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Apple RAID: innovative but limited

Surprise! Apple Computer has entered the storage game. Its new storage array, Xserve, has a lot to offer, but mostly for those with Apple servers.

As unstable as the storage business can be, it's always nice when a new player brings a fresh perspective to the table. That's definitely the case with the newest storage array vendor, Apple Computer Inc. That's right, Apple--a company that some wrote off not so long ago. But you'd have to be living in a cave not to know that Apple is back with a vengeance, and it has brought some smart products to the market. The latest is the Apple Xserve RAID array.

Apple Xserve RAID at a glance

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

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
Rest assured that Xserve RAID from Apple doesn't suffer from the issues mentioned above and offers high availability, high performance and a great deal of manageability and flexibility. Let's start with some numbers.

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. 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.

Apple's first storage product: highlights
All components except the RAID controller are redundant and hot swappable.
Each drive has an independent ATA bus and an independent channel to the RAID controller.
Write-back and write-through cache are both available.
Cache is also available on each drive, but can be disabled.
The system was engineered to continue good performance even if you lose a drive in the array.

Lots of throughput
To truly appreciate the power of the Xserve RAID, you have to understand the difficult target that Apple is aiming at--the high-end, real-time video processing market. These people demand a lot of throughput (well in excess of 100MB/s). When you're talking to someone who edits video, don't mention the maximum transfer rates or seek rates of a disk drive or storage subsystem. Talk to them about minimums. If you drop below 130MB/s sustained throughput, you're going to drop frames--audio or both--and that's simply unacceptable.

The RAID arrays aimed at this target market advertise "up to 130MB/s sustainable throughput," or "over 100MB/s sustainable throughput." With the Xserve RAID, Apple has demonstrated that it can provide a sustained throughput of 209MB/s in a RAID 5 configuration. Although that number alone sounds impressive, Apple says that it engineered the system in such a way that they can support enough throughput for high-definition video editing--even after losing a disk in a RAID array. They say that this applies to both reads and writes, as it claims that their RAID 3 and RAID 5 write performance is approximately 90% of their read performance. Pretty impressive. (Be advised that this article is not a review, but rather a first-look report. Most statements in this article are based on claims made by Apple.)

The Xserve RAID algorithm also minimizes the performance hit caused by so-called soft errors. Soft errors occur when all of the data is not retrieved from the disk in the first pass, and it causes the disk to spin around again for a second, third, or fourth attempt to reread the missing data. Such retries significantly degrade the actual throughput of a RAID set. When a soft error happens on an Xserve RAID array, the RAID controllers actually rebuild the missing data from parity in the cache before the disk can make a quarter turn. This secret sauce also allows them to maintain a nearly constant sustained throughput over the entire platter of a RAID set's hard drives. This means that the read and write performance of the outer track is nearly identical to that of inner track.

As mentioned earlier, the RAID array supports RAID 0, 1, 3, 5 and 0+1. RAID 0 stripes multiple disks into a single virtual disk. RAID 1 mirrors two disks. If you are mirroring two RAID 0 virtual disks, this is called RAID 0+1. RAID 3 and 5 sets can rebuild any single disk within the RAID set using parity. RAID 3 uses a dedicated parity disk and RAID 5 distributes the parity among all disks. RAID 3 and 5 both protect you against the loss of a single disk with a lot fewer disks than RAID 1. However, RAID 3 and 5 must calculate and store parity, resulting in a performance penalty during writes. Although RAID 3 is not used in most data centers, it is used where a constant throughput level is required, like with high definition video editing or real-time recording of data streams such as seismic or telemetry applications.

The way that the Xserve RAID is designed, each RAID controller creates independent RAID 0, 1, 3 or 5 arrays. These arrays can then be used individually, striped together for performance or mirrored for additional availability. Both of these operations would need to be done using Apple's volume manager or Windows dynamic drives. Apple says that it is also quite common for customers to mirror across Xserve RAID arrays.

Two types of write caching
Most RAID arrays--including the Xserve RAID--use caching to help mitigate the performance penalties associated with RAID levels 3 and 5. There are two main types of write caching: write-back and write-through. In write-back cache, data is considered committed, or successfully received, as soon as the RAID controller has written the data to cache. Then the RAID controller tells the host that the write has been completed, even though it is only written to RAM. While this provides fast performance, the volatile nature of RAM means that the data will be lost upon a power loss or other server outage. Those concerned more with data integrity than performance should use write-through cache, which ensures that the data is written directly to disk before it acknowledges that a write request has been completed.

As you can see, write-back cache is designed for high performance and write-through cache is designed for increased data integrity. The mistake that many hardware RAID systems make is deciding which type of write cache is best for their customers, as they often offer only one type of write cache. Apple's Xserve RAID allows you to choose which is more important to you--performance or integrity. If you choose write-back cache for performance reasons, the Xserve RAID will default to write-through cache if it detects that it is running on the UPS. Switching to write-through cache assures that nothing will remain in cache in the event of complete battery loss.

Using an independent ATA bus for every disk is also a good idea. As mentioned earlier, many ATA arrays that the Xserve RAID competes with use two disk drives on each ATA bus. Anyone who has ever tried to burn a CD from another CD on the same ATA bus knows what kind of impact bus contention can have on an ATA bus.

Another important design decision is that of the passive midplane. A lot of server and storage vendors use a back-plane with integrated circuits that are included in the data path. That is, a disk drive is connected to the RAID controller by plugging into the sockets that are connected to the back-plane. The data then travels through the socket, across the back-plane and back out through another socket to the RAID controller. Apple designed its midplane to be passive. In other words, all data passes from each disk to the RAID controller via an independent drive channel. In terms of availability, the loss of one of these channels is no different than the loss of a drive. This removes the single point of failure that back-planes create.

Apple has also removed single points of failure with dual, hot-swappable power supplies, cooling systems and coprocessors. A single power supply can power the entire unit while the other is being repaired. A single cooling system can be instructed to double fan speed if the other cooling system is lost. And the coprocessors monitor and track all of this, automatically notifying administrators via e-mail or pager of any failure. These coprocessors also track self-monitoring analysis and reporting technology (SMART) information from each disk drive, and can warn administrators of prefailure conditions. Such automated notification is essential when using RAID 3 or 5, as the loss of more than one drive results in a complete loss of all data in that RAID set. The Xserve RAID array also supports a global hot spare on each RAID controller, allowing it to step in for any lost drive, mitigating this risk even further.

It's obvious that Apple has put a lot of thought into their first RAID offering and wishes to be the RAID vendor of choice for Apple servers requiring lots of storage. Hopefully in the future, they will resolve the incompatibility issues and make this product more easily available to Windows, Linux and Unix users.

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