On Thu, May 6, 2010 at 2:05 PM, Weijun Li <weiju...@gmail.com> wrote: > Anyway, for mmap, in order for you to access the data in the buffer or > virtual address, OS has to read/page in the data to a block of physical > memory and assign your virtual address to that physical memory block. So if > you use random partitioner you'll most likely force Linux to page in/out all > the time. In this case, disabling mmap and let Cassandra to use random file > access seems to make more sense. mmap should be used when you have enough > ram for OS to cache most or all of your data files. >
You pay the price of disk I/O and cache with or without mmap, don't you? If you're just reading the data, then there is no page-out necessary. Just mmap'ing a file does not cause it to be read in its entirety into the cache.
