How to Configure fs.aio-max-nr for MySQL InnoDB to Prevent AIO Slot Errors

MySQL enables innodb_use_native_aio by default, using asynchronous I/O. When the number of AIO slots required at startup exceeds the system's fs.aio-max-nr value, MySQL cannot start and reports InnoDB: io_setup() failed with EAGAIN after 5 attempts.

In a single‑instance environment this issue is rare, but it becomes common when deploying multiple MySQL instances on the same host.

The article analyzes how to configure the fs.aio-max-nr parameter.

About aio-nr and aio-max-nr

aio-nr is the running total of the number of events specified on the io_setup system call for all currently active aio contexts. If aio-nr reaches aio-max-nr then io_setup will fail with EAGAIN. Note that raising aio-max-nr does not result in the pre‑allocation or re‑sizing of any kernel data structures.

How many event slots does MySQL allocate on startup?

Using strace to trace io_setup calls:

strace -fe trace=io_setup /path/to/mysqld --defaults-file=/etc/my.cnf --daemonize 2>&1 | grep io_setup

The observation shows a total of 4709 slots, calculated as 18 * 256 + 101 .

Each InnoDB I/O thread allocates 256 event slots.

Conclusion

For each instance, fs.aio-nr = (innodb_read_io_threads + innodb_write_io_threads + log thread + insert buffer thread) * 256 + 101 .

If deploying a single instance, fs.aio-max-nr should be greater than the instance's fs.aio-nr .

If deploying multiple instances, fs.aio-max-nr must be greater than the sum of fs.aio-nr for all instances on the host.

Images in the original article illustrate the slot distribution and provide additional visual context.