RAID (redundant array of inexpensive disks) is the technology of grouping several hard disk drives in a server into an array that can be defined as a single logical drive.  This logical drive then appears to the operating system as a single physical drive.  This technology provides access and data transfer rates beyond the physical limitations of existing hard disk drives, greatly enhancing logical-drive capacity and performance.  In addition, if one of the physical drives fails, the system continues to run, with no operator intervention required.  The defunct drive can be replaced without turning off the server (hot-swap) and the new drive contents are rebuilt from the information on the other drives.  This rebuilding process takes place in the background with the system online.  Once the drive contents are rebuilt, full performance and fault-tolerant functions are restored.

  When you install internal SCSI drives in your server, you can connect them to either the Channel 1 or the Channel 2 connector on the RAID adapter (see the User's Handbook for more information).  The external connector on the adapter also is designated "Channel 2".  You cannot connect devices to both the internal Channel 2 and external Channel 2.  This means that you can connect SCSI devices to both the internal channels and not the external channel; or, you can connect SCSI devices to one of the internal channels and the external channel, but never all three channels.

Subtopics:
o Hard Disk Drive Capacities
o Additional Storage Capacity
o Logical Drives
o Improved System Performance
 

Hard Disk Drive Capacities
With a server, it is important to understand the implications of hard disk drive capacities and how they influence the way you create disk arrays.
   Although the drives in the disk array can be of different capacities (for example 1GB or 2GB), they are treated in the disk array configuration as if they all have the capacity of the smallest disk drive.  Therefore, if you have four drives of 1GB, 1GB, 1GB, and 2GB grouped in one disk array, the total capacity of the array will be 1GB times 4, or 4GB (instead of the 5GB physically available).
   By the same logic, if you add a smaller drive to a group of drives, say a 1GB drive to a system containing three 2GB drives, the total capacity of an array that includes all four drives will be 4GB, instead of the 7GB physically available.

Subtopics:
o Hard Disk Drive Mapping

Hard Disk Drive Mapping (F/W-SR)
The drive locations shown on your RAID Configuration utility screen (described by channel and bay number) are shown mapped to the actual physical locations in your server in the following tables.  The SCSI ID for each device is also shown in reference to the channel and bay to which it is attached.  The first table shows the server configuration as it is shipped, with Channel 2 not connected.

One Channel Mapping

DISPLAYED  CHANNEL NUMBER	DISPLAYED  BAY NUMBER	BANK AND  BAY PHYSICAL NUMBER	SCSI ID
1	1	C1	0
1	2	C2	1
1	3	C3	2
1	4	C4	3
1	5	C5	3
1	6	C6	5
1	7	A1 (CD ROM)	6

Two Internal Channels Mapping
The following table shows an example of using both internal channels of the RAID adapter.  The first channel is connected to bank C, and the second channel is connected to the optional backplane on bank D.

DISPLAYED CHANNEL  NUMBER	DISPLAYED BAY  NUMBER	BANK AND  BAY PHYSICAL NUMBER	SCSI ID
1	1	C1	0
1	2	C2	1
1	3	C3	2
1	4	C4	3
1	5	C5	3
1	6	C6	5
1	7	A1 (CD ROM)	6
2	1	D1	0
2	2	D2	1
2	3	D3	2
2	4	D4	3
2	5	D5	4
2	6	D6	5
2	7	Not Used	6

One Internal and One External Channel Mapping
The following table shows an example of using one internal and one external channel on the RAID adapter.  The first channel is connected to bank C, and the second channel is attached to an external DASD storage enclosure.  Refer to the documentation that came with the storage enclosure for physical locations.

DISPLAYED CHANNEL NUMBER	DISPLAYED BAY  NUMBER	BANK AND BAY  PHYSICAL  NUMBER	SCSI ID
1	1	C1	0
1	2	C2	1
1	3	C3	2
1	4	C4	3
1	5	C5	3
1	6	C6	5
1	7	A1 (CD ROM)	6
2	1	External	0
2	2	External	1
2	3	External	2
2	4	External	3
2	5	External	4
2	6	External	5
2	7	External	6

If you install another IBM SCSI-2 Fast/Wide Streaming-RAID Adapter/A in your server, you can use the same combinations shown here, depending upon your space availability.

Additional Storage Capacity
When you add hard disk drives to your server, you must configure a new disk array before you can use the drives.  You can either reconfigure the existing disk array to include the capacity offered with the added drives, or group the added drives into their own array.  You also can create an array with only one drive.

Logical Drives
It is helpful to understand how the system manages logical drives and how many you can define.

When you create an array, you are combining several hard disk drives into one storage area.  The array then can be used as a single logical drive or can be subdivided into several logical drives.  A logical drive of a disk array can be any size you choose within the size limitations of the array.  The RAID Adapter supports up to four independent arrays and a total of eight logical drives.  Each array can be formed from a maximum of eight drives.  And an array can span both channels.  (For information about physical drives supported, see Data Storage Devices.)

For example, if you have only one array, it can be either a single logical drive or divided into as many as eight.  If you have two or more arrays, you can have each one as one logical drive (a total of four), or you can divide them into multiple logical drives, as long as the total number of drives for the arrays is no more than eight.

The operating system considers each of these logical drives just as it does a physical hard disk drive.  That is, the logical drives can be partitioned by the FDISK program (or its equivalent) in the same way that the operating system partitions a hard disk drive.

If you install an operating system, you either can allow the installation program to determine how the FDISK program (or its equivalent) allots the space within a logical drive, or manipulate the FDISK program yourself to partition the available space.  The documentation you receive with your operating system explains how it handles mapping.

Improved System Performance
When hard disk drives are united into a single logical drive, data can be transferred in parallel from the multiple drives in the array.  This parallel transfer yields data-transfer rates that are many times higher than with nonarrayed drives.  This increased speed makes the server better able to meet the throughput (the amount of work in a given amount of time) or productivity needs of the multiple-user network environment.

The ability to respond to multiple data requests provides not only an impressive increase in throughput, but a decrease in response time.  The combination of parallel transfers and simultaneous responses to multiple requests allows disk arrays to provide the highest level of performance in network environments.