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SCSI vs. SATA In a File Server?
turboflux asks: "I'm currently in the process of replacing an aging file server with something more robust. Company-wide, there will be about 100 people who could be using this server, but I don't imagine there being more than 50 concurrent users. Right now, I'm torn between spending alot on SCSI hardware, much like our other servers, or spending less, but getting more space, with SATA II drives. Whatever I decide, the server will be setup with a RAID 1+0 array for the numerous benefits it offers. Does Slashdot have opinions or suggestions on performance, reliability, and stability?"
Have you ever thought about the benefits of RLL?
I don't mean this as a troll, but I was under the impression that USB 1.0 replaced SCSI? Or was that desktop only, and it still has server uses? I mean, I thought USB killed SCSI? Or am I thinking of something different?
SATA is fast and cheap; just make sure you spend a little bit more to get the "nearline" storage drives and not just desktop drives. Put them behind a 3ware 9550 and you'll fly.
Fellowship 9/11
Clearly the answer is SCSI - go with what you know, servers are not the best computers to experiment with random equipment.
I'm starting to think GNU is the problem with "GNU/Linux" these days.
Think about the fact that you need a sufficient Backup. You can buy lots of cheap storage with SATA Disks, but Ultrium 3 Tapes (400/800) are still expensive as fuck. Never forget that cost when calculating.
OTOH, there are 300GB U320 Disks now, which you could use if latency is not an issue. Otherwise, go with lots of disk arms (72GB or 36GB U320 Disks)
We have both SCSI raid (2 1TB arrays with 10k RPM SCSI drives on a dell powervault) and a several arrays with 3ware cards (an 8 way and a 12 way both with 200 or 250GB drives). We run Red Hat WS. We find that the 3ware cards are excellent for large data storage but have latency issues compared to the SCSI raid array. We are happy with both systems, but the price break on the 3ware shows, and I wouldn't recommend for really heavy use.
Laboratree - Scientific collaboration based on OpenSocial.
I use SATA on our smaller, non-mission-critical servers. For our data backend, I wouldn't touch it with a 10-foot pole.
Here are some scenarios where I wouldn't hesitate to use SATA:
- You have redundant servers. Using LVS and/or Heartbeat and your favorite tools, you can get full server redundancy using less expensive hardware. The overall solution can be quite elegant, with hot failover. Why just cover the drives?
- Front-end cluster nodes. You have a powerful, expensive backend server (with a cheaper failover) and you use inexpensive front-end servers for serving client requests. Sounds like overkill for what you want, but with the right server load balancing technology, it can give you a scalable, fault-tolerant and damn fast solution.
- You can live with downtime. Install a server with a couple of SATA disks in a RAID configuration and hope for the best.
Go SCSI. It's pretty amazing that there are so many people out there whose only exposure to SCSI has been a mention in a textbook or having read about it, considering it was the standard for performance for so long. The speed and reliability is definitely better than SATA and as you well know, price isn't much of an issue when the RAID fails is it?
-- Eli Juicy Jones
For any tier 2 storage, SATA is the future. I'd go so far as to say that in about 80% of all instances, good SATA-II drives with NCQ and large caches in a proper RAID setup will be far, far more than adequate for most production servers unless you do tons of non-sequential I/O, need tons of iops, etc.
For a file server, you'll be fine with SATA.
For my tier 2 servers, I am moving a ton of stuff off of my EMC gear (because fibre channel drives are damned expensive) onto SATA-II drives in an iSCSI setup. I'm already running servers with trunked gigabit NICs... might as well let Win/*NIX boot from a local drive and mount block-level iSCSI devices over the gigabit fabric. Save a shit ton of money and get tons of space on the cheap (7.5 terabytes of fast SATA-II in an iSCSI chassis for well under $10K... make your RAID groups, carve up some LUNs, and par-tay!).
