Working with the Background Task Mode

Background Task Mode is specifically designed to address the issue of latency, which can cause problems for some applications, by helping you balance the rate of host I/O and efficient completion of background tasks.

About Latency and the Background Task Mode

Latency is the real or perceived delay in response time seen by the host system when accessing some data. For some applications, such as streaming video and audio application, it is important to minimize the latency of read commands, so that users do not perceive a lag when viewing video or listening to audio.

When background tasks are active (for example, rebuild, verify, and initialization), there can be increased latency, and under some circumstances, this can result in a degraded user experience or in an application failure.

You can set the Background Task Mode to minimize latency. This mode monitors and paces the relationship between host I/Os and background tasks. You can set the Background Task Mode to either Adaptive (the default) or Low Latency.

Low Latency mode minimizes the latency (delay) in reading data from the RAID unit by slowing down the background task process. In contrast, Adaptive mode allows the firmware to adjust the interaction of background tasks with host I/Os to maximize the speed of both host I/O and background tasks.

If latency is not an issue in the applications you use, then using the Adaptive Background Task Mode will probably meet your needs and will result in background tasks that complete faster.

Relationship of Task Mode and Task Rate

The Background Task Mode works in conjunction with the Background Task Rate.

Show Background Task Settings on Controller Settings page

Background Task Rate lets you set the relative performance of background tasks in relation to host I/O activity on a scale of 1 to 5, with 5 being the fastest background task and slowest I/O, while 1 is the fastest I/O and the slowest background task. Separate settings are available for Rebuild/Migrate and for Verify. (Initialization occurs at the Rebuild/Migrate rate.)

5 = fastest background task; slowest host I/O

4 = faster background task; slower host I/O

3 = balanced between background tasks and host I/O

2 = faster host I/O; slower background task

1 = fastest host I/O; slowest background task

If you set the Background Task Mode to Low Latency, it slows down the background task process in the context of the current Task Rate setting, with a graduated pacing of host I/O versus background task I/O, 2 through 5.

Important: It is recommended that if you use the Background Task Mode of Low Latency, you always set the Background Task Rate to at least 2 on the scale of 1 to 5. When used in conjunction with a rate of 1 (fastest host I/O; slowest background task), Low Latency Mode can create a situation in which background tasks never complete, if there is continuous host I/O activity on the unit.

If you do select Low Latency when the Rate is set to 1, 3DM2 will display a warning, and will then continue to display a recommendation to change it until you change either the Rate to 2 or greater, or until you change the Mode to Adaptive.

Show Low Latency Mode with warning when Rate=1


	Note: The Background Task Rate and Background Task Mode settings are only relevant when there is host I/O activity. When there is no host activity, background tasks (rebuild/migrate, initialization, and verify) always run at the maximum.

The table below provides an overview of the impact of selecting different Task Rate and Task Mode settings.

Selecting Appropriate Background Task Rate and Mode
Your priority	Rebuild Task Rate	Task Mode	Comments
Fastest rebuild time	5	Adaptive	Potential for higher latency on host I/Os
Balanced host I/O and rebuild tasks	3	Adaptive	Default setting
Minimized host I/O latency	2	Low Latency	Background tasks will complete, but will be slow when there are host I/Os
Minimum host I/O latency	1	Low Latency	Background tasks may never finish when there are continuous host I/Os. For example, a rebuild may never complete.