As digital storage requirements rise, a Redundant Array of Inexpensive Disks (Raid) array becomes the only method to ensure speed and reliability.

By using multiple disks simultaneously, data read and write times are reduced while the opportunity to build in data protection increases.

The actual choice comes down to the Raid level selected. Choosing the right card can be difficult, so we've reviewed two of the best - Adaptec's 3400S and AMI's Enterprise 1600 - to see what they have to offer.

Architecturally, the two vendors have taken a different approach. While AMI provides a 66Mhz 64-bit PCI interface, Adaptec's offering only supports a 33Mhz 64-bit PCI interface. Potentially this makes the Adaptec card half the speed of the AMI, although this is only likely to be noticed when all four channels are in full use at the same time.

The AMI card is a full-sized board, while the Adaptec is only half-sized.

Part of the AMI size must be attributed to the cache memory, which requires a full-sized 168-pin DIMM to work. The card ships as default with 32Mb, but it can be upgraded to 128Mb.

Adaptec ships its card with 32Mb - in a notebook-type format - which can also be upgraded to 128Mb. However, this card has a slightly easier upgrade path as two memory slots are provided. This means that additional memory can be added without having to dispose of the bundled capacity.

For an additional charge, both companies will provide a battery backup pack for the cache. In the event of a power failure it saves any data in the cache that hasn't been written to disk. When power is restored, the data can be written and so corruption avoided. AMI takes this one step further and sells a transportable battery-backed cache module. This module can be transferred between cards without losing any data.

When it comes to the SCSI channels, Adaptec has the neater configuration as all four channels are available internally and externally.

Internally, it's the standard ultra-high density (UHD) 68-pin connector, while very high-density ports (VHD) provide the external connections.

AMI, on the other hand, only provides access to two of the channels internally via UHD connectors, while all four channels have external VHD connectors.

Capability counts

Adaptec uses its own controller chips to deal with the SCSI channels, but uses Intel's i960 processor for overall control. AMI also uses the Intel i960 processor to provide overall control but turned to Q Logic to provide two dual-channel controller chips.

The quad-channel capability of the cards allows for up to 60 devices to be connected to the cards with the same rules applying as for any SCSI adapter. For example, drives of different speeds can sit on the same channel at the expense of slowing the channel speed down to that of the slowest device.

Adaptec has managed to get around this problem on its SCSI host adapters with the speed-step technology, but this hasn't been implemented here.

AMI hasn't managed to create a workaround for this limitation either. Placing drives in an array shows that AMI has better support for standard Raid levels - 0, 1, 3 and 5 - and then has its own - 10, 30, 50 - on top. These additional levels provide mirroring across logical drives.

Adaptec provides much the same, with the notable exception of Raid 3, but adds its own levels 0/1 and 0/5. Again, this is just mirroring across logical drives. As Raid 3 is really just a poor man's version of Raid 5, we can see why Adaptec decided to drop support for it.

The final choice of card may eventually come down to driver support, which AMI has in abundance. The card supports Netware, Windows NT4/2000, SCO, BSDI, Dos, Red Hat and Solaris. Support will also be there for Win64, Linux 64 and Monterey when the Intel Itanium chip is launched.

Support platforms

In comparison, Adaptec only manages to provide support for Dos, BSDI, Windows 9x, NT4 and 2000, Netware, Red Hat and SCO. No support for Itanium processor-based operating systems has yet been announced.

To see how the cards actually performed when used, we installed them into a Windows 2000 Server. We tested how easy the cards were to set up and how good the management software was. We attached the cards to three Seagate 9Gb 10K LVD drives, which we used for setting up Raid arrays.

The Adaptec card went into the server with no trouble and soon brought up the Post screen that displayed a list of devices connected to it. Our first task was to go into the Bios and disable it from automatically booting, so that it wouldn't conflict with our existing boot drive in the server.

We found that the Bios wasn't quite as clean as the standard Adaptec affair and the required setting was difficult to find. However, from the Bios we could view the current configuration of the drive and even set up a new Raid array.

Storage Manager or Storage Manager Pro is provided on the Adaptec CD. The former looks very dated, and although first on the installation menu, we wouldn't recommend choosing it. Instead we went for Storage Manager Pro, which is a Java-based application.

The neat layout makes it easy to configure arrays online. For lovers of wizards, there's one designed for Raid arrays. It's based on the administrator picking the kind of application that the array will be used for, such as a file server, and then automatically configuring an array. However, you're probably better off selecting the drives, the spares and the Raid level manually. This way, you'll be sure to get what you want.

AMI's installation was a little more difficult. Things started well with the physical installation of the card. It booted up and found all our SCSI drives. We were able to use the Bios to set up Raid partitions and edit settings before an operating system was installed. However, getting into Windows 2000 and installing the software created some problems.

Testing times

First, the machine crashed on installation of the provided drivers, so we had to download the latest set from the website. Then we had lots of trouble getting Storage Manager Pro installed as some DLL files were missing. Eventually we pulled it all together and were able to use the software to completely monitor and manage the Raid array much in the same way as for the Adaptec card.

So how well did the cards perform? We used Intel Iometer version 1998.10.08 to run two test workloads on the cards. The workloads were designed to emulate a web server and a file server. We set each card up to run a single Raid 5 array.

The Adaptec managed 78.21 I/Ops for the file server test and 94.92 I/Ops for the web server test. The AMI card achieved 81.18 I/Ops for the file server test and 100.47 I/Ops for the web server test.

With the problematic installation of the AMI card under Windows 2000 and the small performance difference, Adaptec deserved to win this comparison.

Its lack of a 66Mhz bus, however, may mean that for top-end applications the AMI will become the only choice.

RAID LEVELS EXPLAINED

Configuration of a new Raid array means you must choose the most appropriate level. However, it's not always obvious what each level actually does. There are four well-defined standard levels - 0, 1, 3 and 5 - that you are likely to run into.

• Level 0 provides stripping of data across disks. When a file is written to the Raid drive, it is split up and spread across all the disks in the partition. All the disks are writing at once, improving performance. When the file is read again, all the disks are read at the same time. Level 0 has no support for fault tolerance.
• Level 1 is designed to offer fault tolerance to data by mirroring all data written to one physical disk to another physical disk. This means that an array has to have an even number of drives in it to work and the eventual array size is half the capacity of all of the disks. The result is that any one drive can fail but the data remains intact.
• Level 3 This is basically Level 1 with fault tolerance built on top. One drive is set aside and contains error information that is enough to rebuild any one drive in the event of a failure. In the event that the error drive goes down, however, all of the error information needs to be rebuilt. This level means that the Raid array capacity is one physical disk smaller than the total physical capacity.
• Level 5 is an improvement over the Level 3 design and spreads the error information across all of the disks. It allows any one disk to fail without data loss. It also provides a maximum array capacity that is one physical disk less than the total physical capacity.

On top of these levels, many card vendors have also created their own levels to increase data protection. These are a combination of the standard levels. For example, Raid 5/0 would have two level-5 partitions, each one being a mirror of the other.

Product details

ADAPTEC 3400S

Price: £949

Contact: Adaptec
01276 854 500
www.adaptec.com

Pros: Good management software; easy to get running.

Cons: No 66Mhz bus.

AMI ENTERPRISE 1600

Price: £1687.30

Contact: AMI
020 88488686
www.ami.com

Pros: Full 66Mhz 64-bit PCI card; good Raid support.

Cons: Tricky to install on Windows 2000.