There might be some benefits to you to sticking to SCSI vs. SATA, it really depends on your preference. Both SCSI and SATA offload the main processor from the duties associated with reads and writes. SATA also now has optimized reading patterns just like SCSI. The only real advatages SCSI has right now are the speeds (SATA 150 (there is a newer faster one coming) vs SCSI 320). Also, most SCSI drives are desgined for 24/7 use, whereas most SATA drives are designed for desktop use. Just make sure the SATA drives you buy are made for Enterprise level operation. So it really comes down to compatability/speed vs. cheap/larger. Hope this helps!
Sabre
Why? Why? Why?
At least give us a 2 line explanation, so I don't think you're speaking crap that you read in an advertisement! Please, if you have some justification, I'd love to know what it is... seriously.
the SATA II spec is quite a bit different from the original SATA. SATA II adds port multiplication, hot plugging, native command queuing, external enclosures, and port selection. Also, with a theoretical peak of 3Gbps, it's twice as fast as the old SATA. here is a decent article with more explanation.
Serial-attached SCSI. 15K RPM drives, SAS RAID 1+0. Heaven.
If you like what I've said here, and want to read more, go to http://www.krillrblog.com
USB 1.0 didn't kill SCSI, Steve Ballmer F**KING KILLED SCSI!
Now Ballmer says he is going to F**KING KILL USB.
Man I need a T-Shirt that says that.
"Steve Ballmer says, "I'm going to f**king kill you!"".
Modesty is one of life's greatest attributes
SCSI is very fast, and usually more expensive. You can get really fast, highly cached drives in SCSI with high-RPM spindles, and cool controllers. But they're $$$$. Do you need the speed?
If not, SATA is still pretty fast, much less expensive, less clever controllers, but still very reasonable for things like archiving, steady low-concurrency-demand streaming, and so on.
SATA also has the advantage of not needing loads of austere cables with distance limitations imposed on them; it's a serial rather than a parallel bus-- hence the S in SATA. Use SATA when you don't need the absolute fastest you can get-- and you won't have to spend the most on the controller (which is hopefully a SCSI PCI-X controller or other fast clocker), the drives, the pricey cables, and so on. But if you need the speed, there is no faster than SCSI except for flash drives, which are still hideously expensive.... and not writeable as much as we'd like them to be.
---- Teach Peace. It's Cheaper Than War.
I recently did all this research myself. SATA on Linux is going to get MUCH faster, probably as fast as SCSI, but you'll have to wait for the libATA improvements to take hold. Right now NCQ isn't implemented, and neither are 'multiple sector transfers'. I bought hardware that WILL support those features because I know that NCQ will dramatically improve speed and latency (under high-use conditions) when it is finally fully-baked.
The site to track progress on the library and driver status is here: http://linux.yyz.us/sata/
The project has been moving along quite well. I think their goal is to completely modularize, simplify, optimize, and consolidate the ATA, ATAPI, and SATA kernel pieces into one overarching (underlying?) library. I like this kind of work. I can't see why ALL disk-like I/O isn't under one big modular kernel library, it seems like it would make adding new transport types and drivers a lot simpler and reduce maintainance all-around.
"Sometimes, I think Trent just needs a cup of hot chocolate and a blankie." -Tori Amos on Nine Inch Nails
I'd say Fibre Channel.
One benefit that SATA does have over SCSI is the cabling....it's smaller and blocks less airflow (and easier to do the cabling).
SCSI on the other had has other benefits....like it's used in enterprise servers now. Faster, daisy-chained, more RAID options, etc.
Of course, Fibre Channel is basically SCSI on steroids and has the cabling benefits that SATA has.
With more room thanks to less data cabling, u can add watercooling to reduce the heat generated by the 15k+ rpm drives.
