Installation guide

8
RAID Modes
RAID 1
The RAID 1 storage policy stores all data in duplicate on each drives
to protect against data loss due to drive failure, and is the default
configuration of the DataDock II. One drive mirrors the other at
all times, which means every write operation goes to both drives.
RAID 1 provides the highest level of data fault tolerance, but halves
the amount of storage capacity because all data must be stored
twice. The resulting storage capacity of the virtual volume will be
equivalent to the size of one hard drive. If one drive fails, the volume
is still usable, but it is in a vulnerable state because its mirrored
hard drive is inaccessible. When the offline drive is replaced, the
DataDock II will begins a rebuild process immediately to restore data
redundancy.
Although the volume remains available during the rebuild
process, the volume is susceptible to data loss through
damage to the remaining drive until redundancy is restored
at the end of the rebuild and verification process.
Host access takes precedence over the rebuild process. If you
continue to use the volume during the rebuild, the rebuild process
will take a longer time to complete, and the host data transfer
performance will also be affected.
Span
The Span storage policy concatenates the two physical hard drives
as a single large volume, resulting in a seamless expansion. Hard
drive A and B are concatenated into a single virtual volume with a
storage capacity that is equal to the sum of each of the physical
hard drives A and B. In the event of disk failure the surviving
member of the span is likely recoverable, but because the data is
written sequentially to one disk and then the other the data available
for recovery may vary depending on which member failed.