Quite often, especially for benchmarks, I am trying to limit available memory for a database server (usually for MySQL, but recently for MongoDB also). This is usually needed to test database performance in scenarios with different memory limits. I have physical servers with the usually high amount of memory (128GB or more), but I am interested to see how a database server will perform, say if only 16GB of memory is available.

And while InnoDB usually respects the setting of innodb_buffer_pool_size in O_DIRECT mode (OS cache is not being used in this case), more engines (TokuDB for MySQL, MMAP, WiredTiger, RocksDB for MongoDB) usually get benefits from OS cache, and Linux kernel by default is generous enough to allocate as much memory as available. There I should note that while TokuDB (and TokuMX for MongoDB) supports DIRECT mode (that is bypass OS cache), we found there is a performance gain if OS cache is used for compressed pages.

Today I encountered a situation where MySQL Enterprise Backup caused to much load on the I/O subsystem of the server to cause the application to be so slow that it wasn't usable any longer. So I wanted to limit the mysqlbackup process so it wouldn't cause any more issues.

The mysqlbackup command has settings to for the number of read, write and process threads. The defaults are 1 read, 1 write and 6 process threads. So that isn't really useful for throttling as I was using the defaults.

Using the ionice utility wouldn't work as that requires the CFG I/O scheduler.

I found a solution in this blog post. It is to use cgroups on Linux. I had used cgroups before to test how a galera setup works when one of the three servers had a much slower CPU.

# mkdir /cgroup/blkio
# mount -t …