RAID Types Explained
Below is a description of the different types of RAID that most commonly used in SAN storage arrays.
RAID-0: RAID-0 is called disk "striping". All the data is spread out in chunks across all the disks in the RAID set. RAID-0 has great performance, because you spread out the load of storing data onto more physical drives. There is no parity generated for RAID-0. Therefore there is no overhead to write data to RAID-0 disks. RAID-0 is only good for better performance, and not for high availability, since parity is not generated for RAID-0 disks. RAID-0 requires at least two physical disks.
RAID-1: RAID-1is called disk mirroring. All the data is written to at least two separate physical disks. The disks are essentially mirror images of each other. If one of the disks fails, the other can be used to retrieve data. Disk mirroring is good for very fast read operations. It's slower when writing to the disks, since the data needs to be written twice. RAID-1 requires at least two physical disks.
RAID 1+0: RAID 1+0, which is also called RAID-10, uses a combination of disk mirroring and disk striping. The data is normally mirrored first and then striped. Mirroring striped sets accomplishes the same task, but is less fault tolerant than striping mirror sets. If you lose a drive in a stripe set, all access to data must be from the other stripe set, because stripe sets have no parity. RAID 0+1 requires a minimum of four physical disks.
RAID-2: RAID-2 is no longer used.
RAID-3: RAID-3 uses something called a "parity disk" to store the parity information generated by the RAID controller on a separate disk from the actual data disks, instead of striping it with the data as in RAID-5. This RAID type is not currently used very often, because it performs poorly when there are a lot of little requests for data, as in a database. This type performs well under applications that just want one long sequential data transfer. Applications like video servers work well with this RAID type. RAID-3 requires a minimum of three physical disks.
RAID-4: RAID-4 is good for sequential data access, but is not used much.
RAID-5: RAID-5 uses disk striping with parity. The data is striped across all the disks in the RAID set, along with the parity information needed to reconstruct the data in case of disk failure. RAID-5 is the most common method used, since it achieves a good balance between performance and availability. RAID-5 requires at least three physical disks.
Adaptive RAID: Adaptive RAID lets the RAID controller figure out how to store the parity on the disks. It will choose between RAID-3 and RAID-5, depending on which RAID set type will perform better with the type of data being written to the disks.
RAID-6: RAID-6 increases reliability by utilizing 2 parity stripes, which allows for 2 disk failures within the RAID set before data is lost. RAID-6 is seen in SATA environments, and solutions that require long data retention periods, such as data archiving, or disk-based backup.
Storage specific terms
- RAID – Redundant Array of Inexpensive Disks, also known as Redundant Array of Independent Disks.
- Disk subsystem – A general term that refers to the disks on the server.
- Spindle – Spindles are another way to refer to the physical disk drives that make up the RAID array.
- I/O Ops – Input/Output operations, usually measured per second.
- Queuing – Number of I/O Ops that are pending completion by the disk subsystem.
- SAN – Storage area networks are collections of storage devices and fibre switches connected together along with the servers that access the storage on the device. SAN has also become a generic term, which refers to the physical storage drives such as EMC.
- LUN – Logical Unit Number – This is the identification number assigned to a volume when created on a SAN device.
- Physical drive – How Windows sees any RAID array, single drive or LUN that is attached to the server.
- Logical drive – How Windows presents drives to the user (C:, D:, E:, etc.).
- Block size – The amount of data read from the spindles in a single read operation. This size varies per vendor from 8 KB to 256 MB.
- Hardware array – A RAID array created using a physical RAID controller.
- Software array – A RAID array created within Windows using the computer management snap-in.
- Hot spare – A spindle that sits in the drive cage and is added to the array automatically in the event of a drive failure. While this does not increase capacity, it does reduce the amount of time that the array is susceptible to data loss because of a second failed drive.
- Recovery time – Amount of time needed for the RAID array to become fully redundant after a failed drive has been replaced, either manually or automatically