Figure 60: Ncq Concept - Accusys ExaSAN A08S3-PS User Manual

In the event of performance degradation or delay of a single drive due to aging, the ExaSAN
RAID system reads both data and parity stripes concurrently. It bypasses the slow reads and
returns data to the host with the regenerated data and to provide stabled performance based on
the RAID parity.
Note
Although slow responds of some hard disks can be tolerated in DLP
mode, you may use S.M.A.R.T. function to check the conditions of hard
disks in an array and replace the faulty ones.
Note
This function does not support JBOD and NRAID.
NCQ (Native Command Queuing) – This mode is designed for increasing disk drive access on
SCSI and SATA disk drives. It can do this under certain situations by allowing the individual
hard disk to internally optimize the order in which it executes the read and write requests it
receives. This can reduce the amount of unnecessary drive head movement and resulting in
better performance for work loads where multiple read/write requests and outstanding at the
same time.
This situation most often occurs in server-type applications. However, the current technology
actually slows down disk drive access in certain applications, such as video editing and
sequential reads and writes, because of the added latency induced by NCQ.
The figure above illustrates the access sequence in NCQ and non-NCQ mode. The content
sequences of the two hard disks are the same: 1, 2, 3, 4. However, the access sequence in
NCQ mode may vary to improve the performance.
Note
If Equalization mode is enabled, NCQ is automatically turned off.
User Guide

Figure 60: NCQ concept

4. How to Use
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