What should the high/low watermark cache settings be set to on a CLARiiON array?



CLARiiON CX Series.




Recommended high/low cache watermark settings for CLARiiON CX-Series arrays




The storage processors (SPs) use high and low watermarks to determine when to flush their write caches.  When an SP flushes its write cache, it writes its dirty pages to disk.  A dirty page is a write cache page with modified data that has not yet been written to disk.  According to the EMC CLARiiON Performance and Availability Release 30 Firmware Update Applied Best Practices.pdf for most workloads, the defaults (80% high watermark, 60% low watermark) afford optimal behavior.  Increase the high watermark if data indicates an absence of forced flushes during a typical period of high utilization. Decrease the watermarks if write bursts are causing enough forced flushes to impact host write workloads.  Lowering the watermarks reserves more cache pages to absorb bursts at the cost of fewer read hits from the write cache.  If % Dirty Pages frequently exceeds 80% and reaches 100% for periods of time, it can be adjusted to 60/40.  At 100% SP cache utilization write performance degrades drastically for all LUNs.  For smaller CX-Series arrays, reducing the watermarks to 60/40 can help reduce forced flush conditions.Percentages of dirty pages specify the HIGH and LOW WATER MARKs in the write cache. When the High Water Mark (higher % value) is reached, the SPs begins flushing their write caches. On the other hand, when the Low Water Mark (lower % value) is reached, SPs stop flushing their write cache


Note: The low watermark should be 20 percent less that the high watermark.



When the cache utilization hits the high watermark, the storage system dedicates additional resources to flushing and the High Water Flush process is activated. See the Best Practices guides on Powerlink (http://powerlink.emc.com or https://support.emc.com/) for more information on Watermark flushing:


  • EMC CLARiiON Performance and Availability Release 30 Firmware Update Applied Best Practices.pdf
  • EMC CLARiiON Storage System Fundamentals for Performance and Availability Applied Best Practices.pdf