SCSI vs SATA, Which is Faster?
Written on February 2, 2005 by Jon BachIntroduction
When configuring a server or high end custom computer, we are often asked about the performance benefits of SCSI over SATA. Since SCSI is much more expensive, the common perception is that it must be significantly faster. The short answer we give to that issue is that with the release of 10,000 RPM SATA drives, SCSI simply does not hold the edge it used to, and we do not feel it is worth the sizable increase in cost. Of course, that statement is very general. Surely there are still applications that greatly benefit from SCSI, and it is the goal of this article to take a deeper look at the performance differences in SCSI vs SATA, and to tell you how those differences translate to performance in real world applications.
Background
As with any analysis, we must first look at the benchmarks. As you may know, the biggest difference between SCSI and SATA is that while SCSI has a processor integrated into the controller, SATA makes greater use of the system processor to serve that function. Therefore, the first step we will take in our analysis is to take a close look at how big the controller and rest of the computer plays in the performance of the drives. Will a higher end SCSI controller greatly increase the performance of a SCSI drive, and will a faster system processor help an SATA drive?
SATA Benchmarks
For nearly a year now, we have been benchmarking every system that leaves our door. This gives us access to an incredible wealth of data that we can put to great use here. We will begin by looking at all benchmarks with a 74GB 10k RPM SATA hard drive, to see how the disk performance differs from system to system.
The interesting result is that the CPU speed of the system plays virtually no factor in the speed of the drive. Pentium 4 systems and dual Xeon systems all score in the 5,600 range for overall hard drive benchmarks. It is important to note, however, that a few of our benchmarks using the A8N-SLI motherboard scored in the 6,300 range! This significant increase was due to the fact that the 74GB Raptor drive supports native command queuing, which is used on the A8N-SLI motherboard. Newer controllers and motherboards should also be using this NCQ technology, so if your SATA hard drive supports it, you'll benefit from this increase.
Native Command Queuing
What is native command queuing? It is a technology that has been used in the SCSI world for quite a while, but only recently has been introduced to SATA drives. This explanation is an excerpt from Neoseeker's Nforce 4 Tech Preview.
| Traditionally hard disks on the consumer desktop side process disk requests in a linear fashion. This can potentially be a very bad thing and to understand why, there has to be a basic understanding of the physical structure of a hard disk. Hard disks are made up of platters or disks, much like a compact disk. Each platter is divided into tracks which are concentric circles, tracks are divided into sectors. Each platter is read by one or more heads. Seeking data is fastest when the data resides on the same track. Moving between tracks is time consuming. Consider the case where there are three pieces of data, one on the outermost track, one on the inner most track and one on the outmost track. In a traditional hard disk, the data on the outer track would be read first, then the data on in the inner track second, and finally the third piece of data on the outer track is read. This is not efficient and the time it takes to move the head is the seek time. If the head movement can be minimized, the seek time will decrease accordingly. This is where NCQ comes in - NCQ can rearrange the order of instructions so instead of moving from the outer track to the inner track, both pieces of data may be read from the outer track first before tackling the inner track. |
So, we see that while the speed of the computer plays very little role in the performance of the disk, NCQ does make a difference.
SCSI Benchmarks
Next, we'll do the same comparison technique with various SCSI hard drives. This is a bit more tough since we sell far fewer systems with SCSI drives, but we still see a remarkably predictable trend: while the speed of the SCSI drive makes a large difference, the speed of the system it is in does not make a difference. Note the very similar performance of the maroon and green systems in the graph. The green system is dual Xeon, and the maroon system in a single Xeon, but the performance of the hard drive is nearly identical. In contrast, the slower 146GB SCSI drive shows a significant speed drop, while the 15,000 RPM drive is much faster.
Benchmark Comparison
After taking a look at these benchmarks, we have a very clean outcome: on all cases, the speed of the rest of the system has nearly nothing to do with the speed of the hard drive. This makes the job of comparing hard drives very easy! It means that we can use nearly any benchmark to compare drives, since the rest of the system does not play a role. Of course, everything so far has been interesting...but was just a prerequisite to the rest of the article! The next step is to see how SCSI compares to SATA in performance.
Results
Frankly, I was surprised by the results! They show that SATA has a performance advantage! 10K RPM Raptor SATA drives appear to be on par with the performance of even a 15K RPM SCSI drive, and with NCQ, SATA even holds a very significant lead!
