System information

Introduction
1-6
1.4 RAID Structure Overview
The storage resources are managed as storage
objects in a hierarchical structure. The hard disks, the
only physical storage objects in the structure, are the
essence of all other storage objects. A hard disk can
be a JBOD disk, a data disk of a disk group, or a local
spare disk of a disk group. It can also be an unused
disk or a global spare disk. The capacity of a disk
group is partitioned to form logical disks with different
RAID configurations, and multiple logical disks can be
put together to create volumes using striping,
concatenation, or both. The JBOD disks, logical disks,
and volumes, are virtual disks, which can be exported
to host interfaces as SCSI logical units (LUN) and
serve I/O access from the host systems. Below are
more descriptions about each storage objects.
• JBOD disk
A JBOD (Just a Bunch Of Disks) disk is formed by
single hard disk that can be accessed by hosts as a
LUN exported by the controller. The access to the
LUN is directly forwarded to the hard disk without any
address translation. It is often also named as pass-through disk.
• Member disk
The hard disks in a disk group are member disks (MD). A member disk of a disk group can be a data disk or a local spare
disk. A data member disk provides storage space to form logical disks in a disk group.
• Disk group
A disk group (DG) is a group of hard disks, on which logical disks can be created. Operations to a disk group are applied
to all hard disks in the disk group.
• Logical disk
A logical disk (LD) is formed by partitioning the space of a disk group. Logical disks always use contiguous space, and
the space of a logical disk is evenly distributed across all member disks of the disk group. A logical disk can be exported
to hosts as a LUN or to form volumes.
• Local spare and global spare disk
A spare disk is a hard disk that will automatically replace a failed disk and rebuild data of the failed disk. A local spare disk
is dedicated to single disk group, and a global spare disk is used for all disk groups. When a disk in a disk group fails, the
controller will try to use local spare disks first, and then global spare disks if no local spare is available.
• Volume
A volume is formed by combining multiple logical disks using striping (RAID0) and concatenation (NRAID) algorithms.
Multiple logical disks form single volume unit using striping, and multiple volume units are aggregated to form a volume
using concatenation. A volume can be exported to hosts as a LUN.
• Logical unit
A logical unit (LUN) is a logical entity within a SCSI target that receives and executes I/O commands from SCSI initiators
(hosts). SCSI I/O commands are sent to a target device and executed by a LUN within the target.
• Virtual disk
A virtual disk is an storage entity that can service I/O access from LUNs or from other virtual disks. It could be JBOD disk,
logical disk, or volume. If a virtual disk is part of other virtual disk, then it cannot be exported to LUNs.
• LUN mapping
A LUN mapping is a set of mapping relationships between LUNs and virtual disks in the controller. Computer systems
can access the LUNs presented by the controller after inquiring host ports of the controller.
Hard Disks
Volumes
Logical Disks
Disk Groups
Local
Spare
Logical Units
Unused
Disks
Global
Spare
JBOD
Disks
Figure 1-1 Layered storage objects