Serial SCSI Standard Coming Soon

rchatterjee writes "SCSI is very close to joining ATA in leaving a parallel interface design behind in favor of serial one. Serial attached SCSI, as the standard will be known, is expected to be ratified sometime in the second quarter of this year according to this article at Computerworld. Hard drive manufacturers Seagate and Maxtor have already said that they will have drives conforming to the new standard shipping by the end of the year. The new standard will shatter the current SCSI throughput limit of 320 megabit/sec with a starting maximum throughput of 3 gigabit/sec. But before this thread turns into a SCSI fanboy vs. ATA fanboy flame war this other article states that Serial Attached SCSI will be compatible with SATA drives so you can have the best of both worlds."

SASCSI

[epyT-R]

Well, at least we can get rid of those hard-to-route ribbon cables. That alone is worth the switch, IMHO.

Ok, So I've noticed a couple of corrections.

[taliver]

If the article meant to say 3GBytes, then how in the world will the PCI *at 64bits and 133MHz, it's 1 GB/sec transfer) bus keep up? Or even RAMBUS memory, which, here [kingston.com] says it has a bandwidth of 4.2GB/sec. (So, kinda means you couldn't have more than one SCSI system at a time and get full bandwidth from both.) Now, if you may have to have memory banks for each SCSI component... ick.

Re:3 gigabit/sec!

[Sponge Bath]

Imagine what a Beowulf cluster...

Wouldn't that be called parallel?

good performance.. but at what price?

[thadeusPawlickiROX]

Sure, this definately looks like it could be a great setup: fast, and compatable on multiple systems. But how much will this technology cost? Standard, run of the mill IDE hard drives are about a dollar per Gig. Regular SCSI is a few times higher, especially as drives grow in size. This will be a great advantage if the price range is in the middle of the range, but I doubt that. Now, this won't matter to those with plenty of money to burn on their servers, but would that added price be worth the new types of hard drives? I still don't even see a huge advantage to going Serial ATA right now, so this seemingly good idea could just be another good idea that won't pan out for most users.

It's too bad...

[Quaoar]

...that the speed limitation on data access is mostly the fault of the DRIVE, not the interface. Show me a drive that can achieve 3 gigabytes/sec and I'll be impressed.

Re:good performance.. but at what price?

[Magus311X]

Not just fast, but reliable.

Not to say that ATA disks aren't reliable, but the components that are used in ATA disks are typically those that were outside the absurdly strict tolerances that are required for "enterprise-class" drives.

And yes, when it comes to speed, SCSI tends to rule the roost. Not only because you can throw 320MB/s down each individual channel, but you can toss enough devices on that channel to keep that overall speed sustained over longer periods of time.

Drives have very high burst speeds, but have it do lots of random data access constantly and watch speeds plummet. That's why a 10-disk striped array (with another 10-disks to mirror if you require redundancy, likely on another channel) tends to kick considerable ass. Because even if you're only sustaining say... 10MB/sec per disk, it's now 100MB/sec over the channel.

ATA storage is definitely cheap. If all that is required is just LOTS of storage, and performance and reliability isn't really critical, ATA is a pretty good choice. Of course then you could use robotic tape libraries as well.

SCSI also really ruled the server rooms because those expensive servers and storage systems simply didn't have ATA support. Period.

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Re:bits vs. bytes

[g4dget]

Yeah, but what drive actually delivers 320 Mbytes/second? As long as the connection between the controller and each drive can keep up with the drive, the connection is fast enough.

Of course, a really fast connection may allow you to daisy chain and still get almost full transfer rates from each drive, but that's not really such a big deal, in particular when the cables are as small as they are for serial connections.

Re:It's too bad...

[Anonymous Coward]

Informative? How about ignorant?

That bandwidth can be shared between many drives. The drive itself has cache, so it isn't always returning data from the platters. And it's gigabits, not gigabytes. Get a freakin clue.

Re:Ok, So I've noticed a couple of corrections.

[sedmonds]

The pci and rambus busses don't need to keep up. The peak throughput only needs to be serviced within the scsi chain. Buffering on the scsi adapter could deliver a relatively high sustained transfer rate from the scsi chain to the pci bus, within pci limitations.

Re:bits vs. bytes

[Daniel Phillips]

Yeah, but what drive actually delivers 320 Mbytes/second? As long as the connection between the controller and each drive can keep up with the drive, the connection is fast enough.

