User Manual

Basic LUN Concepts
The volumes on your ReadyNAS can be divided into shares and logical unit numbers
(LUNs), both of which are logical entities on one or more disks. Shares and LUNs enable
you to organize data in a volume by type, group, user, department, and so on. A single
volume can contain multiple shares and LUNs.
LUNs are SAN (storage area network) data sets that allow data transfer and storage over
iSCSI and Fibre Channel devices. The ReadyNAS supports iSCSI devices only. Each
ReadyNAS system supports up to 256 LUNs. The Admin Page displays LUNs in the
following way:
Figure 7. Thin LUN
Figure 8. Thick LUN
Each LUN is configured independently of other LUNs that reside on the same volume.
You can configure settings such as compression, protection, provisioning, LUN size,
and access rights. You can also specify whether and how often a snapshot is created.
These settings are explained in the following sections.
Thin and Thick Provisioning
You can specify the size of a LUN in two ways:
•
Thin. A thin LUN lets you overallocate its size. That is, you can assign a LUN size that
is larger than the size of the volume. Even though you specify the size of a thin LUN
when you create it, storage space is assigned on demand instead of up front. This
method greatly improves the utilization rate of the LUN because storage space is
assigned only as data is written to the LUN. However, the size of the LUN is reported
as the total storage space that you specify when you create the LUN.
You can expand a volume as needed (if necessary, adding disks in the process)
without expanding the size of the LUN and therefore, without disconnecting users.
Make sure that you watch the volume capacity of the volume on which the
overallocated LUN resides so you do not run out of storage space unexpectedly.
Note: We recommend that you do not use an overallocated LUN for storage of
critical data. Instead, use a thick LUN.
Software Manual95LUNs
ReadyNAS OS 6.10