EMC Storage Product Knowledge Sharing

If you open Navisphere Manager and select any Frame/Array and click properties of Array, you will see a cache tab, which will give you cache configuration. There you need to setup configuration like LOW watermark, Hight Watermark. Did you think how CLARiiON behave on these percentage. Lets look close FLUSHING method what CLARiiON does:
There will many situation when CLARiiON Processor has to Cache Flushing to keep some free space in cache Memory.There are different size for cache memory for different series of CLARiiON.

There are three levels of flushing:
IDLE FLUSHING (LUN is not busy and user I/O continues)Idle flushing keeps some free space in write cache when I/O activity to a particular LUN is relatively low. If data immediacy were most important, idle flushing would be sufficient. If idle flushing cannot maintain free space, though, watermark flushing will be used.

WATERMARK FLUSHING The array allows the user to set two levels called watermarks: the High Water Mark (HWM) and the Low Water Mark (LWM). The base software tries to keep the number of dirty pages in cache between those two levels. If the number of dirty pages in write cache reaches 100%, forced flushing is used.

FORCED FLUSHING Forced flushes also create space for new I/Os, though they dramatically affect overall performance. When forced flushing takes place, all read and write operations are halted to clear space in the write cache. The time taken for a forced flush is very short (milliseconds), and the array may still deliver acceptable performance, even if the rate of forced flushes is in the 50 per second range.