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:
For benchmarks I used the sysbench tool and tested different workloads:
Percona’s widely read Percona Data Performance blog highlights our expertise in enterprise-class software, support, consulting and managed services solutions for both MySQL® and MongoDB® across traditional and cloud-based platforms. The decades of experience represented by our consultants is found daily in numerous and relevant blog posts.
Besides specific database help, the blog also provides notices on upcoming events and webinars.
Want to get weekly updates listing the latest blog posts? Subscribe to our blog now! Submit your email address below.