NEC Storage Manager User Manual page 305

(2) RAID1
This method writes data (sent from the host) onto two disks (or into 2 disk groups). (Mirroring)
RAID1 has the same data in dual configuration. Thus, even if one disk fails, the other disk
enables access to the data.
In comparison with a single disk, RAID1 achieves the equivalent performance in write operation,
and up to the double throughput as performance in read operation by accessing two disks
containing the same data.
RAID1 offers data reliability higher than any other RAIDs do, but redundant data takes up 50% of
the disk capacity.
<Advantage>
High reliability
No performance deterioration when a single disk fails
Performance in read operation can be up to twice as high as the throughput of a single disk.
<Disadvantage>
Redundant data takes up a half of the disk capacity.
Data Sent from Host
Block1 Block 2 Block 3
Block 1
Block 2
Block 3
Block 4
Block 5
Physical Disk1
15
Appendix C RAID
Block 4 Block 5
Block 1
Block 2
Block 3
Block 4
Block 5
2