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Going Deep Inside Xserve Apple Drive Modules
adamengst writes "If you've had an Xserve drive fail, you may have considered saving some money by putting a replacement drive inside its Apple Drive Module. That may be a false economy, though. TidBITS explains why, while pinning Apple down on exactly what goes into Apple Drive Modules and why they cost so much more than bare retail drives."
While I hate to copy it, the server being pummeled and reporting errors for 9/10 requests doesn't lead to ad revenue either, so here goes:
About a year ago, we bought an Intel-based Xserve with a pair of 80 GB SATA drives to act as our primary Web server. When the boot drive went flaky on us in October 2008, we were able to recover from the backup on the second drive and off-site backups, if a little shakily (see "TidBITS Outage Causes Editors Outrage", 2008-10-07). But although we were able to bring the machine back online, we didn't trust the drive that had failed. Since the Xserve has three drive bays, the obvious solution was to purchase another drive. Sounds simple, doesn't it? Not so much.
You cannot buy a bare hard drive and insert it into an Xserve, as you can with a Mac Pro (and having just added a drive to my new Mac Pro, I can say that Apple did a stunningly nice job in making it easy to add drives, especially in comparison to the awful approach they used in the Power Mac G5). Instead, Xserves require Apple Drive Modules, which are custom carriers containing drives.
For users accustomed to buying inexpensive hard drives, Apple's pricing on the Apple Drive Modules comes as a bit of a shock. An 80 GB SATA ADM costs $200 from Apple, and a 1 TB SATA ADM costs $450. In comparison, a bare 80 GB SATA drive can be purchased for a measly $35, and a 1 TB drive is only about $100. That would seem to point toward buying a new SATA drive and swapping it into the bad drive's ADM. However, when I started down that path, a number of problems arose, such that I bailed on a quick solution and simply purchased a new 80 GB SATA ADM to replace the bad one.
First, I wasn't sure whether my Xserve had SATA drives, as I thought, because System Profiler on the Xserve shows nothing on the SATA bus, instead including all drives on the SAS bus. (SAS stands for Serial Attached SCSI, and is a high-performance data transfer technology that supports fast SCSI drives and is downward compatible with SATA drives.) After some discussion with knowledgeable folks on the MacEnterprise list and careful reading of the drive details in the SAS section of System Profiler, it became clear that both SAS and SATA drives are shown in the SAS section, with SATA drives having "ATA" as the Manufacturer, and showing "Yes" in the SATA Device line.
Second, once I knew that I had SATA drives in my ADMs, I started investigating if there were any gotchas involved in replacing the drives. There turned out to be surprisingly little hard information about this, with some people having replaced an ADM's drive with no trouble and others experiencing performance or reliability issues. I did find a few discussions about how replacing drives isn't recommended, but giving no solid sources.
Confused, I contacted Apple to discuss why ADMs are so expensive in comparison to bare drives, exactly what an ADM does, what Apple recommends users do with failing ADMs, and whether or not replacing a drive in one is a good idea. That conversation revealed a great deal of interesting information about the ADM and shed some light on what people with flaky ADM drives should do.
Drive Selection -- The most important fact to know about ADMs is that Apple doesn't use just any drives. We've all benefited from the amazingly low cost of storage. But whenever manufacturers compete on price, they cut corners every way they can to reduce costs. Although drive reliability is generally good, everyone who buys bare drives regularly has a drive vendor they refuse to patronize due to bad experiences in the past. (As is often the case, these people all hate different vendors, depending on which one was having a bad run at any given time.)
Since the Xserve is designed to be in constant use - 24 hours a day, 7 days a week, for years at a time - Apple doesn't use the least expensive drives available, since those drives are designed for more normal duty cycles in desktop computers - 8 to 10 hours per day, with variable use during that time. Instead, Apple wor
The pixie dust is in the controller, not the platter.
Wow, those Apple Disk Modules are cheap! A 1TB SATAII 7.2K RPM disk module for an EMC CX3 SAN runs about $1500. But I think they get to high grade the drive makers' inventory since they suggest only 1 hot spare per 30 disks.
The summary makes it seems that there's no rhyme or reason why Apple charges more for their HDs and why can't the consumer simply replace it with a standard SATA HD. If you RTFA, it goes into a long list of reasons why. Whether you accept Apple's reasoning is another matter.
To begin, XServes use a HD module called ADM rather than simple HDs. On the new MacPros, they also use a module but those modules are designed to replace the HDs inside. For the XServe you apparently can't get a bare drive alone, you have to replace the whole module. The author begins to list the reasons:
And then the author concludes:
Well, there's spam egg sausage and spam, that's not got much spam in it.
In theory a SCSI disk should not be much better than ATA but the reality is the best made, more reliable parts go to the high end more profitable products.
There are, typically, huge differences between your average SCSI and ATA disk beyond just manufacturing quality. USENIX paper here. Differences range from disk interface command richness, to reliability under wider operating conditions, to materials and assembly. They do the same thing, but they aren't even close to being the same thing.
Parts of this may be true (it's impossible to say, since they don't specify exactly what they're comparing) -- but even if parts are true, it's misleading at best. In fact, some of it doesn't even seem to make sense. Let's look at real price lists from Apple and dell.
First we note that Dell doesn't seem to offer a 73 GB drive at all, so it's not entirely clear what they're comparing. The most likely possibility appears to be Apple's smallest option, a 73 GB SAS ADM ($300) to Dell's smallest, a 146 GB SAS ($349). While it's certainly true that the prices are comparable, it's also true that the Dell drive is twice as big.
For 300 GB SAS drives, the Apple site shows $650 while the Dell site shows $699. While Apple's price is lower, it's certainly not even close to $200 lower. To get a $200 price difference, it looks like they compared the full price of a 300 GB drive for the Dell to upgrade price for the Apple (i.e. the price difference for changing from the stock drive to the 300 GB drive).
For 1 TB hard drives, they have something of a point, but not a very good one. Apple's price for a 1 TB SATA drive is $450, while Dell's is $639. They fail to note, however, that Dell also lists a 1 TB SAS drive (an option not available for the XServe) for $679. Taking this into account, it looks a great deal as if Dell is simply doing their best to encourage their higher-end customers to use enterprise-class SAS drives by offering them at a purely nominal incremental cost over SATA drives.
The original article attempts to portray the situation as Apple offering prices that are at least as good as, and often better than the competition. In reality, there appears to be only one reasonable configuration where the Apple is likely to be competitive: the one using 300 GB SAS. At the low end, the Dell offers twice as big of a drive as the Apple for a purely nominal price difference. For lots of storage, the Apple offers only SATA drives where Dell offers SAS. If you're storing 1 TB of data (or more) the incremental cost of SAS is usually fully justified. There are undoubtedly exceptions, but they're not particularly common.
The universe is a figment of its own imagination.