Let's dive into the world of Linux IO schedulers and see how we can tune them for modern storage technologies. Buckle up, because we're about to go from 0 to 100K IOPS in no time!

The IO Scheduler Landscape

Before we start tuning, let's take a quick tour of the available IO schedulers in modern Linux kernels:

  • CFQ (Completely Fair Queuing): The old reliable, but showing its age
  • Deadline: A good all-rounder, especially for mixed workloads
  • NOOP: Simple and effective for SSDs
  • BFQ (Budget Fair Queueing): The new kid on the block, promising better latency
  • mq-deadline: Multi-queue version of Deadline
  • Kyber: Designed for fast storage and multi-queue setups

Each of these schedulers has its strengths and weaknesses. The trick is finding the right one for your specific hardware and workload.

Identifying Your Current Scheduler

Before we start tweaking, let's see what scheduler you're currently using. Run this command:

$ cat /sys/block/sda/queue/scheduler

You'll see something like this:

[mq-deadline] kyber bfq none

The scheduler in brackets is the one currently in use.

Choosing the Right Scheduler for Modern Storage

If you're running SSDs or NVMe drives, you might want to consider NOOP, Kyber, or even "none" (which essentially bypasses the scheduler). Here's a quick guide:

  • For SSDs: NOOP or "none"
  • For NVMe: Kyber or "none"
  • For mixed SSD/HDD setups: BFQ or mq-deadline

Tuning Your Chosen Scheduler

Let's say you've decided to go with Kyber for your NVMe drive. Here's how you can tune it:

$ echo "kyber" > /sys/block/nvme0n1/queue/scheduler
$ echo 2 > /sys/block/nvme0n1/queue/iosched/read_lat_nsec
$ echo 10000 > /sys/block/nvme0n1/queue/iosched/write_lat_nsec

This sets Kyber as the scheduler and adjusts the target latency for read and write operations.

Pro Tip: Always benchmark before and after making changes. What works for one system might not work for another.

The IOPS Showdown: Benchmarking Your Changes

Now that we've made some changes, let's see if they actually make a difference. We'll use fio for benchmarking:

$ fio --name=random-write --ioengine=posixaio --rw=randwrite --bs=4k --size=4g --numjobs=1 --runtime=60 --time_based --end_fsync=1

Run this before and after your changes to see the impact.

Beyond Schedulers: Other Tuning Options

IO schedulers are just the tip of the iceberg. Here are some other areas you can explore:

  • I/O Queue Depth: Adjust with nr_requests
  • Read-ahead: Tune with read_ahead_kb
  • I/O Priorities: Use ionice for fine-grained control

The Gotchas: What to Watch Out For

Before you go scheduler-crazy, keep these points in mind:

  • Changing schedulers can impact application behavior
  • Some changes may require a reboot to take effect
  • Always test thoroughly in a non-production environment first

Wrapping Up: The Future of IO Scheduling

As storage technologies continue to evolve, so too will IO schedulers. Keep an eye on developments like:

  • Blk-mq (Multi-queue Block IO Queueing Mechanism)
  • IO_uring for asynchronous I/O
  • Zoned Namespace (ZNS) for NVMe SSDs

These technologies are shaping the future of storage performance in Linux.

Food for Thought

As we wrap up, here's something to ponder: With storage becoming increasingly fast, are traditional IO schedulers becoming obsolete? Or will they evolve to handle new challenges we haven't even thought of yet?

Remember, the best IO scheduler is the one that works best for your specific use case. Don't be afraid to experiment, benchmark, and find the perfect fit for your system. Happy tuning!

Closing Thought: In the world of IO scheduling, there's no one-size-fits-all solution. It's all about finding the right balance between performance, latency, and fairness for your specific workload.