Virtual Memory
Dirty Ratio
Page cache on a Linux system is the area where filesystem based I/O is cached and that these settings affect how the cache is utilized by the kernel.
We can tune pdflush to determine how much RAM based cache to use for (dirty pages) data targeted to disk and how frequently to flush that cached data by writing the pages back to disk.
The “dirty_ratio” is the percentage of total system memory that can hold dirty pages. The default on most Linux hosts is between 20-30%. When you exceed the limit, the dirty pages are committed to disk, creating a small pause. To avoid this hard pause, there is a second ratio: “dirty_background_ratio” (default 10-15%), which tells the kernel to start flushing dirty pages to disk in the background without any pause.
20-30% is a good general default for “dirty_ratio,” but on large-memory database servers, this can be a lot of memory! For example, on a 128GB-memory host, this can allow up to 38.4GB of dirty pages. The background ratio won’t kick in until 12.8GB! We recommend that you lower this setting and monitor the impact to query performance and disk IO. The goal is reducing memory usage without impacting query performance negatively. Reducing caches sizes also guarantees data gets written to disk in smaller batches more frequently, which increases disk throughput (rather than huge bulk writes less often).
A recommended setting for dirty ratios on large-memory (64GB+ perhaps) database servers is: “vm.dirty_ratio = 15″ and “vm.dirty_background_ratio = 5″, or possibly less. (Red Hat recommends lower ratios of 10 and 3 for high-performance/large-memory servers.)
You can set this by adding the following lines to “/etc/sysctl.conf
vm.dirty_ratio=15
vm.dirty_background_ratio=5
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