Smart Array Controller Technology: Drive Array Expansion and Extension
7
RAID migration
Administrators may perform volume extension in order to increase raw data
storage capacity, to improve performance by increasing the number of
spindles in a logical drive volume , or to change fault-tolerance (RAID)
configurations. With ACU or ACU-XE, an administrator can reconfigure a
logical drive to a new RAID level online, without disrupting system operation or
causing data loss. The following situations would require additional drive
space: changing RAID 0 to RAID 1, 5, or RAID ADG; or changing RAID 5 or RAID
ADG to RAID 1. Table 1 summarizes the RAID levels and the amount of space
required for each type of fault tolerance.
table 1. Summary of RAID methods
RAID
0
(striping)
RAID 1
(mirroring)
RAID 5
(distributed
data
guarding)
RAID ADG
(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
drives
1 2 3
4
Tolerant of
single drive
failure?
No Yes Yes
Yes
Tolerant of
multiple
simultaneous
drive failures?
No
If failed
drives 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
*
The values for usable drive space are calculated assuming a maximum of 14
physical drives of the same capacity (or a maximum of 56 for RAID ADG) with no
online spares. HP 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.
For more information about RAID levels, refer to the
Smart Array Controller User
Guide
, or the document titled
Assessing RAID ADG vs. RAID 5 vs. RAID 1+0
.
Both are available from the
smart array controller webpage
.
5
5
The Smart Array controller webpage is available at the following URL:
http://www.compaq.com/products/servers/proliantstorage/arraycontrollers/docs/index.html
