RAID 101

General notions

RAID is implemented either by the driver (this is called "software RAID") or by the special hardware controller ("hardware RAID"). Both Microsoft Windows NT and most flavors of Unix/Linux provide their software RAID implementations. Microsoft Windows 9x series operating systems do not feature a software RAID drivers, but still can be used with a hardware RAID controllers (because of the controller transparency).

RAID does not provide perfect reliability

Consider the following points:

• Multiple drive failures do happen, especially if caused by the single source (for example a power supply unit goes bad and spews 220 volt AC current over the 5V DC power bus - this will take out all the drives powered by that PSU).
• The entire array may be lost due to the controller failure.
• Virus can take out data on the array. The array will be still functional with respect to its hardware, but the data will be of no use.
• Fire or natural disaster (like flood) can take out the whole array.
• Operator error can cause damage to data or RAID misconfiguration.

JBOD (Just a Bunch Of Disks), also called "Span"/"Spanned Volume"

Minimum of two disks is required for a hardware JBOD. Minimum of one disk is required for a software JBOD (Windows NT/2000/2003 "Spanned volume", which allows volume to occupy nonadjacent regions of the same physical disk). There is no disk space overhead. The following exception may or may not apply in your case: hardware RAID controller may support a single-disk JBOD configuration - this is just a trick to allow a single drive to be attached to the controller, without actually RAIDing anything. The same applies to RAID0 consisting of 1 member disk.

RAID 0, also called "Stripe Set"

Minimum of two disks is required for a RAID 0 setup. There is no disk space overhead associated with RAID 0 volume. The following exception may or may not apply in your case: hardware RAID controller may support a single-disk RAID0 configuration - this is just a trick to allow a single drive to be attached to the controller, without actually RAIDing anything. The same applies to JBOD consisting of 1 member disk.

RAID 1, also called "Mirror"

It is possible to have more than two disks in the mirrored set (e.g. three-disk configuration - "primary" with two "shadows"), but such a setup is extremely rare due to the high disk space overhead (200% overhead in three-disk configuration).

Exactly two disks are required for a RAID 1 volume. RAID 1 layout imposes 100% storage space overhead.

RAID 5, also called "Stripe Set with parity"

Let's say for example that the Disk 3 fails in configuration illustrated below.

• Data blocks D₁ and D₂ will be read directly from their corresponding disks (which are operational).
• Parity block P_1,2 is really not needed (does not contain user data) so it will be just discarded.
• Data block D₃ will be read from its corresponding disk (Disk 2).
• Data block D₄, which is missing because its drive is offline will be reconstructed using D₃ and P_3,4 like this: D₄=P_inverse(D₃,P_3,4)

During normal operation, read speed gain is (N-1) times, because requests will be evenly routed to N-1 disks (parity read is not needed during normal operations). Write procedure is more complicated, and actually imposes some speed penalty. Let's say we need to write block D₁. We also need to update its corresponding parity block P_1,2. There are two ways to accomplish this:

1. Read D₂; compute P_1,2=P(D₁,D₂); write D₁ and P_1,2;
2. Read D_1;old and P_1,2;old; compute P_1,2 from these data; write D₁ and P_1,2.

Both of these ways require at least one read operation as the overhead. This read operation can not be parallelized in any way with its corresponding write operation, so write speed should decrease (by the factor of two, assuming equal read and write speed). Most current implementations mitigate this effect by maintaining the entire "row" (D₁, D₂ and D₃) in the cache.

Minimum of three disks is required to implement RAID5. Storage space overhead equals the capacity of a single member disk and does not depend on the number of disks.

RAID 0+1, also called "Mirrored Stripe Set"

For N-disk configuration (with two stripe sets with N/2 disks each)

• N times faster reads, compared to a single member disk (request to read blocks D₁ through D₄ will be routed in such way that each member disk reads one block).
• N/2 times faster writes, compared to a single member disk (write request to blocks D₁ through D₄ will be routed so that Disks 1 and 3 write blocks D₁ and D₃ and Disks 2 and 4 write blocks D₂ and D₄, thus doubling a write speed in 4-disk setup).

The RAID 0+1 array can tolerate a loss of any single disk. Additionally, it can handle half of the dual failures in the four-disk configuration (e.g. if disks 1 and 2 fail in configuration illustrated below, the array will still be online albeit degraded to a stripe set).

Minimum of a four disks is required for a RAID 0+1 volume, with a 100% storage space overhead.

Other (exotic) RAID layouts

Quite a number of other (exotic) types of RAID exist, but they are rarely used and detailed discussion of those is outside the scope of this article.

• RAID 3 and RAID 4 are similar to RAID 5 but use dedicated disk to store parity information. This disk becomes a bottleneck during write.

• RAID 6 is similar to the RAID 5 but uses two different parity functions to maintain redundancy. RAID6 can tolerate a dual failure (simultaneous loss of two drives). RAID6 is useful in high-capacity systems when the rebuild of a RAID5 would take a long time and there is a significant probability that another drive will fail before the rebuild is done, thus causing a loss of the array.

Hot sparing and/or mirror reliability issues

Some fault-tolerant implementations allow the additional drive to be designated as a "hot spare". Once the drive in the array fails, the "hot spare" drive is used to rebuild the array and restore the redundancy as soon as possible, allowing to defer the maintenance action (failed drive replacement) e.g. till working hours. There is a quirk involved. The controller must implement some periodical checks of the "hot spare" drive. If the bad sector develops on the "hot spare" and goes unnoticed, the spare drive will be essentially useless when one of the array drives fail. Should the "hot spare" drive fail completely it will be noticed once the controller discovers it is unable to interrogate the device. Bad sectors cannot be detected this way and will sit there unnoticed until it is too late.

The same consideration applies to RAID 1 (mirror) setup. The controller should interleave the requests so that both drives get their share of reads. This way, if a bad sector develops on one of drives, it will be soon discovered (this is sometimes called "active/active" fault-tolerant implementation). If the "active/standby" scheme is implemented, reading a primary drive and falling back to the shadow drive only when primary one goes bad, it is possible that shadow will develop unnoticed bad sectors and once the primary drive fails, the backup would be already damaged.

Estimating RAID capacity, access speed and fault tolerance.

The RAID planning can be done by hand using the table provided below. We've also created the online RAID estimator tool which will do the job provided just the array layout and member disk sizes.

*Note: speed increase is a very rough estimate based on the assumption that disk traffic consists mostly of linear (sequential) reads of large data chunks. This estimation also assumes controller(s) are capable of overlapped operations and there are no problems with bus throughput (since several RAIDed high-performance drives can easily saturate the bus like PCI).

**Note: disk space overhead calculation is based on the assumption that all member disks are of equal capacity. If the disks are not equal in size, the smallest of all disks will be used as a "column" size.

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