I am a huge fan of efficent, cheap systems. The bulk of our server load is handled by dual opteron machines with 3ware cards and a 10k rpm system spindles and 7.2k rpm data spindles. However, even the best sata drives choke under file system and database loads and our primary data stores are U320. StorageReview.com has a good review of the new 150gig 10k rpm WD drive that shows it gettting spanked by SCSI drives under non-linear server loads. Long story short, if you expect a lot of drive activity you might be able to eek by for a while with a well tuned SATA system but you will have to pony up for a SCSI system at some point and you might as well do it now and save yourself the hassle of migrating later.
SAS hardware is currently a little harder to find than SCSI or SATA stuff, but I'm sure there's a good selection out there if you take the time to look.
I was checking out the Sun Fire 4100 a while ago, and it takes SAS drives, however the form factor is 2.5", and I haven't yet seen any 2.5" SATA drives (I wanted that compatibility). Also, I've heard SATA drives don't work with the Sun Fire 4100's SAS controller anyway. Not sure about that, since the SAS spec says they should work, but just something to keep in mind when you're looking for a server or mobo or controller that supports SAS.
One way to curb some of the cost, I might add, would be to switch to something like RAID 5... you won't have as high throughput, but you'll still see performance gains and end up with more usable drive space. The throughput likely won't be your problem, anyways... typically it would be the drive's ability to handle multiple simultaneous requests, which heavily relies on low access times (which is why SCSI dominates in this type of environment).
Here's a quick reference of some IOMeter benchmarks using a file server test pattern. You'll see what I mean. Wealth of info on drives on that site.
You cannot buy the same performance class of drives in SATA as you can with SCSI. Some people call this a market segmentation scheme, I call it catering to the market. People who demand top class drive performance typically also want the other benefits of SCSI as well. Whether those benefits are needed for your requirements, well, depends on your requirements.
SCSI can (depending on which particular SCSI) provide you with more devices per controller without sacrificing (any noticeable) performance. If you need to shove a ton of drives into one server, this will add up quickly. Since you are talking about RAID 0+1, depending on how much storage you are shooting for, this may be a strong factor (but you may be able to skin by on the 4-6 SATA ports you'll find on most mobo's).
SCSI is more mature. So drivers are likely to be more robust, more efficient, and more stable than those you'll find in your garden variety SATA.
You'll typically find that under heavy load, SCSI performs better. Again, this is mostly due to so called "market segmentation" schemes, but that is why you pay more. If your users are going to be mostly dealing with the usual, periodic saving of word processing documents, spreadsheets, and a couple of light media files - you probably don't need to handle really heavy loads. The RAID controller will eat the peaks of write demand in cache (if you get a decent RAID controller - see later), and you should have fairly smooth performance. Then again, if your users are constantly running large installers (development test environment) or working with large remote files - you should really go SCSI.
All that said: I think you would be served best by investing in a better RAID controller rather than investing in top of the line drives. The RAID controllers they integrate on to most motherboards are crap (for what you are trying to do, desktop use - meh). You want something with a ton of cache, and good management soft/firmware. If you buy a real server class motherboard, you may get a better onboard RAID, but however you go about this - pay the most attention to this detail. Unless you really need low latency for high demand, random access applications, top end drives probably won't give you much over the usual network latencies.
I bought into the whole "SATA is the new less-expensive SCSI" and put in two new file servers using SATA last spring. I can say that I'm unimpressed with the SATA servers as compared with our SCSI servers. I now wish we'd spent the extra $1000/server and gone with SCSI. I recommend SCSI -- you won't second guess your decision down the road.
BTW, we used 3ware controllers with WD RAID Edition HDs. We're supporting approximately 75 users per server.
As a matter of fact, you may not be flying at all. It all depends what you're using it for. The problem with SATA is latency, and there's not much that controller is going to do about it. If you've got a server that is performing latency sensitive tasks, then SATA can cause performance problems.
In my experience, if you've got alot of random I/O, SATA is not a viable solution. That said, even if your I/O is mostly random, if there's not a heavy load on the disk, then you're probably ok. If you've got 200 people hitting a database or email server, you're probably going to have some performance problems. Swap it out with SCSI drives, or a quality disk array, and you'll be doing much better. If you've got a web server, or a database server that is exclusively reading, you can probably get away with SATA. Again, it all depends on how much and how random the disk I/O for your application is.
