What is RAID?

RAID stands for Redundant Array of Inexpensive (or sometimes “Independent”) Disks. RAID is a method of combining several hard drives into one logical unit. It can offer fault tolerance and higher throughput levels than a single hard drive or group of independent hard drives.

The basic idea of RAID is to combine multiple inexpensive disk drives into an array of disk drives to obtain performance, capacity and reliability that exceeds that of a single large drive. The array of drives appears to the host computer as a single logical drive.

The Mean Time Between Failure (MTBF) of the array is equal to the MTBF of an individual drive, divided by the number of drives in the array. Because of this, the MTBF of a non-redundant array (RAID 0) is too low for mission-critical systems. However, disk arrays can be made fault-tolerant by redundantly storing information in various ways.

You may also want to review our Glossary of RAID Terminology

The Benefits of RAID

• Provides real-time data recovery with uninterrupted access when a hard drive fails.
• Increases system uptime and network availability.
• Protects against data loss.
• Multiple drives working in parallel increase system performance.

Types of RAID Architectures

There are five basic types of array architectures that were originally defined as RAID 1 through RAID 5. Each provides disk fault-tolerance but with different compromises in features and performance. In addition to these five redundant array architectures, it has become popular to refer to a non-redundant array of disk drives as a RAID 0 array.

RAID Levels (See below for detailed descriptions)

	RAID 0	RAID 1	RAID 0/1	RAID 5
Description	Data striping (no data protection)	Disk mirroring	RAID 0 and RAID 1 combined	Data striping with distributed parity
Minimum # of Drives	2	2	4	3
Benefit	Highest performance	Data protection through redundancy	Highest performance with data protection	Best balance of cost / performance / data protection

Raid Types

	Description	Advantages
Software-based RAID	Included in NOSs such as NetWare and Windows NT®. All RAID functions are handled by the host CPU which can severely tax its ability to perform other computations.	Low price Only requires a standard controller.
Hardware-based RAID	Processor-intensive RAID operations are off-loaded from the host CPU to enhance performance.	Data protection and performance benefits of RAID More robust fault-tolerant features and increased performance versus software-based RAID.
External Hardware RAID Card	Connects to the server via a standard controller. RAID functions are performed on a microprocessor located in the external RAID storage subsystem.	OS independent Build super high-capacity storage systems for high-end servers.

Disk Striping

Fundamental to RAID technology is striping. This is a method of combining multiple drives into one logical storage unit, or “stripe”. Striping partitions the storage space of each drive into stripes, which can be as small as one sector (512 bytes) or as large as several megabytes. These stripes are then interleaved in a rotating sequence, so that the combined space is composed alternately of stripes from each drive. The specific type of operating environment determines whether large or small stripes should be used.

Most operating systems today support concurrent disk I/O operations across multiple drives. However, in order to maximize throughput for the disk subsystem, the I/O load must be balanced across all the drives so that each drive can be kept busy as much as possible. In a multiple drive system without striping, the disk I/O load is never perfectly balanced. Some drives will contain data files that are frequently accessed and some drives will rarely be accessed.

By striping the drives in the array with stripes large enough so that each record falls entirely within one stripe, most records can be evenly distributed across all drives. This keeps all drives in the array busy during heavy load situations. This situation allows all drives to work concurrently on different I/O operations, and thus maximize the number of simultaneous I/O operations that can be performed by the array.

RAID Level Definitions

RAID 0 is typically defined as a group of striped disk drives without parity or data redundancy. RAID 0 arrays can be configured with large stripes for multi-user environments or small stripes for single-user systems that access long sequential records. RAID 0 arrays deliver the best data storage efficiency and performance of any array type. The disadvantage is that if one drive in a RAID 0 array fails, the entire array fails.

RAID Level 0 is not redundant, hence does not truly fit the “RAID” acronym. In Level 0, data is split across drives, resulting in higher data throughput. Since no redundant information is stored, performance is very good, but the failure of any disk in the array results in the loss of all data. This level is commonly referred to as striping.

RAID 1, also known as disk mirroring, is simply a pair of disk drives that store duplicate data but appear to the computer as a single drive. Although striping is not used within a single mirrored drive pair, multiple RAID 1 arrays can be striped together to create a single large array consisting of pairs of mirrored drives. All writes must go to both drives of a mirrored pair so that the information on the drives is kept identical. However, each individual drive can perform simultaneous, independent read operations. While mirroring doubles the read performance of a single non-mirrored drive, the write performance is unchanged. RAID 1 delivers the best performance of any redundant array type, and in addition, there is less performance degradation during drive failure than in RAID 5 arrays.

Thus, RAID Level 1 is more commonly referred to as mirroring with 2 hard drives. It provides redundancy by way of the duplication of all data from one drive to another drive. Performance of a Level 1 array is slightly better than a single drive, and if either drive fails, no data is lost. This is a good entry-level redundant system, since only two drives are required. However, since one drive is used to store a duplicate of the data, the cost per megabyte is doubled. See also Disk Duplexing.

