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Hot rod thinkin’

By SoupIsGood Food, (, July 4, 2002

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Hey, kids, wanna soup up your spiffy new Xserve? Well, sit back and let me show ya how it’s done ...

Now, I/O is where it’s at. Every time your system has to hit the disk, that’s the slowest part of the operation right there, so it’s only sense that the faster your storage setup is, the faster your application will go. The quad Ultra ATA/100 disks in the Xserve are pretty darn sweet, especially when you compare them to Ultra SCSI. Hell, it even beats SCSI Ultra160 with a big stick when it comes to throughput. Ultra160 maxes out at 160MB/sec, where Ultra ATA/100 tops 266MB/sec. The controllers Apple uses aren’t like the old IDE interfaces ... they can order and execute disk operations on their own quite handily, without bugging the processors for help. If your bag is moving big files on a bitty budget, Ultra ATA/100’s your man.

ATA starts coming up short if your deal is small read/write operations, and lots of ’em ... this is way important if you’re running a database or a Web site with dynamic content. Latency is the name of the game, and with a spindle speed of 7200rpm and a seek time of 9msecs or so, ATA can’t cut it. Then there’s the problem that, well, 266MB/sec just ain’t quick enough when you figure you can’t get anywhere near there with a single drive that can only give you around 25MB/sec of sustained throughput. Apple has four independent busses, one for each drive, which means that the system is never waiting for an access operation to be queued. This is a good thing. The four disks together might get you the full 266MB/sec in “burst mode” for short, unpredictable spurts, but can only guarantee you around 25MB/sec solid throughput from each disk – 100MB/sec. This is a bad thing.

Apple in its infinite wisdom has seen fit to provide three PCI slots, and we’ll only need one to feed the need for speed. We can populate one of ’em with some serious tech to bring our bouncing baby Xserve to the bleeding edge. The T10 group was supposed to come up with the successor to SCSI2, but kind of got sidetracked and brought us, among other things, Ultra SCSI, FireWire, SSA (don’t ask, it’ll only hurt your head) and, ultimately, the T11 group. The T11 group then brought us Fibre Channel, and it was good.

Fancy British spelling or no, this is the storage interface of the biggest of the big boys. Current FC-AL cards can move more than 2 gigabits a second – enough to saturate your Ethernet port twice over. This translates to 200MB/sec practical throughput, which is more than comparable to ATA/100, because you are gonna get 200MB/sec, come hell or high water, “burst mode” be damned. What’s more, the quickest Fibre Channel disks come with a spindle speed of 15000rpm. That means the disk can get to your data when you want it (ie: right freakin’ now!) This gives you a seek time under 4 msecs and a latency of less than 2 msecs, twice as spry as the fastest Ultra ATA drive.

Even the speediest drive by itself can only give you 60MB/sec or so. It’s when you get a bunch of drives together that the glory of high-performance storage shines down upon your throughput. Think about setting up your drives in a RAID 5 configuration. Its striping will move bits to and fro with alarming alacrity, and give you redundancy for reliability. Lose a disk, and keep on keeping on. This amazing speed and robustness comes with a price tag, and that’s capacity. With RAID 5, you need roughly three disks to hold the information of two. If you’re willing to sacrifice some robustness for speed and capacity, or your FC-AL controller does not do the hardware RAID thang, RAID 0 will fill your bill. It’s the fastest of the fast, and RAID 0 software is bundled with OS X.

What? What’s that, you say? Still not fast enough? Well, hang onto your handbags, ladies, here’s where we kick it to the next level. After Fibre Channel, the next frontier in speed is Ultra320 SCSI. Three hundred and twenty megabytes a second, albeit without the hot-swap features and cabling flexibility of Fibre Channel. If you slapped two of these interfaces in your Xserve and ran ’em at full tilt, you’d saturate the PCI bus. How cool is that?

Not cool enough? It must be because those 15000rpm disks are throwing off more heat than the Xserve was designed for. Never fear, the aftermarket has seen fit to address your concerns. Size is the inevitable trade-off for speed. Still, you can have your Xserves share external drive enclosures, meaning you can fit two servers and a drive enclosure in a 5U space, and trick each Mac out with 120GB of 15000rpm drives, or 600GB of 10000rpm drives. Can’t beat that with a stick.

You could also buy an external hardware RAID box, complete with hot-swappable drive bays. This may be your best bet for RAID 5 setups, where uptime is everything: Not only will you be spared the hassle of rolling your own RAID, you don’t have to take down the system(s) to replace dead drives. A Fibre Channel hardware RAID can be shared by multiple systems, too. This setup would be a SAN, and it is some seriously sexy stuff I’ll cover in a future essay.

Three hundred and twenty megabytes a second of throughput with a latency of three point nine miliseconds. Say that out loud, long and slow, with a Southern drawl. Then say to your amazed colleages in the same voice while patting the rack your Xserve sits in, “It ain’t exactly a stock setup ...”

How you gonna hot-rod your box? Let us know below.

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