HP Dc5750 WebPAM User Manual - Page 97

RAID 1 - Mirror, Data Mirror

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Chapter 6: Technology Background RAID 1 - Mirror When a logical drive is mirrored, identical data is written to a pair of disk drives, while reads are performed in parallel. The reads are performed using elevator seek and load balancing techniques where the workload is distributed in the most efficient manner. Whichever drive is not busy and is positioned closer to the data will be accessed first. With RAID 1, if one disk drive fails or has errors, the other mirrored disk drive continues to function. This is called Fault Tolerance. Moreover, if a spare disk drive is present, the spare drive will be used as the replacement drive and data will begin to be mirrored to it from the remaining good drive. Data Mirror 1 1 2 2 3 3 4 4 Disk Drives Figure 2. RAID 1 Mirrors identical data to two drives Due to the data redundancy of mirroring, the capacity of the logical drive is only the size of the smallest disk drive. For example, two 100GB disk drives which have a combined capacity of 200GB instead would have 100GB of usable storage when set up in a mirrored logical drive. Similar to RAID 0 striping, if disk drives of different capacities are used, there will also be unused capacity on the larger drive. RAID 1 logical drives on FastTrak consist of two disk drives. 91

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Chapter 6: Technology Background
91
RAID 1 – Mirror
When a logical drive is mirrored, identical data is written to a pair of disk drives,
while reads are performed in parallel. The reads are performed using elevator
seek and load balancing techniques where the workload is distributed in the most
efficient manner. Whichever drive is not busy and is positioned closer to the data
will be accessed first.
With RAID 1, if one disk drive fails or has errors, the other mirrored disk drive
continues to function. This is called Fault Tolerance. Moreover, if a spare disk
drive is present, the spare drive will be used as the replacement drive and data
will begin to be mirrored to it from the remaining good drive.
Figure 2.
RAID 1 Mirrors identical data to two drives
Due to the data redundancy of mirroring, the capacity of the logical drive is only
the size of the smallest disk drive. For example, two 100GB disk drives which
have a combined capacity of 200GB instead would have 100GB of usable
storage when set up in a mirrored logical drive. Similar to RAID 0 striping, if disk
drives of different capacities are used, there will also be unused capacity on the
larger drive.
RAID 1 logical drives on FastTrak consist of two disk drives.
1
2
3
4
1
2
3
4
Data Mirror
Disk Drives