RAID 2 arrays sector-stripe data across groups of drives, with some drives assigned to store ECC information. Because all disk drives today embed ECC information within each sector, RAID 2 offers no significant advantages over other RAID architectures and is not supported by most popular RAID controller manufacturers such as Adaptec.

RAID Level 2 uses Hamming error correction coding, and is intended for use with drives which do not have built-in error detection. All SCSI drives support built-in error detection, therefore this level would be of little use when using SCSI drives.

RAID 3, as with RAID 2, sector-stripes data across groups of drives, but one drive in the group is dedicated to storing parity information. RAID 3 relies on the embedded ECC in each sector for error detection. In the event of drive failure, data recovery is accomplished by calculating the exclusive OR (XOR) of the information recorded on the remaining drives. Records typically span all drives, which optimizes the disk transfer rate. Because each I/O request accesses every drive in the array, RAID 3 arrays can satisfy only one I/O request at a time. RAID 3 delivers the best performance for single-user, single-tasking environments with long records. Synchronized-spindle drives are required for RAID 3 arrays in order to avoid performance degradation with short records. Because RAID 5 arrays with small stripes can yield similar performance to RAID 3 arrays, RAID 3 is not supported by most popular RAID controller manufacturers such Adaptec.

RAID Level 3 stripes data at a byte level across several drives, with parity stored on one drive. It is otherwise similar to level 4. Byte-level striping requires hardware support for efficient use.

RAID 4 is identical to RAID 3 except that large stripes are used, so that records can be read from any individual drive in the array (except the parity drive). This allows read operations to be overlapped. However, since all write operations must update the parity drive, they cannot be overlapped. This architecture offers no significant advantages over other RAID levels and is not supported by most RAID controller manufacturers such as Adaptec.

RAID Level 4 stripes data at a block level across several drives, with parity stored on one drive. The parity information allows recovery from the failure of any single drive. The performance of a level 4 array is very good for reads (the same as level 0). Writes, however, require that parity data be updated each time. This slows small random writes, in particular, though large writes or sequential writes are fairly fast. Because only one drive in the array stores redundant data, the cost per megabyte of a level 4 array can be fairly low.

RAID 5, sometimes called a Rotating Parity Array, avoids the write bottleneck caused by the single dedicated parity drive of RAID 4. Under RAID 5 parity information is distributed across all the drives. Since there is no dedicated parity drive, all drives contain data and read operations can be overlapped on every drive in the array. Write operations will typically access one data drive and one parity drive. However, because different records store their parity on different drives, write operations can usually be overlapped.

This level is more commonly referred to as striping with distributed parity. RAID Level 5 is similar to level 4, but distributes parity among the drives. No single disk is devoted to parity. This can speed small writes in multiprocessing systems. Because parity data must be distributed on each drive during reads, the performance for reads tends to be considerably lower than a level 4 array. The cost per megabyte is the same as for level 4.

In summary:

• RAID 0 is the fastest and most efficient array type but offers no fault-tolerance. RAID 0 requires a minimum of two drives. Any application which requires very high speed storage, but does not require redundancy, such as Photoshop temporary files as an example.
• RAID 1 is the best choice for performance-critical, fault-tolerant environments. RAID 1 is the only choice for fault-tolerance if no more than two drives are used. Applications which require redundancy with fast random writes; entry-level systems where only two drives are available such as small file servers.
• Level 0/1 or 10 (mirroring and striping) – Dual level raid, combines multiple mirrored drives (RAID 1) with data striping (RAID 0) into a single array. Provides highest performance with data protection.
• RAID 2 is seldom used today since ECC is embedded in all hard drives. RAID 2 is not supported by most RAID controller manufacturers.
• RAID 3 can be used to speed up data transfer and provide fault-tolerance in single-user environments that access long sequential records. However, RAID 3 does not allow overlapping of multiple I/O operations and requires synchronized-spindle drives to avoid performance degradation with short records. Because RAID 5 with a small stripe size offers similar performance, RAID 3 is not supported by most RAID controller manufacturers.
• RAID 4 offers no advantages over RAID 5 and does not support multiple simultaneous write operations. RAID 4 is not supported by most RAID controller manufacturers.
• RAID 5 combines efficient, fault-tolerant data storage with good performance characteristics. However, write performance (and performance during drive failure) is slower than with RAID 1. Rebuild operations also require more time than with RAID 1 because parity information is also reconstructed. At least three drives are required for RAID 5 arrays. Similar to level 4, but may provide higher performance if most I/O is random and in small chunks such as would be experienced with database servers.
• Level 0/5 or 50 – Dual level raid, combines multiple RAID 5 sets with data striping (RAID 0). Increased reliability and performance over standard RAID5 that can stand a multiple drive failure; one hard drive per RAID5 set.

