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.
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 Levels For the exam, you should be familiar with RAID levels 0, 1, 5, and 1+0. These are the only levels that can be used with Solaris Volume Manager.
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 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 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.
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.