User manual

iSCSI GbE to SAS/SATA II RAID Subsystem
User Manual
9
1.2 Terminology
The document uses the following terms:
RAID
RAID is the abbreviation of “Redundant Array of Independent
Disks”. There are different RAID levels with different degree of
the data protection, data availability, performance to host
environment.
PD
The Physical Disk belongs to the member disk of one specific
volume group.
RG Raid Group. A collection of removable media or physical disks.
One RG consists of a set of VDs and owns one RAID level
attribute.
VD Virtual Disk. Each RG could be divided into several VDs. The VDs
from one RG share the same RAID level, but may have different
volume capacity.
CV Cache Volume. The iSCSI RAID controller uses the on-board
memory as cache. All RAM (except for the part which is occupied
by the controller) can be used as cache.
LUN Logical Unit Number. A logical unit number (LUN) is a unique
identifier used on iSCSI connection which enables it to
differentiate among separate devices (each one is a logical unit).
GUI Graphic User Interface.
WT Write-Through cache write policy. A caching technique in which
the completion of a write request is not signaled until data is
safely stored on non-volatile media. Each data is synchronized in
both data cache and the accessed physical disks.
WB Write-Back cache write policy. A caching technique in which the
completion of a write request is signaled as soon as the data is in
cache and actual writing to non-volatile media occurs at a later
time. It speeds up system write performance but needs to bear
the risk where data may be inconsistent between data cache and
the physical disks in one short time interval.
RO Read-Only. The volume can be set as Read-Only.
DS Dedicated Spare disks. The spare disks are only used by one
specific VG. Others could not use these dedicated spare disks for
any rebuilding purpose.
GS Global Spare disks. GS is shared for rebuilding purpose. If some
RGs need to use the global spare disks for rebuilding, they could
get the spare disks out from the common spare disks pool for
such requirement.