User`s guide

24 Chapter 2 Understanding Concepts and Terms
2.1 Overview
The term “storage virtualization” refers to the process of grouping together independent storage
devices found across a network to create what seems to be a single large storage entity that can
be centrally managed.
The purpose of XStack Storage is to virtualize disk storage for use by a customer’s host computers
(servers). With a SAN, servers connected to that network can access any SAN device (such as an
xStack Storage unit) wherever that device is attached to the storage area network. The user is no
longer aware of, and no longer needs to know, which physical devices contain which data. The
storage has become virtualized. The virtualization of storage provides a more intuitive use of
storage, while the XStack Storage unit quietly manages the storage network in the background.
At its front end, the XStack Storage unit uses the Internet small computer system interface
(iSCSI) protocol over Ethernet to connect to the customer’s servers. At its back end, the XStack
Storage unit contains a bank of SATA disk drives. The XStack Storage unit performs its
virtualization task by presenting volumes that the servers see as disks or drives, depending on the
server operating system’s terminology. Volumes are created by organizing blocks of storage from
the SATA disk drives.
2.2 Understanding iSCSI
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 Request for
Comments (RFCs) are published by the Internet Engineering Task Force (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 is officially sanctioned by them (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 small computer
system interface/Internet Protocol (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 unit will act
as the target. A local SCSI bus usually connects a single initiator to up to seven targets, but a SAN
allows an unlimited number of each. The initiator's iSCSI stack packs (or encapsulates) SCSI
commands and data into IP packets, which are then unpacked by the target for processing as if
they had originated locally.