RAID

Objective:

Analyze and explain RAID 0, 1, 5.

When describing SVM volumes, it's common to describe which level of RAID the volume conforms to. RAID is an acronym for Redundant Array of Inexpensive (or Independent) Disks. Usually these disks are housed together in a cabinet and referred to as an array. There are several RAID levels, each referring to a method of organizing data while ensuring data resilience or performance. These levels are not ratings, but rather classifications of functionality. Different RAID levels offer dramatic differences in performance, data availability, and data integrity depending on the specific I/O environment. Table 10.1 describes the various levels of RAID.

Table 10.1. RAID Levels

RAID Level	Description
0	Striped disk array without fault tolerance.
1	Maintains duplicate sets of all data on separate disk drives (mirroring).
2	Data striping and bit interleave. Data is written across each drive in succession one bit at a time. Checksum data is recorded in a separate drive. This method is very slow for disk writes and is seldom used today since Error Checking and Correction (ECC) is embedded in almost all modern disk drives.
3	Data striping with bit interleave and parity checking. Data is striped across a set of disks one byte at a time, and parity is generated and stored on a dedicated disk. The parity information is used to re-create data in the event of a disk failure.
4	This is the same as level 3 RAID except data is striped across a set of disks at a block level. Parity is generated and stored on a dedicated disk.
5	Unlike RAID 3 and 4, where parity is stored on one disk, both parity and data are striped across a set of disks.
6	Similar to RAID 5, but with additional parity information written to recover data if two drives fail.
1+0	Combination of RAID 1 mirrors that are then striped.

RAID level 0 does not provide data redundancy, but is usually included as a RAID classification because it is the basis for the majority of RAID configurations in use. Table 10.1 described some of the more popular RAID levels; however, many are not provided in SVM. The following is a more in-depth description of the RAID levels provided in SVM.

RAID 0

Although they do not provide redundancy, stripes and concatenations are often referred to as RAID 0. With striping, data is spread across relatively small, equally sized fragments that are allocated alternately and evenly across multiple physical disks. Any single drive failure can cause the volume to fail and could result in data loss. RAID 0, especially true with stripes, offers a high data transfer rate and high I/O throughput, but suffers lower reliability and availability than a single disk.

RAID 1

RAID 1 employs data mirroring to achieve redundancy. Two copies of the data are created and maintained on separate disks, each containing a mirror image of the other. RAID 1 provides an opportunity to improve performance for reads because read requests will be directed to the mirrored copy if the primary copy is busy. RAID 1 is the most expensive of the array implementations because the data is duplicated. In the event of a disk failure, RAID 1 provides the highest performance because the system can switch automatically to the mirrored disk with minimal impact on performance and no need to rebuild lost data.

RAID 5

RAID 5 provides data striping with distributed parity. RAID 5 does not have a dedicated parity disk, but instead interleaves both data and parity on all disks. In RAID 5, the disk access arms can move independently of one another. This enables multiple concurrent accesses to the multiple physical disks, thereby satisfying multiple concurrent I/O requests and providing higher transaction throughput. RAID 5 is best suited for random access data in small blocks. There is a "write penalty" associated with RAID 5. Every write I/O will result in four actual I/O operations, two to read the old data and parity and two to write the new data and parity.

RAID 1+0

SVM supports both RAID 1+0 (mirrors that are then striped). This combines the benefits of RAID 1 for redundancy and RAID 0 for performance. If a device fails, the entire stripe or concatenation is not taken offline, only the failed devicea characteristic of 1+0.