Disclaimers
Now keep in mind that we're strictly talking about hard drive performance. Take a look at the "XP Booting" benchmarks in the last graphic -- you'll notice that is the area in which SCSI holds one of the only advantages to SATA. This is due to CPU utilization -- SCSI drives simply don't use as much CPU power to run, leaving more CPU time for the rest of the system. Based on those numbers, if you are looking to build the fastest possible computer, it does appear that SCSI holds onto a very marginal performance lead. If you are only concerned with getting the highest disk throughput possible, then SATA with NCQ is the way to go! I should also add that given the right SCSI drive (ie, a 146GB 15k RPM), you can still beat the performance of the 74GB Raptor with NCQ, but your costs will be three to four times higher.
Jon,
I really enjoyed reading this article. It looks like you were using the HD test suite from PCMark04 for benchmarking? (Just curious.) The part I was most interested in had to do with NCQ, which I've been looking forward to on principle, even if I didn't see any significant performance increase -- I just like the idea of my HDs operating in a more efficient, "intelligent" manner. ;-) I've also read that HDs supporting NCQ can have a greater longevity since they're operating more efficiently, and that such drives are also useful for people like myself who do a lot of multitasking. Nice! :-)
I'm a little confused in that I was under the impression that WD's 74GB Raptor doesn't support NCQ but rather TCQ (tagged command queueing), since it's not a native SATA drive but has a PATA <-> SATA bridge. Is there any significant difference between NCQ and TCQ, or are they just different acronyms for pretty much the same thing?
Anyway, what I found most helpful and exciting about your article is that you demonstrated the following: a) TCQ/NCQ enhances the performance of a single HD when paired with a quality motherboard that supports it. So, you don't have to have some type of RAID array to see benefits; and b) A person can get the benefits of command queueing at a reasonable cost by using any nForce4 Ultra-based motherboard along with a 74GB Raptor. That's terrific, since other NCQ-ready HDs, like those from Seagate and Maxtor, seem to be pretty pricey at the moment.
I'd like to add a note for anyone looking for motherboards that can support HDs with NCQ: Chances are, you won't find the term "NCQ" anywhere in the motherboard specs. So, you need to look for terms like "SATA II" or "SATA 3.0Gb/s." I think the latter term is what Asus uses.
Again, thanks for an interesting article! :thumbsup
-- Max
Thanks for the correction, Max! You're right, it should have read "TCQ." I'm not too up on the differences...and it appears that most places treat the two as pretty near the same. TomsHardware.com tried to do a comparion at http://www6.tomshardware.com/s... , but didn't end up with very conclusive results...the Raptor blew 'em all away :-D
In my experience, there is little diference between individual drives when comparing ATA, SATA, and SCSI.
Where the diferences start to add up, is when you are comparing multiple drive configurations, where a volume is striped across several (3 or better) drives.
This is where SCSI starts to destroy ATA and SATA.
i'm just curious, what if SCSI raid 0 vs SATA raid 0, assume both of the set up is based on individual raid controller card? :thumbsup
allen
For relative sizes i suppose the test was accurate but what about the smaller scsi drives which provide more performance (typically) then the larger drives or what was the difference in cache sizes. I dont wanna bash the article im just wondering the difference in speeds of different sizes and/or similar cache sizes.
This is perhaps the dumbest artical I've ever encountered. Not only are the tests faulty and ill-conceived, but the conclusions are all wrong.
SCSI benefits most when multiple drives are in use. SCSI drives and SATA drives are fundamentally the same mechanically. The test fails to establish a control, which would be to use a similar front end on the same motherboard. What the first few tests "establish" is that all systems will yield different results, so you can't compare one drive on one MB with another drive on a different MB.
The test that show 4 SCSI drives being slower than 1 illustrated the stupidity of the tests. Clearly the test isn't exercising the drives in a way that would be considered a proper benchmark.
Supermicro makes several MBs that have both SATA and SCSI, but even in that scenario the controllers are different so who's to say you are not testing the controller more than the drive? SCSI often appears faster because of the front end controller used for the scsi while SATA usually uses a raw controller. Using a SCSI-like controller for SATA (such as Areca products) can change that and make SATA function more like a SCSI with front-end buffering. Of course for the price of the controller you could buy a faster processor which may change results in the other direction.
The bottom line is that there is no way to really compare SCSI to SATA directly. You can try a lot of system combinations and say that the fastest SCSI system is faster than the fastest SATA system. But there are too many other variables to isolate one drive against another. Just like you can't really compare AMD vs Intel, because they don't run on the same chipsets. You can only compare one collection of components against another collection of componants.
DT
This is perhaps the dumbest artical I've ever encountered.
I wonder if that's hyperbole?
http://www.google.com/search?s...
That's a GREAT idea for an article. ;-)
For official purpose where we need
higher disk throughput and higher speed which one should we prefer SCSI or SATA?