I/O schedulers

SUSE Linux Enterprise Server 11 SP1 provides a number of I/O scheduler alternatives to optimize for different I/O usage patterns. You can use the elevator= option at boot time to set the scheduler for I/O devices or you can assign a specific I/O scheduler to individual block devices.

Completely Fair Queuing (CFQ) scheduler

The Completely Fair Queuing (CFQ) scheduler is the default I/O scheduler for SUSE Linux Enterprise Server 11 SP1. The CFQ scheduler maintains a scalable per-process I/O queue and attempts to distribute the available I/O bandwidth equally among all I/O requests. The effort balancing of I/O requests has some CPU costs.

Deadline scheduler

The Deadline scheduler is one alternative to the CFQ scheduler. The deadline scheduler uses a deadline algorithm to minimize I/O latency by attempting to guarantee a start time for an I/O request. The scheduler attempts to be fair among multiple I/O requests and to avoid process starvation. This scheduler wills aggressively re-order requests to improve I/O performance.

NOOP scheduler

The NOOP scheduler is another alternative, that can help minimize the costs of CPU utilization of managing the I/O queues. The NOOP scheduler is a simple FIFO queue that uses the minimal amount of CPU/instructions per I/O operation to accomplish the basic merging and sorting functionality to complete the I/O operations.

I/O scheduler test results

In this test, twenty-three of the spinning hard disks attached to System B are replaced by fifty-two 73 GB SAS SSD devices for main database space.

Table 1 compares the database performance with the 3 different I/O schedulers using all of the workloads. We see that the in the mixed read/write workloads (2 and 4) the NOOP scheduler has a negative impact on performance and so should not be used for these workloads. The Deadline I/O scheduler shows a performance benefit when run against the smaller workloads on lesser performing storage while having no impact on the larger workloads with better performing storage.

Note: Values shown in the following table are the Performance metric result.
Table 1. Comparing I/O schedulers
Scheduler Workload 1 Workload 2 Workload 3 Workload 4
CFQ 100 100 100 100
Deadline 105 105 100 100
NOOP 107 95 100 98

I/O scheduler conclusions

For the workloads used in this test, the Deadline scheduler was a better choice overall than the default CFQ scheduler. It yielded a higher throughput in the smaller workloads and performed equally to the CFQ scheduler in the larger workloads. While these particular workloads performed best with the deadline scheduler, not all workloads benefit equally from the same scheduler. If optimal performance is an requirement, it is worthwhile to investigate the benefits of each I/O scheduler for your workloads.