How does a CLARiiON array flush cache?

        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.