APC RAID Subsystem SCSI-SATA II - Manual (Page 69)

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When rebuilding, the status of PD/VG/UDV is “R”; and “R%” in UDV will display
the ratio in percentage. After complete rebuilding, “R” and “DG” will disappear.
VG will become complete one.
Sometimes, rebuild is called recover; these two have the same meaning. The
following table is the relationship between RAID levels and rebuild.
RAID 0 Disk striping. No protection of data. VG fails if any hard drive
fails or unplugs.
RAID 1 Disk mirroring over 2 disks. RAID 1 allows one hard drive fails
or unplugging. Need one new hard drive to insert to the
system and rebuild to be completed.
N-way
mirror Extension to RAID 1 level. It has N copies of the disk. N-way
mirror allows N-1 hard drives fails or unplugging.
RAID 3 Striping with parity on the dedicated disk. RAID 3 allows one
hard drive fail or unplugging.
RAID 5 Striping with interspersed parity over the member disks. RAID
5 allows one hard drive fail or unplugging.
RAID 6 2-dimensional parity protection over the member disks. RAID
6 allows two hard drives fails or unplugging. If it needs to
rebuild two hard drives at the same time, it will rebuild the first
one, then the other by sequence.
RAID 0+1 Mirroring of the member RAID 0 volumes. RAID 0+1 allows
two hard drives fails or unplugging, but at the same array.
RAID 10 Striping over the member RAID 1 volumes. RAID 10 allows
two hard drives fails or unplugging, but at different arrays.
RAID 30 Striping over the member RAID 3 volumes. RAID 30 allows
two hard drives fails or unplugging, but at different arrays.
Tips
The list box doesn’t exist if there is no VG or only VG of RAID
0, JBOD. Because user cannot set dedicated spare disk for
these RAID levels.