Dual-Level Raid in Brief

We use Adaptec RAID Controllers, and here’s why! In addition to the standard RAID levels, Adaptec RAID controllers can combine multiple hardware RAID arrays into a single array group or parity group. In a dual-level RAID configuration, the controller firmware stripes two or more hardware arrays into a single array.

NOTE: The arrays being combined must both use the same RAID level.

Dual-level RAID achieves a balance between the increased data availability inherent in RAID 1 and RAID 5 and the increased read performance inherent in disk striping (RAID 0). These arrays are sometimes referred to as RAID 0+1 or RAID 10 and RAID 0+5 or RAID 50.

Creating Data Redundancy

RAID 5 offers improved storage efficiency over RAID 1 because only the parity information is stored, rather than a complete redundant copy of all data. The result is that three or more drives can be combined into a RAID 5 array, with the storage capacity of only one drive dedicated to store the parity information. Therefore, RAID 5 arrays provide greater storage efficiency than RAID 1 arrays. However, this efficiency must be balanced against a corresponding loss in performance.

The parity data for each stripe of a RAID 5 array is the XOR of all the data in that stripe, across all the drives in the array. When the data in a stripe is changed, the parity information is also updated. There are two ways to accomplish this: See also System Fault Tolerance.

1. The first method is based on accessing all of the data in the modified stripe and regenerating parity from that data. For a write that changes all the data in a stripe, parity can be generated without having to read from the disk, because the data for the entire stripe will be in the cache. This is known as full-stripe write. If only some of the data in a stripe is to change, the missing data (the data the host does not write) must be read from the disks to create the new parity. This is known as partial-stripe write. The efficiency of this method for a particular write operation depends on the number of drives in the RAID 5 array and what portion of the complete stripe is written.
2. The second method of updating parity is to determine which data bits were changed by the write operation and then change only the corresponding parity bits. This is done by first reading the old data which is to be overwritten. This data is then XOR’ed with the new data that is to be written. The result is a bit mask which has a 1 in the position of every bit which has changed. This bit mask is then XOR’ed with the old parity information from the array. This results in the corresponding bits being changed in the parity information. The new updated parity is then written back to the array. This results in two reads, two writes and two XOR operations. This is known as read-modify-write.

The cost of storing parity, rather than redundant data as in RAID 1, is the extra time required for the write operations to regenerate the parity information. This additional time results in slower write performance for RAID 5 arrays over RAID 1. Because Adaptec RAID controllers generate XOR in hardware, the negative effect of parity generation is primarily from the additional disk I/O required to read the missing information and write the new parity. Adaptec RAID controllers can generate parity using either the full- or partial-stripe write algorithm or the read-modify-write algorithm. The parity updated method chosen for any given write operation is determined by calculating the number of I/O operations needed for each type and choosing the one with the smallest result. To increase the number of full stripe writes, the cache is used to combine small write operations into larger blocks of data.

Technology Comparison

	UDMA	SCSI	Fibre Channel
Best Suited For	Low-cost entry level server with limited expandability	Low to high-end server when scalability is desired	Server-to-Server campus networks
Advantages		Performance: up to 160 MB/s Reliability Connectivity to the largest variety of peripherals Expandability	Performance: up to 100 MB/s Dual active loop data path capability Infinitely scalable

Handling I/O Errors

Adaptec RAID controllers maintain two lists for each RAID 5 array: a Bad Parity List, and a Bad Data List. These lists contain the physical block number of any parity or data block that could not be successfully written during normal write, rebuild or dynamic array expansion operations. These lists alert the controller that the data or parity in these blocks is not valid. If the controller subsequently needs data from a listed block and cannot recreate the data from existing redundant data, it returns an error condition to the host.

Blocks are removed from the Bad Parity List or the Bad Data List if the controller successfully writes to them on a subsequent attempt.

Degraded Mode

When a drive fails in a RAID 0 array, the entire array fails. In a RAID 1 array, a failed drive reduces read performance by 50%, as data can only be read from the remaining drive. Write performance is increased slightly because only one drive is accessed. A RAID array operating with a failed drive is said to be in degraded mode.

RAID 5 arrays synthesize the requested data by reading and XOR’ing the corresponding data stripes from the remaining drives in the array. For RAID 5, the magnitude of the performance impact in degraded mode depends on the number of drives in the array. An array with a large number of drives will experience more performance degradation than an array with small number of drives.

Rebuilding a Failed Hard Drive

A failed drive can be replaced in a RAID 1 or RAID 5 array by physically removing the drive and replacing it or by a designated Hot Spare. Adaptec RAID controllers will rebuild the data for the failed drive onto the new drive or Hot Spare. This rebuild operation occurs online while normal host reads and writes are being processed by the array.

RAID 1 arrays are rebuilt relatively quickly, because the data is simply copied from the duplicate (mirrored) drive to the replacement drive. For RAID 5 arrays, the data for the replacement drive must be synthesized by reading and XOR’ing the corresponding stripes from the remaining drives in the array. RAID 5 arrays that contain a large number of drives will require more time for a rebuild than a small array.

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