HP D5970A - NetServer - LCII Installation And User Manual page 154

Appendix B
Consistency Check
Refers to a process where the integrity of redundant data is verified. For example,
a consistency check of a mirrored drive will make sure that the data on both drives
of the mirrored pair is exactly the same. For RAID Level 5 redundancy, a
consistency "connect" is a function that allows a target device (typically a disk
drive that received a request to perform a relatively long I/O operation) to release
the bus so that the controller can send commands to other devices. When the
operation is complete and the bus is needed by the disconnected target again, it is
"reconnected."
Disk Failure Detection
The controller automatically detects disk failures. A monitoring process running
on the controller checks elapsed time on all commands issued to disks )among
other things). A time-out causes the disk to be "reset" and the command to be
retried. If the command times out again, the disk could be "killed" (taken
"offline") by the controller (its state changed to "dead").
Disk Media Error Management
Disk array controllers transparently manage disk media errors. Disks are
programmed to report errors, even ECC-recoverable errors.
When a disk reports a media error during a read, the controller reads the data from
the mirror (RAID 1 or 0+1), or computes the data from the other blocks (RAID 3,
RAID 5) and writes the data back to the disk that encountered the error. If the
write fails (media error on write), the controller issues a "reassign" command to
the disk, and then writes the data to a new location. Since the problem has been
resolved, no error is reported to the system.
When a disk reports a media error during a write, the controller issues a "reassign"
command to the disk, and writes the data out to a new location on the disk.
Drive Groups (or Drive Packs)
A drive group is a group of individual disk drives (preferably identical) that are
logically tied to each other and are addressed as a single unit. In some cases this
may be called a drive "pack" when referring to just the physical devices. Up to
eight (8) drives can be configured together as one drive group.
All the physical devices in a drive group should have the same size; otherwise
each of the disks in the group will effectively have the capacity of the smallest
member. The total size of the drive group will be the size of the smallest disk in
the group multiplied by the number of disks in the group. For example, if there are
4 disks of 400MB each, and 1 disk of 200MB in a pack, the effective capacity
available for use is only 1000MB (4*200), not 1800MB.
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Glossary