RAID Systems (page 3)

• Introduction
• RAID Levels
• Extended Data Availability & Protection (EDAP)
• New RAID classification

RAID Performance Characteristics

* Availability is equal to MTBF of one disk divided by the number of disks in the array. With the falling price of the hard drives, RAID systems become affordable enough for home use also.  RAID-0 and RAID-1 are the simplest to integrate then the other, more complex systems.

Hot-spare drives

A hot-spare drive is a special drive that is designated for automatic use if any drive within an array fails.  The hot-spare has a storage capacity greater than or equal to that of the smallest drive in an array. It is possible to define as many hot spares as you want.  A RAID-1 array on an adapter can use the hot-spare disk drives on that adapter.  If a drive within an array fails, the adapter will automatically engage a hot spare instead of the failed disk drive, and rebuilds the data that was on the failed disk on to the hot spare.

   Extended Data Availability & Protection (EDAP)

EDAP is the ability of a disk system to provide timely, continuous, on-line access to reliable data under certain specified abnormal conditions. These conditions, as described by RAB,  include (this is the RAB's exact description):

• Internal Failures
  Failures within the disk system

• External Failures
  Failures of equipment attached to the disk system including host I/O buses and host computers

• Environmental Failures
  Failures resulting from abnormal environmental conditions, including: External power source out of operating range, temperature out of operating range, natural disasters such as floods and earthquakes, accidental disasters such as fires and unlawful acts such as sabotage, arson, terrorism, etc.

• Replacement Periods
  Replacement periods are the intervals required for replacement of a failed component. If "hot swap" is not supported by the disk system, then the component replacement period is tantamount to disk system down time. If "hot swap" is supported, then down time due to a replacement period is eliminated; however, until the failed component is replaced, the disk system is in a vulnerable period.

• Vulnerable Periods
  Vulnerable periods occur when the disk system has invoked its ability to circumvent a failure, rendering the system vulnerable to additional failures and causing the system to operate at something less than optimum performance until the fault is corrected.

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