HP StorageWorks Scalable File Share System Installation and Upgrade Guide Version 2.2

EVA4000 storage — overview and planning 5–3
Logical Units (LUNs) and Virtual Disks
A virtual disk is a storage subdivision that is allocated from some of the available capacity of a
disk group. When a virtual disk is created on an EVA4000 array, it is assigned a 32–character
WWID that is unique across all RAID arrays from any vendor.
A virtual disk cannot be accessed by a server until it is presented to the server. When the virtual
disk has been presented to a server, the virtual disk is called a logical unit (LUN).
On the SAN, the server is identified by the WWIDs of the HBA ports that connect the server to
the SAN. Hosts are used to represent the HBA ports on the servers (see the description of Host).
To allow a server pair to access a virtual disk, you present the virtual disk as a LUN to the host
associated with the server pair. The LUN is then given a LUN number that is unique within the
array.
Within the HP SFS system, each LUN is assigned another LUN number; this LUN number is
unique within the HP SFS system. The system assigns the LUN numbers as the LUNs are
discovered, and there is no implicit order to, or relationship between, these LUN numbers.
Virtual Disk
See the description of Logical Units (LUNs) and Virtual Disks.
5.1.2 EVA4000 configuration rules for HP SFS systems
When configuring storage on the EVA4000 arrays in an HP SFS system, you must take account of certain
rules. Failure to follow these rules may result in incorrect system operation or poor system performance. The
rules are as follows:
• In a typical HP SFS system, there is one EVA4000 array for each Object Storage Server in the system
(that is, two EVA4000 arrays for each Object Storage Server pair), and one EVA4000 array for the
administration and MDS server pair.
It is possible to have a system configuration where EVA4000 arrays are attached to more than two
Object Storage Servers. This configuration is described in Appendix I.
• Each EVA4000 array that is attached to an Object Storage Server must have a minimum of eight
disks per shelf.
• You must have two disk groups, and you must have an equal number of disks in each disk group. As
mentioned earlier, one disk group (the default disk group) is created when the EVA4000 array is
initialized. You must create an additional disk group; this task is explained in Section 5.2.4.3 (using
the SSSU utility) and in Section 5.2.5.3 (using the Command View EVA Web interface).
Two disk groups are needed because the EVA4000 array has two controllers; for best performance, a
one-to-one ratio between disk groups and controllers is required. Measurements with the EVA4000
array have shown an unbalanced performance distribution between LUNs when a single disk group
is used. For this reason, HP SFS configurations with EVA4000 arrays always have two disk groups.
• The LUNs in one disk group must be preferred to the first controller. The LUNs in the other disk group
must be preferred to the second controller. You must specify the preferred controller in two places, as
follows:
• On the EVA4000 array itself (where you set the preferred path/mode—that is, the controller)
when you configure the EVA4000 arrays.
• In the HP SFS software, when you create the file systems.
• You must use a spare disk policy of single.
When the single spare disk policy is used, storage capacity equivalent to the capacity of two disks
is kept as spare storage, and is not used for LUN storage. When another disk fails, the spare capacity