Hitachi Compute Rack CR 210H User Manual page 31

Functions of Disk Array
Degraded mode
NOTICE
Even if one of the HDDs in RAID1, RAID5, RAID6, or RAID10 disk array fails*,
you can continue to use the disk array as usual. It is said that the disk array with
one HDD failing* is operating in degraded mode. In response to a read/write
request, the array controller uses the data and array parity (for RAID5 and
RAID6) of the active disks to compute the data of the failed disk and continue
operation as it is. However, the data computation required causes a decrease in
processing performance as compared to the normal status.
*: RAID6 can operate even if two HDDs fail.
Rebuilding data
In the disk array of RAID1, 5, 6, or 10, after the failed disk is replaced, the array
controller restores and stores data on the replaced disk. This is referred to as
rebuilding data. Although the disk array can continue operating while rebuilding
data, its processing performance lowers kind of degraded mode as compared to
normal status.
If another HDD failure occurs while the disk array is operating in degraded mode,
the disk array cannot rebuild data, thus losing all data. In such a case, replace
the failed disk as soon as possible and rebuild data.
If another HDD failure occurs while the disk array is operating in
degraded mode, the disk array cannot rebuild data, thus losing all
data. Replace the failed disk as soon as possible and rebuild data.
If a HDD fails in the drive group having multiple logical drives, all the
disks under the drive group will operate in degraded mode.
RAID0 and JBOD cannot rebuild data because neither data duplication
nor parity generation is conducted on their drives.
System Unit Functions
Hitachi Compute Rack 210H User's Guide
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