Looking to benefit from iSCSI storage?

Computer Technology Review, May, 2004 by Augie Gonzalez

Everything you've heard suggests that using iSCSI over an Ethernet LAN will bring inexpensive capacity expansion. With your internal drive bays filling up quickly and the need for capacity growing in urgency, you need to make some smart decisions fast.

Few Choices

Despite all of the commotion around iSCSI, the list of practical storage solutions is surprisingly short. Some startups have introduced specialty appliances, while a few of the established disk array vendors have retrofitted their products with iSCSI channels. You'll even find some NAS boxes getting an iSCSI facelift. Most of them approach iSCSI from the standpoint of hardware. They lock in a controller, disk drives, and network ports into sheet metal--hoping it fits your needs and price point.

Intriguing software alternatives exist as well. On the host side of the link, Microsoft, Red Hat and others provide iSCSI initiator drivers that run right on top of the operating system's TCP/IP stack, making Windows and Linux machines with an Ethernet port "iSCSI-ready."

On the storage end of the connection, shrink-wrapped software available today can turn a PC into an iSCSI "disk server." These software products enable a PC to imitate iSCSI disk arrays inexpensively with the hardware and administrative tools you are already familiar with.

Purpose-Built vs. General Purpose

The essential differences between software-enabled iSCSI disk servers and purpose-built storage devices revolve around several dimensions:

* Cost

* Performance

* Price/performance

* Choice of hardware

* Configuration flexibility

* Upgradeability

Although it's easy to concede that disk servers configured from PCs would have cost, configuration and upgradeability advantages over purpose-built appliances, you might guess that purpose-built iSCSI subsystems outperform disk servers built on general-purpose components. Nothing could be further from the truth.

[FIGURE 1 OMITTED]

The same factors that made hardware-centric database machines obsolete in favor of portable database software are now at play in the storage market.

Back to Basics

iSCSI provides a fantastic means to access disk drives over a LAN, removing the barrier of buying, installing and learning an entirely new networking infrastructure for the purpose of storage. Unfortunately, real disk drives don't come with Ethernet plugs. Instead, they have EIDE/ATA, SCSI, Fibre Channel or SATA cables coming out of them. To complete the iSCSI connection, something must convert SCSI packets sent over the GigE cable into a language and wire compatible with the disk drive. That "something" essentially pretends to be an iSCSI target device--this is the role played by intelligent controllers in a storage array or appliance.

[FIGURE 2 OMITTED]

With cost competitiveness in mind, suppliers of iSCSI solutions often use slower, cheaper disk drives with relatively poor access times. Good controllers compensate for such drive characteristics by caching I/Os in electronic memory to mask mechanical latencies from the applications. Caching thus improves the apparent response time from disk.

To avoid wasting capacity and provide more multi-user flexibility, the controller offers features that carve up each physical disk drive into smaller logical drives that better match each application's capacity needs. For example, an 80-GB disk can be sliced into eight separate 10-GB LUNs (logical unit numbers). Each 10-GB logical drive can be independently allocated among various application hosts, just as if they were eight physical disks.

Embedded software (sometimes known as firmware or microcode) in the controller makes protocol translation, caching and fine-grain LUN allocation possible. Like any other software, firmware runs on a computer--usually a custom motherboard designed around the CPUs that were current at the inception of the development project. Depending on the original design date, controller CPUs will be two or three generations behind processors shipped with current PCs. The same is true for other components that make up the memory subsystem and the network ports. Hence, the purpose-built appliance starts with a performance disadvantage relative to off-the-shelf PCs running the latest and greatest architecture.

Another business consideration further handicaps purpose-built hardware: lack of substantial sales volume. The number of disk arrays sold by a storage vendor pales in comparison with the large volume of PCs sold each year. Component costs for specialty parts bought in smaller lots runs much higher than for the mass-market equipment. The specialty devices suffer from higher prices and fewer inventory turns, forcing storage suppliers to stretch out their investment over more years, further compounding an already stale hardware platform.

What we are experiencing with storage subsystems today is pretty much what intelligent database machines were running up against several years ago. The purpose-built database hardware could not keep up with rapidly evolving off-the-shelf PCs. When the database vendors came to that realization, they conceded the hardware platform to the server suppliers and shifted their attention to portable database software, focusing on the quality and richness of the solution. Each year SQL Server, Oracle, Sybase, as well as other database implementations, deliver better value in part because Intel and AMD OEMs offer faster, smaller and cheaper systems on which to run the software.