Dell PERC 4/SI - Manual (Page 20)

Maximizing Performance
A RAID disk subsystem improves I/O performance. The RAID array appears to the host computer as a single storage unit or as multiple logical units. I/O is
faster because drives can be accessed simultaneously.
Table 2 -11 describes the performance for each RAID level.
Table 2-11. RAID Levels and Performance
Maximizing Storage Capacity
Storage capacity is an important factor when selecting a RAID level. There are several variables to consider. Mirrored data and parity data require more
storage space than striping alone (RAID 0). Parity generation uses algorithms to create redundancy and requires less space than mirroring.
Table 2 -12
explains the effects of the RAID levels on storage capacity.
Table 2-12. RAID Levels and Capacity
RAID Availability
that requires fault tolerance and minimal capacity.
5 Combines distributed parity with disk striping. Parity provides redundancy for one drive failure without duplicating the contents of entire disk
drives. If a drive fails, the RAID controller uses the parity data to reconstruct all missing information. In RAID 5, this method is applied to entire
drives or stripes across all disk drives in an array. Using distributed parity, RAID 5 offers fault tolerance with limited overhead.
10 Provides complete data redundancy using striping across spanned RAID 1 arrays. RAID 10 works well for any environment that requires the 100
percent redundancy offered by mirrored arrays. RAID 10 can sustain a drive failure in each mirrored array and maintain drive integrity.
50 Provides data redundancy using distributed parity across spanned RAID 5 arrays. RAID 50 includes both parity and disk striping across multiple
drives. If a drive fails, the RAID controller uses the parity data to recreate all missing information. RAID 50 can sustain one drive failure per RAID 5
array and still maintain data integrity.
RAID
Level
Performance
0 RAID 0 (striping) offers the best performance of any RAID level. RAID 0 breaks up data into smaller blocks, then writes a block to each drive in the
array. Disk striping writes data across multiple disk drives instead of just one disk drive. It involves partitioning each drive storage space into stripes
that can vary in size from 8 KB to 128 KB. These stripes are interleaved in a repeated sequential manner. Disk striping enhances performance
because multiple drives are accessed simultaneously.
1 With RAID 1 (mirroring), each drive in the system must be duplicated, which requires more time and resources than striping. Performance is
impaired during drive rebuilds.
5 RAID 5 provides high data throughput, especially for large files. Use this RAID level for any application that requires high read request rates, but
low write request rates, such as transaction processing applications, because each drive can read and write independently. Since each drive
contains both data and parity, numerous writes can take place concurrently. In addition, robust caching algorithms and hardware based exclusive-
or assist make RAID 5 performance exceptional in many different environments.
Parity generation can slow the write process, making write performance significantly lower for RAID 5 than for RAID 0 or RAID 1. Disk drive
performance is reduced when a drive is being rebuilt. Clustering can also reduce drive performance. Environments with few processes do not
perform as well because the RAID overhead is not offset by the performance gains in handling simultaneous processes.
10 RAID 10 works best for data storage that need the enhanced I/O performance of RAID 0 (striped arrays), which provides high data transfer rates.
Spanning increases the size of the logical volume 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 eight.) 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 array.
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 size of the logical volume 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 eight.) 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 array.
RAID
Level
Capacity
0 RAID 0 (disk 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 physical disks.
1 With RAID 1 (mirroring), data written to one disk drive is simultaneously written to another disk drive, which doubles the required data storage
capacity. This is expensive because each drive in the system must be duplicated.
5 RAID 5 provides redundancy for one drive failure without duplicating the contents of entire disk drives. RAID 5 breaks up data into smaller blocks,
calculates parity by performing an exclusive-or on the blocks, then writes the blocks of data and parity to each drive in the array. The size of each
block is determined by the stripe size parameter, which is set during the creation of the RAID set.
10 RAID 10 requires twice as many drives as all other RAID levels except RAID 1. RAID 10 works well for medium-sized databases or any environment
that requires a higher degree of fault tolerance and moderate to medium capacity. Disk spanning allows multiple disk drives to function like one big
drive. Spanning overcomes lack of disk space and simplifies storage management by combining existing resources or adding relatively inexpensive
resources.
50 RAID 50 requires two to four times as many parity drives as RAID 5. This RAID level works best when used with data that requires medium to large
capacity.