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Introduction | Origins of Raid | Original Raid Levels | Raid Today

Raid Summary:

Raid 0: Striped Disk Array | Raid 1: Mirrored Disk Array | Raid 10: Mirroring and Striping | Raid 0,1: Striping and Mirroring | Raid 3: Striping with dedicated parity disk | Raid 5: Striping and Parity

Comparing Raid Configurations | Raid Set-up Considerations

IV. RAID Today
The most widely used RAID levels today in 2002 are:
RAID 0: Stripping
RAID 1: Mirroring
RAID 10 Mirroring and Striping
RAID (0+1) Striping and Mirroring
RAID 3: Striping with Dedicated Parity Disk
RAID 5: Stripping with Distributed Parity
As noted eailier, the different RAID levels offer a variety of performance, data availabilfty, and data integrity depending on the specific I/O environment, however ft is important to remember:
• RAID levels are not progressive - In other words, increasing the RAID level from 0 to 1 to 2 to 3 etc. does not give progressively better data integrity, performance, or cost. Each RAID level is independent and the numbering is arbitrary (use of the term RAID level creates some confusion).
• Not all RAID levels are redundant- RAID 0 provides no data redundancy, in fact, it is more prone to data loss than individual disk drives, because if any drive fails in a RAID 0 group, all data is lost.
• There are no standards for RAID - Each vendor has its own implementations, and may use different
terminology. Some vendors have invented their own RAID terminology (e.g., EMC’s RAID-S and Storage
Computer’s RAID 7). Vendors who claim to implement RAID 3 are actually implementing a modified RAID
3. Combinations such as 10, 0+1, and 53 are all vendor defined. Storage users must be closely examine
RAID implementations.
• RAID can be implemented in various places in the computer system - The storage devices, the Host Bus Adapter (HBA) and the host operating system (e.g., Windows 2000) can all implement RAID. It is possible to use a combination of these, for example, RAID 0 (striping) in the storage array combined with RAID 1 (mirroring) in the operating system. Each location has benefits and shortcomings, which need to be, understood by computer system architects.
• Physical vs. Logical drive numbering- Physical numbering refers to the physical components in the storage array. Logical (or virtual) numbering refers to the “disks” or “volumes” that the host operating system “sees” in the storage device. These two can be very confusing to new storage users.
• Logical disks do not always map I-to-I with physical disks - In RAID, several physical disk drives (or portions of several physical drives) can be grouped into a logical disk or Logical UNit (LUIV). Each LUN can be broken into logical blocks of 512 bytes each, numbered 0 through “n” (the Logical Block Number or LBIV). For example, a 100GB LUN has approx. 200,000,000 logical blocks.
• Logical volumes are very similar to logical drives - A logical volume is composed of one or
several logical drives, the member logical drives can be the same RAID level or different RAID levels.
The logical volume can be divided into partitions. During operation, the host sees a non-partitioned logical volume or a partition of a partitioned logical volume as one single physical drive.