OK, hear mine then.
We have several terabytes of SATA storage at work to hold our main business-critical digital asset archive.
We've been using a ATA/SATA disk-only strategy for over 5 years now. It's worked great, and eliminated our slow and unreliable tape robot, which has greatly improved productivity.
Back in 1999/2000 SCSI wasn't an option for the main archive because a terabyte of SCSI would have broken the bank. We went ATA back then. It was a mess trying to route 24 ATA cables in a case, I admit. SATA fixes that nicely.
We keep three copies of our data, two onsite and one offsite. We use rsync-incremental snapshots to do disk-based incremental backups. Because the cost of SATA is less than 1/3rd the cost of SCSI, we get a high reliability solution for less than the price of a single SCSI RAID.
One more advantage of SATA is that the disks are so cheap, it's easy to just replace all of them every two or three years. The disks you replace them with generally are twice as large after 2 or 3 years, so every cycle your RAIDs get more reliable as the number of disks is slashed in half.
Most companies wouldn't replace every SCSI disk every two years, it would cost way too much. And considering the slow pace of SCSI size growth, you wouldn't see as much gain, a double hit against SCSI.
So basically unless you need the excellent latency performance of SCSI, higher than even the WD Raptor can offer, I see no compelling reason to use SCSI for anything anymore.
I've had enough abrasive sigs. Kittens are cute and fuzzy.
SATA's peak raw transfer rate (150 MB/sec) is half that of the peak raw transfer rate of SCSI (320 MB/sec), but you're going to be limited by the individual hard drive's transfer rate anyway. Keep in mind that a proper SATA implementation will be 150MB/sec PER DRIVE, since each drive is on its own channel. SCSI is 320 MB/sec per channel, but you're in for a cabling nightmare if you want only one drive per channel. Note that there is a 300 MB/sec SATA standard, although few drives and controllers seem to support it.
If you buy the right model, you can get SATA drives that have gone through the rigorous quality control testing that has historically been reserved for SCSI drives. Many of the higher end server-grade SATA models are warrantied for 24/7 operation. SCSI has lost its advantage there.
SATA has Native Command Queueing, formerly a SCSI-only performance feature. Note that it's optional for SATA drives though, so make sure you get a controller and drives that support NCQ. Again, one of SCSI's few advantages has disappeared.
Last, but most definately not least, SATA cabling is far simpler and robust than SCSI cabling. SCSI cabling is a finicky nightmare where even high-end cables can cause data corruption if you're not careful, whereas even the cheapest SATA cables I've seen worked reliably. I've had hardware related data loss on hard drives twice in my life. One case was an IBM Deathstar, the other was a SCSI cable that started flaking out and corrupted data on three drives at once. I haven't touched SCSI with a ten foot pole since that incident.
retrorocket.o not found, launch anyway?
For some things you NEED SCSI, for others you don't. That much is obvious.
Large files/streams that require heavily mixed-mode I/O beat the balls off of SATA. E.g. Correct me if I'm wrong, but my partial understanding of SATA is that if many writes are cached and a read enters the queue, the cached writes are trashed.
so if you are working with check-in/check-out I/O type such as Samba profiles, SVN stuff, or (Samba|N)FS on a small-medium number of small-medium size files, or web stuff, SATA offers best price/performance ratio, with RAID or whatnot.
If you are working with large files that get a lot of unpredictable I/O, or databases, you really want SCSI.
RAID 5 is noticably slower disk performance for writes, and radically slower performance for reads and writes if you lose a disk. In many cases, the performance during a failure for RAID 5 can reduce system capacity below the required level, and thus RAID 5 is simply not an acceptable technology for those environments.
If just sticking more data on disk is the requirement, and you don't care how slow it gets if you have a drive fall over, then RAID 5 is great. But real world enterprise environments exist where losing half your disk throughput will cause the company's service to go down, and then you're out of business. Those guys don't RAID 5 if they know what's good for them.
