Report on Iscsi

Report on iSCSI

==========

Introduction

For this iSCSI part, we have followed this approach: as the main objective was not to make a tutorial, we decided to split this report into four parts: first we will give a theoretical introduction about iSCSI. After that, we will cover and explain all the single steps involved in configuring and managing OpenFiler and iSCSI. And finally we will explain the packets that navigate in the network, as well as the performance of the iSCSI protocol.

What is iSCSI ?

Have you ever heard about the OSI layer? If not, please check here[1]: Conceptually, iSCSI alongside TCP+IP provide an equivalent of Layer 3/4 network transport, rather than alternatives of either parallel SCSI[2] cable or Fiber Channel Protocol (FCP) - which is SCSI over Fiber Channel. The basic idea of iSCSI is to take advantage of the existence of IP Networks to facilitate and extend SANs[3]. How is this accomplished?

Well, by using the TCP/IP protocol to transport SCSI commands and data among host and SAN nodes. But, how is it different from Fibre Channel Network? Hum, it’s easy. Although they almost fit the same purposes, the use of iSCSI over IP networks does not necessarily replace a Fibre Channel network. Instead, it provides a transport for an IP-attacked host to reach Fibre Channel bases SANs.

So, what does iSCSI brings new to the world, compared to SCSI, Fibre Channel and other similar approaches?

By operating on existing IP networks, users can connect hosts to storage facilities without any additional host adapters, better use of storage resources, and eliminate the need for separate parallel WAN infrastructures. Because iSCSI uses TCP/IP as its transport for SCSI, information can be passed over existing IP-based host connections typically via Ethernet. Additional value can be obtained by being able to better utilize existing storage resources. Because hosts can utilize their existing IP/Ethernet network connection to access storage elements, it is now easier to consolidate storage and, therefore, optimized the usage of all the storage.

But, for what is it needed?

It serves many purposes. For example, it allows applications like SQL Server Exchange to access raw storage blocks in remote systems. It also allows a iSCSI disk to appear to the OS as a local disk.

So you might be asking right now. What is the purpose of iSCSI if we have an IP network we can access anything thought common file share? In order to explain this, we must first explain two things: an access to an extern hard disk can be file level or block level.

The first (File Level) is much more common, which lead us to ask the question. In order to access a remote shared folder thought a machine being a client, you just inform the UNC of this share. For example, \\server\share01. We call this kind of access ‘File Level’. One of the differences to the block level is that, the file system is in the side of the file server, as well as the protocol to share this file system (CIFs, FTP, NFS).

The iSCSI access is block level type. What we basically have is a iSCSI Target (storage) serving an iSCSI Initiator (host) in block level. The term Target and Initiator come from SCSI protocol. Practically speaking, what happens is the same that happens when you present a SCSI disk to the Windows. The disk is identified and presented as a new disk inside the Disk Management. To the SO is really transparent. To it, it is a mere local disk being used. In opposition to file level, in the block level the file system is being managed by the host. Only the SCSI packages nagivate between the host and the storage. With this solution, no more file shares, and no more file system overhead.

How iSCSI works?

When an application sends a request, the operating system generates the appropriate SCSI commands and data requests, which then go, depending on the situation, through encapsulation and or encryption procedures if necessary. A package header is added before the resulting IP Packages are transmitted over the Ethernet connection.

When a package is received, it is decrypted, and disassembled, separating the SCI commands and request. The SCSI commands are sent to the SCSI controller, and from there to the SCSI storage device. SCSI is bi-direction, so the protocol can also be used to return data in responde to the original request.

http://img.tomshardware.com/us/2005/02/04/flexible_data_storage_across_networks/iscsi-san.gif

The diagram above details the working principles behind iSCSI. In a tradition system, the storage systems are supposed to be able to utilize network infrastructure independently of servers. A storage system can be accessed from several servers, with minimal management complexity. iSCSI allow the possibility to offer additional storage space in existing systems, and attach them.

The advantages of this approach are many, and mostly pretty obvious. Many businesses already have an efficient network infrastructure in place, usually consisting of mature and reliable technology such as Ethernet. No new technologies need to be introduced, tested and validated for the incorporation of iSCSI systems or others such SANs (Storage Area Networks). Additionally, there is usually no need to hire expensive specialists for implementation.

This means any network administrator can manage iSCSI clients and servers with very little training. Also, iSCSI can be considered a high-availability solution, since iSCSI servers can be connected to several switches or network segments. Finally, the architecture is scalable by design, thanks to Ethernet switching technology.

Through an iSCSI client (initiator) the storage resources on the iSCSI-server can now be integrated into the client system as a device, which can be used as if it were a local drive. The great advantage of this compared to a classic network share is in the area of security, as iSCSI puts great emphasis on proper authentication of the iSCSI packets, which are transported through the network encrypted.

Of course, the attainable performance will be slightly lower than that of a local SCSI system due to the network's higher latencies. Still, today's networks, with their bandwidths of up to 1Gbit/s (=128 MB/s), already have a lot of capacity, most of which is not being fully utilized.[4]

iSCSI in reality

That’s it with the theoretical part, in which we explained textually what is iSCSI, how it works and for what it is used. Now we will try to explain every