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RAID-5 Array States
A RAID-5 array can be in one of several states. A knowledge of those states is useful
when you are configuring your arrays. The states are described here. A flowchart for the
RAID-5 array states is shown in Figure 15 on page 35.
Good State
A RAID-5 array is in the Good state when all the member disk drives of that array are
present.
Exposed State
A RAID-5 array enters the Exposed state when a member disk drive becomes missing
(logically or physically) from that array. When an array is in the Exposed state, you can
reintroduce the missing disk drive, or exchange it for a new one. If the missing disk
drive is reintroduced, the array returns to the Good state. The array management
software does not need to rebuild the data. If a new disk drive is exchanged for the
missing disk drive, the array management software rebuilds the data that was on the
original disk drive before it became missing, then writes that rebuilt data to the
replacement disk drive. When the data is correct, the array management software
returns the array to the Good state.
Read Operations while in the Exposed State
When a read operation is performed on an array that is in the Exposed state, the array
management software recreates the data that was contained on the missing disk drive.
On the Advanced SerialRAID Adapter, the array management software immediately
exchanges a hot spare disk drive for the missing disk drive, if a hot spare disk drive is
enabled and available when the read command is sent.
Write Operations while in the Exposed State
When a write command is sent to an array that is in the Exposed state, the array
management software does the following:
v If a hot spare disk drive is enabled and available when the write command is sent,
the array management software immediately exchanges the hot spare disk drive for
the missing disk drive, and returns the array to the Rebuilding state.
v If no hot spare disk drive is enabled and available, the first write operation causes
the array to enter the Degraded state. The written data is not protected. If the power
fails during a write operation, data might be lost (64 KB) unless the array is
configured to allow read-only operations while in the Exposed state. Most application
programs, however, cannot be run when write operations are not allowed.
Degraded State
A RAID-5 array enters the Degraded state if, while in the Exposed state, it receives a
write command. If a hot spare disk drive is available, the array management software
immediately exchanges the hot spare disk drive for the missing disk drive, and returns
the array to the Rebuilding state. If no hot spare disk drive is available, and a write
operation is performed on the array, the array remains in the Degraded state until you
take action to return that array to the Good state.
Chapter 3. RAID Functions and Array States
33
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