Redundant Array Of Independent Disks (RAID) is a mechanism to group hard disks to form the foundation for logical units of storage. Logical disk partitions (drives) or LUNs with file systems or RAW LUNs can be hosted on RAID groups. Every RAID group has two main characteristics: performance and fault tolerance. Having said that, it is important to understand that there are different types of RAID groups based on two principles that dictate how the data destined to the disk group will be written, and they are striping and mirroring:
- Striping refers to the act of distributing the blocks of data to be written onto all the disks in the RAID group
- Mirroring refers to the act of writing identical copies of data to more than one disk
Striping and mirroring form the basis of all the other type of RAID groups that can be created. Although, understanding every possible RAID level is beyond the scope of this book, we will review a few that are commonly used in a modern day data center. With RAID space, utilization is at a maximum if you have identically sized hard disks, or else the size of the smallest disk will become the usable segment size from logical disk units (drives) using this RAID group. For example, if you have three hard disks with the sizes 50 GB, 100 GB, and 200 GB, then the usable segment size is reduced to 50 GB, therefore leaving unused space on the 100 GB (50 GB unused) and 150 GB (100 GB unused) hard disks.
RAID-0 or striping requires a minimum of two disks. The data blocks are sequentially distributed (written) onto the available disks. This is a performance-only setup and there is zero fault tolerance, as the loss of a single disk would make the data on the disk group unusable.
RAID-1 or mirroring requires a minimum of two disks. Identical copies of the data blocks are maintained on a disk pair. This RAID is fault tolerant; it delivers a doubled READ rate and a single hard disk's WRITE performance.
RAID-10 requires a minimum of four hard disks. This RAID level is a striped group of a mirrored set of disks. It is a RAID-0 (striped) grouping of RAID-1 (mirrored) disks. That is, if there are two mirrored sets of disks then the data is striped across the mirrored set and not within the mirrored set of disks. This RAID level offers excellent performance and data redundancy making it fault tolerant.
RAID-5 requires a minimum of three hard disks. It uses striping to distribute data blocks on to the hard disks in the group, but also distributes parity information across. This RAID level can sustain the failure of a single hard disk and offers good performance.