Storage Pools And Drives - D-Link DSN-500 Software User's Manual

The xStack Storage system performs its virtualization task by presenting volumes that the
servers see as disks or drives or SCSI Logical Units, depending on the server operating
system's terminology. Volumes are created by organizing blocks of storage from the drives.
iSCSI is an end-to-end storage block protocol that makes it possible to transfer storage data
reliably over any IP-based network, including the Internet. The iSCSI drafts and RFCs are
published by the IETF, but based on the SCSI specifications from the ISO's Technical
Committee Ten (T10), the ANSI-accredited body responsible for developing and maintaining
the core SCSI standard. To the committee, iSCSI is another SCSI transport and just as
officially sanctioned (though technically it's a superset of SCSI, providing additional
functionality through unique commands and data formats used for secondary services such as
authentication).
From the network's perspective, iSCSI is just another service that runs over TCP/IP. It can
use the same networking stack as other applications, with clients requesting data from
servers. The main difference is that its function is more specialized. Whereas other Layer 7
protocols such as SMTP are agnostic toward the technologies used at their endpoints, iSCSI is
designed as a way to extend an existing storage technology across IP networks.
For iSCSI purposes, the SCSI protocol is conceptually similar to TCP/IP's client/server
architecture. Every SCSI link involves a host adapter, called an initiator, and a storage
device, called a "target." The customer's server will act as the initiator and the xStack
Storage system will act as the target. Traditionally, a local SCSI bus connects a single initiator
to up to seven targets, but a SAN allows an unlimited number of each. The initiator's iSCSI
stack packs SCSI commands and data into IP packets, which are then unpacked by the target
for processing as if they had originated locally.
The iSCSI protocol defines two types of iSCSI Nodes:
The initiator node

The target node

The iSCSI initiator node consists of an initiator name and a set of properties. The server's
operating system provides software to define and configure its iSCSI initiator node. The
xStack Storage system serves as one or more iSCSI target nodes and automatically assigns
target node names using the standard naming convention defined in the iSCSI protocol.
The xStack Storage system provides the customer a method to create Volumes. Each Volume
is a unit of storage of a specific size with a RAID organization, as described later in this
document. Every volume creation automatically includes the creation of an iSCSI target
node; i.e. there is a one-to-one relationship between iSCSI Target Nodes and Volumes in an
xStack Storage system. An iSCSI target node name is automatically created by the xStack
Storage system using the standard naming convention with the Volume Name in a well-
known position in that name. The xStack Storage system manages initiator access to a
target node as part of volume management. For example, the customer grants an initiator
access to a target node by granting access to the Volume. The server (iSCSI initiator) can
find its storage by finding a known volume name in a list of discovered target node names.

1.3 Storage Pools and Drives

Storage space is managed on the xStack Storage system through storage pools. The
placement of a drive in a storage pool controls the way that drive can be used. There are
four types of storage pools (see Table 1-1).
xStack Storage Management Center Software User's Guide 3