Explain SCSI to me

I was at Alienware.com pricing a new system for myself and was looking at the SCSI drives. I noticed they operate at 10,000 rpm as opposed to the standard 7200. They are also a bit more expensive. Is speed the only advantage? If one has a non-negligable amount of money to spend on a system and wants every speed tweak available, is SCSI the way to go? Will that extra 2800 RPM really make a difference assuming everything else on the system is also top of the line?

Yes SCSI is definitely the way to go for high performance.

First, like you mentioned, the speed of the disks go much higher than normal IDE disks. The performance is noticable, and will shave a fair amount of time from loading programs.

Second, unlike IDE which can only have four devices (Pri master/slave, Sec master/slave), SCSI can have 14 devices attached, both internally and externally. No more running out of room when trying to install that new DVD player!

Third, the SCSI card has its own processer on it, so your CPU can devote its full attention to getting work done. Watch your CPU usage when copying huge files before and after SCSI. You’ll see what I mean. Major difference.

Hope this helps.

Whoops. SCSI can have 17 devices. Not 14.
My bad.

Ohhh… SCSI. One of my favorite Geek topics.

** SCSI - Small Computer System Interface **

To start off with let me explain that SCSI is fast as He11. It easily outstrips even the new Ultra 133 IDE devices that you see advertised.

SCSI has been a staple technology in file servers for years. Fast, ultra reliable, and (at the time) capable of dealing with hard-drives so large even the best IDE contollers would choke. The secret to SCSI’s speed? It’s controllers. SCSI takes the load off of the processor and places it on the controller itself. Each drive contains an EPROM (Electroncially Programable Read Only Memeory) that explains how to best use the drive to the controller.

SCSI comes in a number of flavors. SCSI, SCSI II, SCSI II WIDE, SCSI III, SCSI LVD. That’s the problem most people have with SCSI. Which drives goes with which controller? If you’re new to High-End computing I recommend an Adaptech 2940U2W controller card. It will support standard 50 pin SCSI for your CD-ROMs and DVD-Drives. It will also support Ultra Wide, the most common Hard Drive connection in the SCSI world. To be honest, if you’re not comfortable with working on your own PC, I would avoid SCSI. Ask you’re vendor. Thats the best answer I can give you.

This also allows multiple drives to be accessed at once, allowing you to saturate the SCSI bus (up to 160 Mb/s - 320 is on the way). IDE only allows one device to access the bus at once. This makes SCSI very good for server applications.

About the only downside to SCSI is the price - usually about twice the price of the equivalent IDE capacity. Oh, you can get 15000RPM drives now too.

You can also move into even more extreme territory with SCSI drives using a RAID controller (Redundant Array of Inexpensive Drives). An array controller can take many drives and make them appear to the computer as one gigantic drive. The purpose of this is it allows for redundancy (in RAID 5 one drive can completely die and your system will keep working) as well as some ease of use (hot swappable drives…you can remove and replace a dead drive without turning off the computer). You also get a noticeable speed increase on any drive I/O as the drives can read and write from multiple sources at once (i.e. instead of one drive having to write 8 bits in series you can have 8 drives each writing 1 bit of information simultaneously).

Of course, all of this comes with a price so array controllers are generally left to server class machines or very highend workstations. IDE RAID also exists but every performance review I’ve seen gives them no gain in performance (although it can allow some redundant features).

I guess I should note that RAID-5 requires a minimum of three disk drives to work. You also lose a not inconsiderable amount of drivespace in order for RAID-5 to work (e.g. with 3, 20GB drives in RAID-5 configuration you net about 40GB of usable space).

Still, if you want the max performance and have money to burn RAID-5 is the way to go.

[sub]NOTE: There are also other flavors of RAID such as 0, 1 and so on each with its own benefits and drawbacks…RAID-5 is the best fo the lot.[/sub]

Consider that harddisks optimized for performance and mostly intended to be stacked away in a server room aren’t built with the same requirements in mind concerning noise level as your average desktop drives.
At least that’s what I remember from back in the dark ages when 10Gb meant big and 7200rpm meant zoom.

Hmm. Tricky one.

SCSI is cool because its rather fast, and allows you to stack a lot of disk drives together, and has bi-directional data transfer and stuff. However, its expensive.