The very definition of RAID is "Redundant Array of INEXPENSIVE Disks". Emphasis mine.
I've already read a bunch of posts about how SCSI is more reliable than SATA. Well, they actually mean SCSI drives are generally more reliable than SATA drivers (and some actually say so). They're quite correct for the most part.
Here's what storage vendors don't want you to know: It doesn't matter.
Use RAID. With SCSI or FC disks, you'll have to use RAID5. At that point, two disk failures in a given array and you're screwed. You REALLY care that two disks don't fail at the same time. And when you're using low-end or even mid-range drives, it happens.
Why do you have to use RAID5? Because with SCSI or FC disks, RAID5 is the only economical option. With a 300GB SCSI drive going for at least $1200USD, and FC drives of that size going for $2500USD, even the biggest corporations end up using RAID5.
Of course, RAID5 isn't the only level of RAID. It's the least redundant of any level of RAID, as a matter of fact.
Go SATA with RAID10, at least 4 drives, ideally six or more. With six drives, the likelyhood of having two drives fail before you can replace the first one is somewhat higher than if you're using SCSI, but the likelyhood of that second drive causing you data loss due to a failed array is infinitesimally smaller. It's guaranteed with RAID5, and the chance for RAID10 is inversely proportional to the number of disks in the array. So first the first drive has to fail, then the second drive which fails has to be of the same RAID1 set. Add onto that that drives do indeed "go old", and the heavier you work them, the faster they get old. With RAID5, disks tend to get worked a lot harder (without any cache, or if the cache misses, each write requires n-2 reads, and 2 writes).
Of course, you've pretty much decided that RAID10 is the way to go. At that point it's cost. If you're looking for 50GB of fast redundant storage, SCSI is going to be slightly cheaper. If you need any amount of storage though, SATA is going to be a whole lot cheaper for the same level of reliability (which requires more spindles), and typically better speed (more spindles means more seeks per second and more megs per second, though one needs to be mindful that big SATA disks are only 7200RPM, while the slowest SCSI disks you're going to get are 10kRPM).
Summary? I'm value-concious. I'd go the SATA route. RAID10, four disks minimum to start, a pair of 4-port 3ware SATA cards with 128MB+ of battery-backed cache. I'd do the RAID entirely with software (Linux MD), with each RAID1 set split across two controllers. We get cheap disk redundancy, cheap disk speed, cheap I/Os, and cheap controller redundancy. I'd consider using less fancy controllers, the 3ware jobbies tend to be expensive, but when you're doing big writes the cache makes a massive difference (75MB/s across four disks of RAID10 versus 20MB/s). I've considered putting together a dedicated storage appliance, exporting via SMB/NFS/NBD/GFS/what-have-you, without the battery-backed cache, but with a pair of 1U UPS units (one for each power supply). Then I'd go around turning off all the application-level fsync()ing, and see what happens with 4GB of disk cache. Bet it'd be fast. And with shutdown initiated via UPS trigger, almost as safe as a battery-backed cache. Remember: "Redundant Array of INEXPENSIVE Disks."
God I ramble.
Barclay family motto:
Aut agere aut mori.
(Either action or death.)
All hardware (and software) sucks, and it breaks, it's a fact of life. No matter if you go with SCSI or SATA, the important thing is that you can find out when a drive dies so that it can get replaced.
Many low to mid range SCSI raid cards (most? all?) either don't have any sort of interface to find the raid status when the server is up (they just beep at you and expect that somehow that's going to be hard over the AC and server noises when you're walking by the machine), or the tools for checking the raid status are so poor that they'll lock up the shared memory segment after checking a certain amount of times (ADAPTEC, I'M TALKING TO YOU). Since being certain about your raid status means checking it via something like nagios, that means that it gets checked many times, and will thus eventually lock up.
