Posted by Vadim

As you know MyISAM does not cache data, only indexes. MyISAM assumes OS cache is good enough and uses pread/pwrite system calls for reading/writing datafiles. However OS is not always good in this task, my benchmarks show Linux/Solaris aren’t scalable on intensive pread calls (I believe the same for Windows, but I did not test it).
In 5.1 I implemented a new feature: memory mapping for the datafiles. That can be enabled by –myisam_use_mmap=1 startup option.
In this case instead of systems call MyISAM will use memcpy function. There is a memory addressing limit for 32bit platforms - 2Gb, so the datafiles over 2GB will be used the old way - pread/pwrite functions. Mmap is available on all POSIX-compatible platforms. It will work faster for SELECT/UPDATE/INSERT inside file queries, and no performance gain (maybe a bit slower) for INSERT at the end of file. In case with INSERT at the end of file we have to use a remap technic - resize memory mmaped area to new extended size. Currenlty we call remap once per 1000 inserts at the end of file and on an exclusive operation (DELETE/UPDATE/INSERT inside file), for work with non-mmaped area we use pread/pwrite functions.
To approve effectiveness of memory mapping several benchmarks:

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