Overview of Microsoft iSCSI Software Target

Article
09/28/2010

Applies To: Microsoft iSCSI Software Target

Internet Small Computer Systems Interface (iSCSI) is a protocol that supports access to storage devices over a TCP/IP network, which facilitates storage consolidation and sharing of storage resources across an organization. You can use Microsoft iSCSI Software Target to provide centralized management of disk systems in a storage area network (SAN) by using the iSCSI protocol. Microsoft iSCSI Software Target uses the Windows® TCP/IP network stack and works in conjunction with the Windows-based file system and Logical Disk Manager to make iSCSI disk subsystems available to iSCSI initiators. Storage is provided on the disk subsystem by using virtual disks that are created through Microsoft iSCSI Software Target. Additional functionality provided in iSCSI Software Target includes support for taking snapshots of virtual disks. It also includes wizards for setting up and maintaining iSCSI target.

IP-based storage network

A storage server that uses internal disks for storage requires a common protocol, such as SCSI (SAS), or ATA (SATA), to communicate between the disks and the controller. When a storage subsystem is part of a consolidated storage area network (SAN), and it is attached using a local area network (LAN), the connection of storage disks to controllers requires an appropriate network protocol. Microsoft iSCSI Software Target uses the iSCSI protocol to enable communication between components of an IP-based storage network.

At its highest level, the iSCSI protocol is simply an encapsulation of SCSI commands within a TCP/IP–formatted package. This encapsulated package is called the Protocol Data Unit (PDU). There are two primary types of PDUs: a request PDU and a response PDU. A request PDU is sent from the iSCSI initiator to the iSCSI target to initiate a command. The iSCSI target executes the requested command and returns the result in the response PDU. Each PDU consists of two parts: a header and a data segment. The header contains iSCSI–specific information, such as the command to be executed and the data length. The data segment carries the actual storage information for the command.

The primary components of an IP-based storage network include:

iSCSI target, which is the storage provider. An iSCSI target is a set of disks in a disk subsystem.
iSCSI initiator, which is the client that consumes the storage. The function of the iSCSI initiator is analogous to the disk controller of a locally-attached disk in a computer.
Storage fabric, IP-based network, which connects the iSCSI target to the iSCSI initiator. This is typically a gigabit Ethernet LAN.

For more information about iSCSI targets, see Creating and Managing iSCSI Targets.

Disk storage

Microsoft iSCSI Software Target creates storage devices as iSCSI virtual disks, which are files in the virtual disk(.vhd) format. These iSCSI virtual disks are assigned to specific iSCSI targets, and only the iSCSI virtual disks that are assigned to an iSCSI target are available to an iSCSI initiator. After the iSCSI initiator logs on to the iSCSI target, these virtual disks appear as locally attached hard disks. These disks can be locally mounted to support backup and recovery operations.

For more information about iSCSI virtual disks, see Creating and Managing Virtual Disks for iSCSI Targets. For more information about making a virtual disk available locally, see Mounting a Virtual Disk Locally.

Snapshots

You can use Microsoft iSCSI Software Target to create point-in-time, read-only snapshots of virtual disks on an iSCSI subsystem. These iSCSI virtual disks are useful for providing data recovery for data that was modified since the most recent backup. Snapshots can be taken frequently because they are space efficient and they can be taken without stopping the system or disrupting service. You can create a snapshot manually or set up automatic snapshots to be taken at specific times.

There are two methods for creating snapshots:

Create a snapshot on the iSCSI target server by using the Schedule Snapshot Wizard. Snapshots that are created on the iSCSI target server are “crash consistent.” This means that if the iSCSI initiator computer stops responding, the state of the snapshot will be the same as the state of the virtual disk.
Create a snapshot on the iSCSI initiator computer by using the Volume Shadow Copy Service (VSS) service, VSS Hardware Provider, and non-Microsoft VSS-aware backup software. Snapshots created on the iSCSI initiator computer with this method are “application consistent.” This means the data is in a state that an application can use for recovery.

When scheduling snapshots, you can optionally specify that the snapshots be mounted locally as read-only disks on the server running iSCSI Software Target. You can also export snapshots to a centrally located server. This makes the data on the disks available for backup as part of standard backup procedures.

For more information about snapshots, see Creating and Managing Snapshots and Schedules. For more information about using snapshots for backup and recovery, see Mounting a Read-Only Snapshot Locally, Exporting a Snapshot, and Rolling Back a Virtual Disk to a Snapshot.

Wizards

To simplify creating and managing iSCSI targets, the following wizards are available in the Microsoft iSCSI Software Target console:

Create iSCSI Target Wizard helps you create an iSCSI target on an iSCSI subsystem, and then assign an iSCSI initiator to it. For more information, see Creating an iSCSI Target.
Create Virtual Disk Wizard helps you create an iSCSI virtual disk on an iSCSI subsystem and supports assigning it to an iSCSI target at the time the disk is created. For more information, see Creating a Virtual Disk for an iSCSI Target.
Import Virtual Disk Wizard helps you import one or more iSCSI virtual disks that you previously created using iSCSI Software Target so that they are available for assignment to iSCSI targets. For more information, see Importing a Virtual Disk.
Extend Virtual Disk Wizard helps you add space to an iSCSI virtual disk. For more information, see Extending a Virtual Disk for an iSCSI Target.
Schedule Snapshot Wizard automates the creation of snapshots by using schedules to specify when the snapshots are to be taken and whether they should be mounted locally. For more information, see Creating and Scheduling Snapshots of Virtual Disks.
Export Snapshot Wizard helps you make snapshots available on a centrally managed server for backup and recovery purposes. For more information, see Exporting a Snapshot.

Integration of Windows technologies

Microsoft iSCSI Software Target works with Windows technologies to simplify and optimize storage management as follows:

Uses the Windows TCP/IP stack to implement an IP-based storage network, so it supports the use of advanced networking capabilities such as IPsec, and hardware offload, if the features are available on the network adapter.
Makes use of the storage capabilities that are built in to Windows operating systems, so it can be used to manage any Windows-certified device.
Relies on the Logical Disk Manager to manage disk storage, so it works with hardware and software redundant array of independent disks (RAID).
Works with Windows Failover Clustering server clusters, providing integrated setup and management of highly available clusters for iSCSI storage by using iSCSI Software Target and Failover Cluster Management. For more information about Failover Cluster Management, see Failover Clusters.