Tuning I/O resources

Tuning disk I/O-bound systems

If the system is I/O bound because of disk activity, there are a number of things that can be done:

Replace the existing disks with faster versions.
Filesystems that are used to hold temporary files can be implemented as ramdisks in memory. (See ramdisk(HW) for more information.) This has the disadvantage of taking memory away from applications but it can be extremely effective in improving I/O throughput.
Upgrade the disk controller to a type that supports block or track caching, and scatter-gather read/writes.
For SCSI disks, upgrade the host adaptor to one that supports caching, scatter-gather, and tagged command queuing. Where possible, use fast SCSI subsystems with wide data paths.
If the system is running a disk-intensive application such as a database, having multiple host adapters (for SCSI), disk controllers and disks will help speed up access to data by reducing contention.
Spread filesystems and swap areas across different disks and/or buses to help spread the load. Alternatively, you can use hardware RAID or virtual disk software to balance the load across several disks. For a comparison of the various configurations that are available using virtual disks see ``About virtual disks''.

You may find that the performance of the system can be improved slightly by increasing the values of the BDFLUSHR and NAUTOUP kernel parameters. This will reduce the number of times the disk will be accessed because blocks can be updated more often in memory before they are written to the disk. The inherent risk is that more data will be lost if the system crashes because it will be longer since it was last written to the disk. It is considered good practice to protect mission-critical systems against power failure using a UPS or similar device.

Various disk organization strategies are discussed in ``Overcoming performance limitations of hard disks'' which includes suggestions for optimizing your current hardware configuration.

Disk manufacturers implement various hardware and firmware (software in the disk controller) strategies to improve disk performance. These include track caching and varying the number of disk blocks per track across the disk surface. Usually, you have no control over such features.