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From Vick Khera <>
Subject Re: performance tuning - where does the slowness come from?
Date Thu, 06 May 2010 20:11:47 GMT
On Thu, May 6, 2010 at 2:05 PM, Weijun Li <> 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.

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