more cores, can help improve performance with the same data, application workload and set of compressed tables. You may also benefit by increasing the size of the InnoDB buffer pool, so that more uncompressed pages can stay in memory, reducing the need to uncompress pages which exist in memory only in compressed form.

A large number of compression operations overall (compared to the number of INSERT, UPDATE and DELETE operations in your application and the size of the database) could indicate that some of your compressed tables are being updated too heavily for effective compression. You may want to choose a larger page size, or be more selective about which tables you compress.

If the number of successful compression operations (COMPRESS_OPS_OK) is a high percentage of the total number of compression operations (COMPRESS_OPS), then the system is likely performing well. However, if the ratio is low, then InnoDB is being caused to reorganize, recompress and split B-tree nodes more often than is desirable. In this case, you may want to avoid compressing some tables or choose a larger KEY_BLOCK_SIZE for some of the tables for which you are using compression. You may not want to compress tables which cause the number of compression failures in your application to be more than 1% or 2% of the total (although this may be acceptable during a data load, for example, if your application does not encounter such a ratio during normal operations).

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