Accusys ExaSAN A12S2-PS - Manual (Page 75)

4. How to Use
User Guide Page 4-24
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.