Advanced Data Guarding Option Software Reference Guide

Overview of RAID ADG
1-2 Compaq Advanced Data Guarding Option Software Reference Guide
COMPAQ CONFIDENTIAL
Writer: John Turner File Name: b-ch1 overview of RAID ADG
Codename: adg2 Part Number: 283954-001 Last Saved On: 5/22/02 12:41 PM
Theoretically, RAID ADG can support a logical volume that spans over 200 hard
drives without any great risk of data loss. However, currently existing array
controllers and storage systems support no more than 14 drives per channel. Thus, the
largest obtainable logical volume, using a 4-channel controller, spans 56 hard drives.
Comparing RAID ADG to Other RAID Methods
Table 1-1 summarizes the most important features of the different RAID methods
that are supported by Compaq array controllers. For more detailed information, refer
to the user guide for the Smart Array controller that is installed in the system.
Table 1-1: Comparison of RAID Methods
RAID 0 RAID 1+0 RAID 5 RAID ADG
Alternative name Striping (no
fault tolerance)
Mirroring Distributed
Data Guarding
Advanced Data
Guarding
Usable drive space* 100% 50% 67% to 93% 50% to 96%
Usable drive space
formula
n n/2 (n-1)/n (n-2)/n
Minimum number of
physical drives
1 2 3 4
Tolerates failure of
one physical drive?
No Yes Yes Yes
Tolerates
simultaneous failure
of more than one
physical drive?
No Only if no two
failed drives are
in a mirrored pair
No Yes
Read performance High High High High
Write performance High Medium Low Low
Relative cost Low High Medium Medium
*Values for usable drive space are calculated with these assumptions: (1) All physical drives in the
array have the same capacity; (2) Online spares are not used; (3) No more than 14 physical drives
are used per array for RAID 5; (4) No more than 56 drives are used with RAID ADG.