Avago Technologies MegaRAID Fast Path Software - Manual (Page 43)

LSI Corporation Confidential | July 2011 Page 43
MegaRAID SAS Software User Guide Chapter 2: Introduction to RAID | RAID Configuration Strategies
2.6.3 Maximizing Storage Capacity Storage capacity is an important factor when selecting a RAID level. There are several
variables to consider. Striping alone (RAID 0) requires less storage space than mirrored
data (RAID 1) or distributed parity (RAID 5 or RAID 6). RAID 5, which provides
redundancy for one drive failure without duplicating the contents of entire drives,
requires less space than RAID 1.
Table 17 explains the effects of the RAID levels on
storage capacity.
10 RAID 10 works best for data storage that need the enhanced I/O performance of RAID 0 (striped drive groups), which provides high
data transfer rates. Spanning increases the capacity of the virtual drive and improves performance by doubling the number of spindles.
The system performance improves as the number of spans increases. (The maximum number of spans is 8.) As the storage space in the
spans is filled, the system stripes data over fewer and fewer spans, and RAID performance degrades to that of a RAID 1 or RAID 5 drive
group.
50 RAID 50 works best when used with data that requires high reliability, high request rates, and high data transfer. It provides high data
throughput, data redundancy, and very good performance. Spanning increases the capacity of the virtual drive and improves
performance by doubling the number of spindles. The system performance improves as the number of spans increases. (The maximum
number of spans is 8.) As the storage space in the spans is filled, the system stripes data over fewer and fewer spans and RAID
performance degrades to that of a RAID 1 or RAID 5 drive group.
60 RAID 60 works best when used with data that requires high reliability, high request rates, and high data transfer. It provides high data
throughput, data redundancy, and very good performance. Spanning increases the capacity of the virtual drive and improves
performance by doubling the number of spindles. The system performance improves as the number of spans increases. (The maximum
number of spans is 8.) As the storage space in the spans is filled, the system stripes data over fewer and fewer spans, and RAID
performance degrades to that of a RAID 1 or RAID 6 drive group.
RAID 60 is not well suited to tasks requiring a lot of writes. A RAID 60 virtual drive has to generate two sets of parity data for each write
operation, which results in a significant decrease in performance during writes. Drive performance is reduced during a drive rebuild.
Environments with few processes do not perform as well because the RAID overhead is not offset by the performance gains in handling
simultaneous processes.
Table 16: RAID Levels and Performance (Continued)
RAID
Level
Performance
Table 17: RAID Levels and Capacity
RAID
Level
Capacity
0 RAID 0 (striping) involves partitioning each drive storage space into stripes that can vary in size. The combined storage space is
composed of stripes from each drive.
RAID 0 provides maximum storage capacity for a given set of drives. The usable capacity of a RAID 0 array is equal to the number of
drives in the array into the capacity of the smallest drive in the array.
1 With RAID 1 (mirroring), data written to one drive is simultaneously written to another drive, which doubles the required data storage
capacity. This situation is expensive because each drive in the system must be duplicated. The usable capacity of a RAID 1 array is equal
to the capacity of the smaller of the two drives in the array.
5 RAID 5 provides redundancy for one drive failure without duplicating the contents of entire drives. RAID 5 breaks up data into smaller
blocks, calculates parity by performing an exclusive-or on the blocks and then writes the blocks of data and parity to each drive in the
drive group. The size of each block is determined by the stripe size parameter, which is set during the creation of the RAID set. The
usable capacity of a RAID 5 array is equal to the number of drives in the array, minus one, into the capacity of the smallest drive in the
array.
6 RAID 6 provides redundancy for two drive failures without duplicating the contents of entire drives. However, it requires extra capacity
because it uses two parity blocks per stripe. This makes RAID 60 more expensive to implement. The usable capacity of a RAID 6 array is
equal to the number of drives in the array, minus two, into the capacity of the smallest drive in the array.
00 RAID 00 (striping in a spanned drive group) involves partitioning each drive storage space into stripes that can vary in size. The
combined storage space is composed of stripes from each drive. RAID 00 provides maximum storage capacity for a given set of drives.