Ncq; Smart Mode - Accusys ExaSAN User Manual

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.
S.M.A.R.T. is a monitoring system of disk drives to detect and report on various indicators of
reliability, in the hope of anticipating failure. ExaSAN RAID system supports S.M.A.R.T. Once
this function is selected, you can select the check interval from the drop-down list. Choose from
1 minute to 8 hours for SMART Mode to be active. The RAID controller will command each hard
disk to perform S.M.A.R.T. according to the check interval selected. The check results will be
shown as an Event message in the main menu.
When running S.M.A.R.T., the performance of the system will be slightly affected. The higher
the check frequency, the more the sequential access is influenced. It is recommended to turn off
S.M.A.R.T. if high frequency is needed. On the other hand, running S.M.A.R.T. constantly
allows you to monitor the conditions of hard disks at any time.
Different hard disk models and brands will have different attributes in the reporting of S.M.A.R.T.
Event message will show all messages generated by the hard disk.
User Guide

4.1.6.2 NCQ

NCQ concept

4.1.6.3 SMART Mode

5. RAID