While SATA is nowhere near the performance of scsi (despite what SATA fanboys will tell you), 3ware cards are actually really good at:
a) letting you know when a drive has failed
b) letting you check with their tools as many times as you want without locking it up
And since the SATA stuff is so much cheaper, you can buy multiple servers, so even if the card fails, you have a hot backup.
If you absolutely have to have the fastest, go for a raid 10 of 15krpm drives.
If you don't, and want peace of mind, get at least 2 SATA setups with 3ware cards.
Serial Attached SCSI takes the best of both world together:
SAS has:
- lean SATA cables
- 3Gbps transfer, soon to be 6Gpbs. Better than U320
- 15,000 rpm disks
- NCQ like SATAII
- RAID-capable controllers
- SATA on SAS possible
50 concurrent users is a LOT. You may not really mean concurrent, as in "50 people actually reading from or writing to this drive at the same time". If you DO mean that, you desperately need SCSI, the fastest you can find. You'll need seek time more than anything else; the drives need to respond as fast as possible to multiplexed requests for data. Rotation speed, which improves seek time and transfer rate, is good too, but it's seek time that's most crucial in heavy multitasking environments. If by 'concurrent' you mean '50 people occasionally hitting the disk', then yeah, you could probably do SATA.
However, you already have SCSI. Management is used to paying for SCSI machines. If you have 50-100 people depending on something, and it's slow, that's a productivity drag. If you assume that all those people cost $100k/year each (not at all unreasonable with benefits), 50 people are getting paid about 2,500 bucks an hour, or about 20,000 dollars a day. In other words, if you speed them up by just 5% with better hardware, you're saving the company a thousand dollars a day. Even if it's a tiny 1% speed gain, that's still 200 bucks a day. Saving six grand a month for an upfront investment of ten grand is a total no brainer.
Buy SCSI.
.. if you do end up with SATA, make sure to get some neon lighting for inside the case.
--
Current setup - 4x Seagate 400GB SATA, NVRAID-0, ThermalTechno 4000 1U Case w/ Ground-FX, 3x Zalmat 80mm SilentKiller Fans (soon)
If my limited experience has taught me anything about computer reliability, it's that a single mis-set bit somewhere can bring down a system. Maybe the bit got there by user error, maybe it got there because of RAM or disk failure, maybe it got there from a bug in the application, OS, or firmware. Maybe a component on the motherboard shorted out. Maybe it's the climate. Maybe it's the phase of the moon.
I've seen it happen with discount ghetto hardware, I've seen it happen with high end hardware. I've seen it happen on Windows. On Linux. On FreeBSD. On Solaris. I've seen servers go down due to catastrophic hardware failure and I've seen them go down because a $2 fan died. I've seen people come inches from major power supply caused injury working on a desktop PC.
Everything will break.
There's just too much freaking complexity. Now I just buy whatever's cheapest so I can buy way more than I need. Mix up the configurations a bit so you get some bio-diversity; if one drive manufacturer has a bad year, you don't want all of your eggs invested in them.
Most important of all, at the first sign of trouble, throw it away.
Try to resist the urge to fix it. I mean it. You cost more than that piece of junk. Put in a purchase request and move on.
It's not necessarily for "internal components". It's also for entry-level servers and raid arrays. You can get hotplug bays that fit in 5 1/4" slots on a machine. This provides an easy way to swap out the drives in case of failure. If you're running a server you nor your users want downtime. If you're running RAID 1, RAID 5 or RAID 10, you want to be able to rebuild to the replacement drive before you lose another drive and lose the whole array. Its a lot faster to rebuild a single drive from the existing data on the other drives, than having to restore from tape or other backup media.
Also motherboard manufacturers are now starting to include external SATAII ports to "hotplug" external SATA drives.
SCSI still tears the alternatives to shreds for price/performance at the heavy end of the load curve, no doubt about it.
If you doubt it, try both.
