User`s guide

26 Chapter 2 Understanding Concepts and Terms
If a volume is reduced (for example, if a physical drive fails), the firmware
automatically moves a drive from the available pool to the Blade A Base Pool
and starts the rebuild operation for that volume.
2.4 Volumes
A volume is a set of blocks of storage that are organized and presented for use by a customer’s
server (an iSCSI initiator node). Every volume must be associated with a storage pool, which
limits the drives that can be used to hold data for that Volume (only drives in that pool can be
used for this volume). Only the Blade A Base Pool can be used for volumes.
The iSCSI initiator node sees the volume as a contiguous series of numbered blocks, called Virtual
Logical Block Numbers (VLBNs), in the same way that it would see the storage space on a single
disk drive. The XStack Storage unit constructs a volume from extents, where each extent is a
block of storage from a single drive. A volume typically consists of extents from several drives. A
volume of n blocks is shown in
Figure 2-1.
Volume
VLBN
Extent 1
. . .
Extent x
0
n-1
Figure 2-1 Internal Structure of a Volume
Volume can be organized in several ways (refer to Table 2-2).
Table 2-2 Ways to Organize Volumes
Organization Definition Redundant Striped
Storage
Cost
JBOD One copy of the data is written to the selected Extents. No No 1x
Mirror Two copies of all data are written to independent Extents. Yes No 2x
Stripe Distributes one copy of the data among several drives to improve
the speed of access.
No Yes 1x
Stripe Mirror Distributes the data among several drives and then keeps a mirror
copy of the blocks on each drive
Yes Yes 2x
Parity Distributes one copy of the data among several drives and adds
parity blocks spread throughout the volume to protect against the
loss of any single drive.
Yes Yes 1x-1.5x