By using Leading() to define nested pairs of joins and specifying access methods for each table, you can fully hint your queries and avoid leaving any decisions up to the query planner. However, be aware that too many parentheses in a hint may make it harder to read and understand. To troubleshoot errors with hints, set `pg_hint_plan.debug_print` to “verbose” and turn on logging for client messages using `set client_min_messages = log;`.
In terms of I/O schedulers in modern Linux implementations, there are several options available that prioritize disk access requests differently. Each scheduler has its own unique method of handling input-output operations, which can impact performance depending on the workload being used. To change the I/O scheduler at runtime, you can modify a file located within /sys or pass Kernel parameters via the Grub boot loader for changes that persist across reboots. It’s essential to understand your environment and select the right scheduler based on testing multiple options and choosing based on results.
In our case study, we tested three different I/O schedulers: CFQ (Complete Fairness Queueing), Deadline, and Noop. The CFQ scheduler provides a fair priority for each process’s disk access requests but may not be optimal for read-heavy workloads due to its round-robin approach. On the other hand, the Deadline scheduler prioritizes reads over writes by setting timeouts for I/O operations and is ideal for environments that require high read throughput. The Noop scheduler simply places all disk requests into a FIFO queue without any priority or optimization but can be useful in scenarios where an external I/O scheduler is already being used, such as within Virtual Machines.
To change the I/O scheduler at runtime, we modified the /sys/block//queue/scheduler file to specify our desired scheduler. For example: `echo “cfq” > /sys/block/sda/queue/scheduler`. To make this change permanent across reboots, we added the elevator parameter to the GRUB_CMDLINE_LINUX option in the /etc/default/grub configuration file and ran the update-grub2 command.
We used pgbench to test each I/O scheduler’s performance on our PostgreSQL instance by running a benchmark with 100 clients, 2 threads per client, and 1,000 transactions per client for 100 seconds. The results showed that the Deadline scheduler provided the best overall performance, reaching 2,141 transactions per second compared to CFQ’s 1,644 tps and Noop’s slightly better 2,156 tps. However, each environment is unique and may require different I/O schedulers based on workload requirements.