Advanced Data Guarding User Guide

1-2 Advanced Data Guarding for the Smart Array 5300 Controller Reference Guide
Compaq Confidential – Need to Know Required
Writer: John Turner Project: Advanced Data Guarding for the Smart Array 5300 Controller Reference Guide Comments: final draft
Part Number: 188270-xx1 File Name: b-ch1 RAID ADG.doc Last Saved On: 12/11/00 12:04 PM
The main advantages of RAID ADG are:
■
High read performance
■
High data availability – any two drives can fail without loss of critical
data
The only significant disadvantage is a relatively low write performance (lower
than RAID 5), due to the need for two sets of parity data.
The following table summarizes the most important features of the different
varieties of RAID available. The decision chart in Figure 1-2 may help you
determine which RAID option is best for your situation.
Table 1-1
Summary of RAID Features
RAID 0 RAID 1 / RAID 0+1 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 hard drives
12 34
Tolerant of single
hard drive failure?
No Yes Yes Yes
Tolerant of multiple
simultaneous hard
drive failure?
No In RAID 0+1, only
when the failed
drives are not
mirrored to each
other
No Yes
Read performance High High High High
Write performance High Medium Low Low
Relative cost Low High Medium Medium
*Note: The value for usable drive space is calculated assuming a maximum of 14 hard drives of the same
capacity (or a maximum of 56 for RAID ADG) with no online spares. Compaq recommends that these
maximum figures (excluding any allowable online spares) are not exceeded when configuring a drive array,
due to the increased likelihood of logical drive failure with more hard drives.