Storage Management Strategies for the CIO
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Apple Xserve RAID at a glance | |
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Apple's Xserve RAID has two independent RAID controllers, each supporting up to 512MB of cache. The subsystem has seven drive channels per controller, which are connected to 180GB, 10,000 rpm ATA drives. One subsystem supports up to 14 drives for a total raw capacity of 2.5TB. Source: Enterprise Storage Group |
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Array vendors all over the world are making RAID arrays based on ATA disks connected to Fibre Channel (FC) or SCSI bridges. It's a wonderful idea that offers customers a tremendous amount of storage at a fraction of what they're used to paying for FC or SCSI-based storage. However, not all of these products offer the kind of availability and performance that today's storage administrators have come to expect. Many of these arrays suffer from a number of issues. They include:
- Little or no cache: If they offer cache, they often use the same caching algorithm for all customers
- Multiple drives on each ATA bus, resulting in bus contention
- Many single points of failure, including active back-planes that aren't hot swappable
- Throughput nowhere near the speed of FC, let alone 2Gb FC
- Severe performance degradation if you're using RAID 5
- Little or no on-site service available from the manufacturer
- Lack of hot spare support
- Little or no automated notification of drive failures
It's a 3U high rack storage subsystem containing from four to 14 180GB, 7,200 rpm, ATA/100 disk drives (Yes, ATA/133 is newer and faster than ATA/100). You can buy as few as four drives, and later upgrade to as many as fourteen as your needs grow. This means that a standard 42U rack could hold over 35TB of Xserve RAID storage (see "Apple Xserve RAID at a glance," on this page for the subsystem specifications).
Great price, but who can use it?
One downside is that Apple chose to develop a copper interface for FC. They did this because FC host bus adapters (HBAs) are expensive. Unfortunately, this means it will take extra effort to use the Xserve RAID with anything but an Apple server. It can be connected to your Windows or Unix system, but you'd have to configure it with a server first. You can also connect the Xserve RAID to a hub or a switch, but only with a special cable. The same is true for the Java management application that only runs on MacOS. It's obvious that Apple only wants to sell its subsystem to Apple customers at this time. Perhaps this will change as they recognize the potential market they are ignoring.
A base system of 720GB is $5,999, and a fully configured system with 2.52TB raw is only $10,999. That's just more than $4 per gigabyte. No matter how you slice it, that's a pretty good price, although it's not quite as earthshattering as Apple's Web site might suggest. Apple.com compares the price of the Xserve RAID to SCSI and FC-based systems from Dell, Hitachi and others. Based on this comparison, Xserve RAID looks incredibly inexpensive, and it is. However, it's not a fair comparison. Because Xserve RAID is based on ATA drives, it should be compared to the price of others in that class of storage, such as Nexsan Technologies' ATAboy line, which is roughly the same price as the Xserve RAID.
This was first published in October 2003
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