## Glossary

#### Auto-synchronization

If the controller supports auto-synchronization, automatically synchronizes all of the RAID level-5 logical drives, in the background, when they are created. RAID level-0 and RAID level-1 logical drives do not need to be synchronized when they are created.

#### Battery backup cache

An optional device that protects your data during write-back mode. This device is not optional if you have either of the following:

- ServeRAID-3HB controller
- ServeRAID-3H or 3L controller with the new firmware installed.

#### Blocked logical drive state

#### Clustering

A set of two or more computers that work as a group for greater performance, reliability, and prevention of failure.

#### Critical logical drive state

#### Critical migrating logical drive state

#### Critical system logical drive state

####

#### Data scrubbing

If the controller supports data scrubbing, provides automatic, continuous synchronization during system use.

#### Defunct physical drive state

#### Enclosure

A SCSI management device that requires the use of an ID, such as an EXP15.

####

#### Free space

The space in an array currently not assigned to a logical drive.

#### GB

When referring to hard-disk-drive capacity, GB means 1 000 000 000 bytes, or 1000 MB. Total user-accessible capacity may vary depending on operating environment.

#### High-availability

A system designed and implemented with components that satisfy the system's functional requirements, but also has hardware and software component redundancy to eliminate any unexpected behavior.

#### Hot-spare drive

A hot-spare drive is a physical drive that is defined for automatic use when a similar (that is, with equal or smaller capacity) drive that is part of the array fails.

#### Hot-spare physical drive state

#### Hot-swap rebuild

If the controller supports hot-swap rebuild, you can replace a failed drive that is part of an array with a good new drive. Then, the new controller automatically begins rebuilding the failed drive's image on the new drive.

If you replace a failed drive that is not part of the array with a good new drive, the new drive becomes a hot-spare drive.

#### MB

When referring to hard-disk-drive capacity, MB means 1 000 000 bytes. Total user-accessible capacity may vary depending on operating environment.

#### Migrating logical drive state

#### Offline logical drive state

#### Okay logical drive state

#### Online physical drive state

#### Parity

The information stored on a RAID level-5 or level-5 Enhanced logical drive that holds the redundant data necessary to rebuild a failed drive.

#### PFA error

Predictive Failure Analysis® event that announces the physical drive is about to fail.

#### RAID level-0

#### RAID level-1

#### RAID level-1 Enhanced

#### RAID level-5

#### RAID level-5 Enhanced

#### Read-ahead cache mode

#### Ready physical drive state

#### Rebuilding physical drive state

####

#### SAF-TE ( * )

A processor that manages small computer system interface (SCSI) accessed fault-tolerant enclosures (SAF-TE). An example of such an enclosure is the IBM® Netfinity® EXP15.

#### Standby physical drive state

#### Stripe-unit size and interleave depth

With RAID technology, data is striped across an array of hard disk drives. This data-distribution scheme complements the way the operating system requests data.

The granularity at which data from one file is stored on one drive of the array before subsequent data is stored on the next drive of the array is called the interleave depth.

You can control the interleave depth and maximize the performance of your ServeRAID controller by setting a stripe-unit size that is close to the size of the system I/O requests. You can set the stripe-unit size to 8 KB, 16 KB, 32 KB, or 64 KB. For example, performance in transaction-based environments, which typically involve large blocks of data, might benefit when using a setting of 32 KB or 64 KB; however, performance in file and print environments, which typically involve multiple small blocks of data, might perform best when set to a stripe-unit size of 8 KB or 16 KB.

The collection, in logical order of these stripe units, from the first drive of the array to the last drive of the array, is called a stripe.

#### System logical drive state

#### Total disk space

Hard disk drives in an array can be of different capacities (1 GB, or 2 GB, for example). The sum of the hard disk drives' capacity grouped in an array is the total disk space.

For example, if you group two 2 GB drives and one 3 GB drive into an array, the total disk capacity is the 7 GB physically available.

Similarly, if you group three 2 GB drives and 1 GB drive into an array, the total disk space is the 7 GB physically available.

#### Unusable capacity

Hard disk drive capacities influence the way you create arrays. Drives in an array can be of different capacities (1 GB, or 2 GB, for example), but RAID controllers treat them as if they all have the capacity of the smallest disk drive.

For example, if you group two 2 GB drives and one 3 GB drive into an array, the usable capacity of the array is 2 GB times 3, or 6 GB, not the 7 GB physically available. The 7 GB is the total disk space. The remaining space on the 3 GB drive is unusable capacity. In the following diagram, usable capacity is labeled as 1 and unusable capacity is labeled as 2.

Similarly, if you group three 2 GB drives and 1 GB drive into an array, the usable capacity of that array is 4 GB, not the 7 GB physically available. The 7 GB is the total disk space. The remaining space left on the three 2 GB drive is unusable capacity.

The optimal way to create arrays is to use hard disk drives that have the same capacity. Doing so avoids unusable capacity.

#### Usable capacity

Hard disk drive capacities influence the way you create arrays. Drives in an array can be of different capacities (1 GB, or 2 GB, for example), but RAID controllers treat them as if they all have the capacity of the smallest disk drive.

For example, if you group two 2 GB drives and one 3 GB drive into an array, the usable capacity of the array is 2 GB times 3, or 6 GB, not the 7 GB physically available. The 7 GB is the total disk space. In the following diagram, usable capacity is labeled as 1 and unusable capacity is labeled as 2.

Similarly, if you group three 2 GB drives and 1 GB drive into an array, the usable capacity of that array is 4 GB, not the 7 GB physically available. The 7 GB is the total disk space. The remaining space left on the three 2 GB drive is unusable capacity.

The optimal way to create arrays is to use hard disk drives that have the same capacity. Doing so avoids unusable capacity.

#### Write-cache mode

Determines, on a logical drive basis, whether the controller writes data to the hard disk drive before or after sending the operating system a successful reply to the write command. You can select from write back or write through.