Thursday, 30 November 2017

SQLServer Buffer Manager Buffer cache hit ratio, Page Life,Lazy Write/sec, Checkpoint Pages/sec

Without any shadow of a doubt, one of the procedures to follow in order to monitor and diagnose performance issues of a database server is to analyse meaningful performance counters related to SQL Server. As a Database Administrator, we need to get used to dealing with this sorts of problems so it truly helpful to know how to analyse and interpret them by contrasting with other windows performance counters.

To begin with, we can analyse SQLServer Buffer cache hit ratio performance counter which gives us the percentage of cache memory usage. This percentage should mostly be above 99% for OLTP database servers. Therefore, nowadays 4GB or 8GB of RAM is not good enough for the vast majority of them. The knock-on effect of this lack of RAM is that there will be a huge increase on the I/O disk activity as a result of using much more paging (data pages are moving in and out from memory very frequently because there is not enough space to locate new ones). This will also cause a huge disk queue which will keep disk subsystem busy, consequently, it will impact directly on the database performance detrimentally. In this context, we also need to look into other SQL Server performance counters like SQLServer: Buffer Manager: Page Life Expectancy, SQLServer: Buffer Manager: Lazy Write/sec, and SQLServer: Buffer Manager: Checkpoint Pages/sec. 

As I mentioned before, there are other useful performance counters to diagnose and tune cache memory issues like Buffer Manager: Page Life Expectancy which indicates the time in seconds that a page has been inside of memory. This time should mostly be above 300 seconds, that is, 5 minutes. So, values below it should be considered as an alert, which means that SQL Server is under high memory pressure because bad-written queries may be using a lot. Clearly, the final solution is not to add more memory but identify and optimise those problematic queries. Only after that may you opt to add memory.

The other performance counter to check is SQLServer: Buffer Manager: Lazy Write/sec which gives the quantity of pages moved out per second from memory. Not only could it mean a lack of memory, but also there are copious amounts of Checkpoints which are not good at all because it will cause recompilation of stored procedures at the same time.The value of this counter should mostly be below 20. So, if you notice this value above 20, you may need to check SQLServer: Buffer Manager: Checkpoint Pages/sec counter as well. Checkpoints move out all pages from memory to disk.

Finally, keep in mind that it really important to do diagnostic tasks regularly which allow to take preventive actions so as to improve the whole performance over time. I do hope you find this post helpful. Let me know any remarks you may have. Thanks for reading. Stay tuned.

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