The standatd IDE (intergrated digital electronics) system only allows two channels, with two devices on each channel. Ergo, four devices. And its slower than SCSI. But, however, few people need more than 4 devices, and IDE devices are notably cheaper. (Like my 80GB drive was only around £70).

At the moment IDE is better for desktop home systems and SCSI is a must for servers. Ill be getting SCSI on my next computer, but thats a long way in the future.

First of all, the disk interface and the disk hardware are two seperate issues. You can often get an almost identical disk drive with either a SCSI controller or an IDE controller attached to it.

IDE stands for Integrated Drive Electronics. What did they integrate? It’s basically the architecture of a very old PC disk controller that is built into the disk drive (although in all fairness it has evolved over the years a bit). As the name suggests, and evident even more so when you call it by its other name of ATA (AT, as in the 286 PC made by IBM, attachment), it is very closely coupled to the PC architecture. Hence it was designed for a single user machine.

SCSI is the successor to DSSI, an old disk interface found in many old VAX computers. VAX computers were old multi-user mainframe type computers, and could do all kinds of neat things like clustering and pre-emptive multitasking back in the 70’s (these are all “new” features according to Microsoft in the PC world). SCSI stands for Small Computer System Interface, and it’s designed for relatively small systems (i.e. not huge mainframes), but historically it’s also designed for systems that are multi-user.

Ok, now fast forward a few dozen years. PCs use IDE disk drives, and the rest of the world basically uses SCSI. PCs at the time also basically take over the world, so IDE drives get really, really cheap. It’s also not a complete division, since you can get SCSI interfaces for a PC and occasionally an IDE interface on something that is not a PC. High end servers tend to stick with SCSI, because since it was designed for multi-tasking, multi-user systems to begin with, it tends to perform better in these areas. This gives SCSI a reputation of being “faster” but in a single user environment this isn’t necessarily so.

So, to finally get around to answering your question, it depends on what you use the computer and disk for. SCSI disks aren’t produced in such large quantities, so you don’t get as much bang for your buck. All other things being equal, you can probably expect to pay an extra 20 to 50 percent more just to have the SCSI interface. However, if money is no object, then the higher end disk drives are going to be made for higher end systems, which means they will most likely have a SCSI interface.

There are a lot of things that factor into disk performance. The two main ones are the RPM of the disk and the track to track seek time of the disk head. Look for the specs on the max sustained data transfer rate. This should give you a relative comparison of the disk performance. Beware of burst transfer rates. Disk drives will often cache data, and reading data out of a cache makes for a very fast burst (and then of course the data transfer drops as soon as the cache is empty and it has to read the physical disk).

“Cray has a solid state drive that transfers data at 80 Gigabytes a second. The fastest SCSI drive tops out around 60MB/s.”


I want a Cray :slight_smile:

one thing I have noticed about scsi in the past (i don’t know if it is overcome yet) is that having a scsi card in your computer means longer boot times (I guess it’s because the PC must initilize the card which then must start the drive).

If cost is not an option then scsi is the way to go - BUT you must also consider if you stick with eIDE drives (and they do make 9600 rpm eIDE drives now) you will save a butt load of money that yyou could put towards more ram/ faster cpu, faster video card, etc. that would most likely be a better increase in performance for the $.

Since I’m a Mac person, I’ve had SCSI as long as I’ve had computers. (Yeah, I never owned a Mac prior to the SE).

On the one hand, it’s a fast bus and daisy-chaining is cool. I once had my SCSI-1 bus filled (7 devices plus the computer itself, NOT 15 or 16, that’s a more modern implementation of SCSI) – a flatbed scanner, a SyQuest drive, a Jaz drive, a DaynaFile drive (5.25" floppy for reading elderly PC 5.25" diskettes on the Mac), a Zip drive, an external hard drive, and a Polaroid digital camera.

On the other hand, we used to say “It’s called ‘scuzzy’ for a reason.” I’ve spend entire days switching the order of the chain around, changing SCSI ID#s, trying the chain with and without active and/or passive terminators.

Going back to the first hand, though, SCSI is indeed fast (far more so for modern implementations) and you can boot from a SCSI drive regardless of location (even on a PC, which is far pickier about what it will boot from than a Mac, which will boot from a “slave” ATA drive).

And if you have multiple empty card slots, you can set up multiple SCSI busses, one for fast devices and one for slower and more elderly devices.

But expect to pay for it. You don’t even want to know how much a 100 gig SCSI drive will cost you.