For going on twenty years it's been the same: those who haven't tried SCSI claim that there's no or little difference. Those who have used both SCSI and [MFM,RLL,IDE,ATA,SATA] in high-load environments hate to try to make due with anything but SCSI.
For performance and reliability reasons both, you want SCSI if you're dealing with high-random-access-load or high-throughput situations. ATA/SATA is fine if you're just offering up noncritical bulk network storage but for the rest you want the real deal, and you will notice the obvious difference if you try both in a stressed environment.
STOP . AMERICA . NOW
No, choose RAID 5 instead of RAID 1+0. Here is why:
To give you a datapoint, I have set up multiple Linux software RAID 5 arrays on various servers with 10+ SATA disks, and the I/O throughput is over 500+ MB/s (enough to saturate 2 full-duplex GigE links !). At my previous work we had about 200 servers, all using Linux software RAID 5. And we have been MUCH MORE HAPPY than the previous setup where all of them were using hardware RAID 5. Moreover, Linux's software RAID 5 is more flexible (create arrays on ANY disk on ANY SCSI/SATA card in the system), more consistant (one and only one control software to learn: mdadm(8), no need to use crappy vendor tools or reboot into vendor BIOSes), cheaper (no hardware to buy), more reliable (no hardware card = 1 less hw component that can fail), easier to troubleshoot (plug the disks on ANY linux server and it works, no reliance on any particular hw card) and more scalable (spread the load across multiple disk controllers, multiple PCI-X/PCIe busses, or even multiple SAN devices).
It's amazing the amount of misinformation and misconceptions about RAID that is spread around the world. I hate to say it but 95% of IT engineers don't make good choices regarding RAID servers because of all those misconceptions.
If you want reliability for the disk you had better check what the manufacturer claims for the MTBF (mean time between failure).
Many SATA drivers have a MTBF of around 0.6 to 1 where as SCSI have between 1 and 2. Your SCSI disk therefore has about twice the life expectancy. If you couple this with the speed of the SCSI I guess for the moment if your budget allows for it then go for SCSI
If your budget doesn't allow for it...just make sure you have good redundancy in your RAID with at least 2 redundant disks
perl -e 'print $i=pack(c5, (41*2), sqrt(7056), (unpack(c,H)-2), oct(115), 10);'
Assuming equal storage sizes, SCSI drives would have way better throughput and latency than a SATA drive because you can get 15K SCSIs. However, the sizes are NOT equal. Fact is that for the price of a 147GB 15K SCSI drive, you can get about 2TB of 7200RPM SATA space.
What you end up with is the following throughput when disks are empty:
1x147GB 15K SCSI -- 150MB/s
8x250GB 7200 SATA -- 275MB/s to 550MB/s depending on exact RAID configuration
Now fill up both configurations with 140GB of data and the throughput of the 15K SCSI has dropped in half to 75MB/s because the heads are now positioned at the "slower" inner portion of the disk. Meanwhile, the 2TB SATA config is 7%-15% slower depending on the RAID config.
Latency also benefits from many disks for the same reason. Fill up a disk and you possibly have to traverse the entire disk. So while a 15K drive has a seek time of 2-3 times faster, you end up having to move 10X-15X farther than in a mega array where the heads pretty much just hover over the 2X faster outer portion.
The big advantage for SCSI is the better TCQ algorithms for multi-user access. This can be mostly negated if you use a SATA RAID controller with enough onboard RAM to reorder IO at the controller level versus depending on the drive's NCQ.
This is the route we've taken -- we went from a LSI MegaRAID 320-1 + 4-drive SCSI RAID config to an Areca 1170 + 1GB RAM + 24-drive SATA RAID. Every aspect of performance is up by big amounts -- throughput, latency, multi-user access. The drive array is actually TOO fast for our 2x244 Opteron server to drive. We ended breaking the array into 3 8-drive volumes and mirroring 2 volumes against each other for more redundancy. One of these days, we'll upgrade to faster CPUs and retest a 16-drive volume.