Before we dive into RAID Levels, we need to define a few terms that will be used
throughout this paper. The purpose of this is so that when a term is used, you
will know what it is intended to mean.
- Concatenated array
- This is an array where multiple disk drives or arrays are logically connected
together, end-to-end. This was the earliest method to combine multiple disks.
This type of array has no redundancy. It is not one of the RAID levels. Some
manufacturer's incorrectly call this JBOD.
- Data Drive
- A data drive is a disk drive that is dedicated to storing data, as opposed
to parity, Hamming code, or a hot standby. In RAID levels where the data and
parity move around, the space equivalent of this many drives is available
to store data.
The number of data drives in a RAID array is designated by the first number
in the size description. In some cases, this is the only number.
- Hamming Code
- A Hamming Code is
an algorithm that can be used to determine if an error exists in a data stream,
and sometimes (dependent on the exact code used) correct that error. This
is sometimes referred to as an Error Correction Code (ECC).
The number of Hamming Code drives in a RAID array is designated by the
number after the first plus sign ('+'). If there is no first plus sign,
then there are no Hamming Code drives. This is the same location that the
parity number is found in. Except for RAID-1, if the value is one, then
it is a parity drive; if it is greater then one, then they are Hamming Code
- Hot Standby
- This is a disk drive that is reserved for use, in case a failure occurs
in one of the active drives on the array. If such a failure occurs, the hot
standby drive will start being rebuilt to replace the failed drive. This usually
happens automatically, but some RAID implementations may require human intervention.
The number of hot standby drives in a RAID array is designated by the
number after the second plus sign ('+'). If there is no second plus sign,
then there are no hot standby drives.
- This is an acronym for 'Just a Bunch Of Disks'. Most disk controllers don't
have any RAID functionality built into them. In these cases, the Operating
System sees the disk drives connected to the controller as JBOD.
In addition, many RAID controllers default to a JBOD configuration before
being configured for RAID.
be noted that some manufacturers use the term JBOD to refer to a Spanned or Concatenated array.
- Logical Disk
- This is what a RAID array is. Although the RAID array is multiple disks,
it appears to the Operating System as a single disk.
- Mirror Disk
- These disks are used in RAID-1. A mirror disk contains an exact duplicate
of the disk that it mirrors. More than one mirror disk is sometimes used.
- Parity is used in RAID-3, RAID-4 and RAID-5 to validate the data written
to the RAID array. Parity across the array is computed using the XOR (Exclusive
OR) logical operation. This is a very fast operation, but it transfers a great
deal of information to and from memory. It should be noted that this is a
special case of a Hamming
Except for RAID-1, the number of parity drives in a RAID array is designated
by the number after the first plus sign ('+'). If there is no first plus
sign, then there are no parity drives. This is the same location where the
Hamming Code number is found. In general, if the value is one, then it is
a parity drive; if it is greater then one, then they are Hamming Code drives.
- Physical Disk
- A physical disk is a disk. This term is sometimes used to distinguish it
from a logical disk.
- This is an acronym for 'Redundant Array of Independent Disks'. Originally,
the I stood for Inexpensive, and is sometimes still seen
written that way.
- Segment size
- This is the number of blocks (sometimes expressed in bytes) that are written
to one disk drive, before moving on to the next disk drive in the array.
does not apply to RAID-1 or RAID-3.
This is sometimes called Stripe Size. The term Stripe Size is
only valid for RAID-0 arrays. Other RAID levels do not have stripes. All RAID
levels (except RAID-1 and RAID-3) have segments.
- Spanned array
- This is another name for a Concatenated array.
- Spindle Sync
- Spindle Sync is a feature that allows multiple disk drives to operate in
sync with each other. When enabled, and properly cabled, a series of disk
drives will spin at the same speed, and a given sector will pass under the
heads of all the drives at the same time.
This feature will result in a performance improvement in virtually all
RAID arrays. This is because it has an impact on the statistical rotational
latency of a RAID array. As an example, in a RAID-3 array using 7200RPM
disk drives, configured as 8+1, the statistical rotational latency will
be 4.17ms if the drives are in sync, but 8.30ms if the drives are not in
This feature is available on many SCSI and Fiber disk drives, but I have
never seen it on an IDE disk drive.
- Stripe size
- This is similar to Segment size, except that it is only valid
for RAID-0 arrays. Many manufacturers use this term when they mean Segment
- Stripe width
- This is the number of blocks that must be written to the array, so that
every data drive has had a complete segment written.
- The Exclusive OR (XOR) logical function is used to generate parity. It is
also used in Hamming codes. The XOR function is similar to a binary addition,
without any carry operations. The binary truth table for an XOR is as follows:
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