WebPAM User v1.5 - Promise Technology, Inc.

Cache Settings
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Chapter 6: Technology Background
There is a data cache on the FastTrak controller and another one on each
physical disk drive. A cache holds data in volatile memory during RAID
management and data transfer activity. The right combination of cache settings
for your needs can improve FastTrak’s efficiency and performance.
Physical Drive Write Cache – Activates the write cache on the disk drive.
Write Back Cache – Activates write-back feature of FastTrak’s controller cache.
A write back cache holds data after an I/O operation and tells the controller the
data has been written. The data is written as soon as the disk drive is no longer
busy.
This action increases availability of the logical drive. If the power fails before the
data is written to the disk drive, the data will be lost.
The alternative is a write-through cache policy, which reports that the data has
been written to the disk only when it actually has.
To make Cache settings, see “Create a Logical Drive” on page 62 and “Logical
Drive Settings” on page 68.
Migration and Expansion
Migration is the process of changing the RAID level of an existing logical drive.
Expansion is the process of adding more physical drives to a logical drive while
keeping the same RAID level. See “Logical Drive Migration and Expansion” on
page 69 for instructions how to migrate or expand a logical drive.
Migration and Expansion are not available for JBOD.
The Migration feature applies to certain combinations of RAID level and number
of physical drives, as described in the table below.
From To Result
RAID 0: 1 or 2 drives RAID 5: 3 or 4 drives Increased performance
RAID 10: 4 drives
and capacity, adds
redundancy
RAID 0: 3 drives RAID 5: 4 drives Increased performance
and capacity, adds
redundancy
RAID 1: 2 drives RAID 5: 3 or 4 drives Increased performance
RAID 10: 4 drives
and capacity
RAID 10: 4 drives RAID 5: 4 drives Increased capacity