Scsi is a bus. I have a box here with 5x10K drives, at 49 MB/s each, easily able to saturate its ultra 160 bus. These days, that box is nothing special.

Re:SASCSI

[iotaborg]

If you've taken a look inside Apple's cases, you would notice how much better ribbon cables can be than rounded. With the ribbon cables routed on the side of the case, under the motherboard, completely out of your way due to the flat nature, it's much more cleaner than what you get with rounded cables (and esp ribbon cables just dangling in mid air). However I do not think this is easy to do in an ATX format.

Re:good performance.. but at what price?

[g4dget]

Not to say that ATA disks aren't reliable, but the components that are used in ATA disks are typically those that were outside the absurdly strict tolerances that are required for "enterprise-class" drives.

The future of reliable, enterprise-class hardware is not delicately engineered systems that cost a premium, but a large number of inexpensive, simple servers and drives. For disks, we already have that in the form of RAIDs. If a drive, or two, or three, fail, you just replace them.

And yes, when it comes to speed, SCSI tends to rule the roost. Not only because you can throw 320MB/s down each individual channel, but you can toss enough devices on that channel to keep that overall speed sustained over longer periods of time.

That is circular reasoning. If you pick separate channels for each device, then each channel can be slower. Besides, "tossing enough devices on that channel" makes the overall system less reliable because if there is a problem with any one of them, it may kill the whole channel. And, besides, the more devices you toss onto a serial bus, the less efficiently it will be utilized relative to having a single device with the same total bandwidth requirements. Overall, you are probably better off using five separate USB2 or IEEE1394 connections than one of these serial SCSI connections.

Re:Mbit != MByte

[Cramer]

That doesn't matter. Almost everything else in the post is in error. SCSI already has a serial interface standard (and has for a very long time.) -- ever heard of Fibre Channel or Firewire? SATA and SSCSI compatiblity? F***ing duh! What do you think ATAPI is? The SCSI command protocol moving in packets across an IDE physical interface.

I don't know what stupid scheme they are trying to create here -- interface-wise. SATA is a point-to-point configuration. SCSI has always been a bus configuration. If they go the p-t-p route, then it depends on the controller to be able to support the device on the other end -- SCSI crossing the pyhical interface or IDE/ATA/ATAPI crossing it. (Think parallel port ethernet dongle.) I'll have a hard time accepting p-t-p SCSI.

If they want to make SCSI more attractive, they should stop significantly over charging for the technology. They can bulk test "desktop" SCSI drives just as cheaply as IDE drives. They all use the same servo assemblies -- and in some cases, the same basic interface logic (obviously with different microcode.)

Re:good performance.. but at what price?

[drsmithy]

For disks, we already have that in the form of RAIDs. If a drive, or two, or three, fail, you just replace them.

And if/when these drives go down and take your 2TB RAID array with them, who wears the blame for buying crap disks ?

RAID gives you some added security, it is *not* a silver bullet - even with hot-spares and several replacement drives handy, a simultaneous failure of 3 drives could potentially bring down nearly any RAID array.

Here's a shot

[0x0d0a]

Okay. SCSI lets you do command queuing and reordering. Serial ATA will have this too. Theoretically, if you have a bunch of things doing sequential accesses at once, this can help. SCSI can have outstanding requests to multiple devices on a bus at once, and ATA cannot. This is a pretty big deal for environments where you can have heavy disk load, since if you have two drives on an ATA bus, one can get starved if the other is doing lots of work. I'm not sure if Serial ATA addresses this. For Average Joe's desktop, it's not a big deal because he's usually only doing one thing at once -- loading a game up, or copying a file.

SCSI is generally used to allow price discrimination by vendors. SCSI drives have a reputation for being more reliable, and much more expensive.

SCSI supports many more devices on a bus. This is a big deal to me -- it's a royal pain to buy another controller to add another device or two.

It's unlikely that the two will be merged any time soon, because there's tremendous financial incentive to prevent "enterprise-class" drives from becoming commoditized. SCSI is one of the industry's last useful tools to avoid this.

If you're getting a desktop, use ATA, almost certainly. If you're getting a server with a lot of drives, it may be worth your while to get SCSI, for the abovementioned benefits.

If I had some extra money and just wanted some extra reliability, I'd probably have a mirrored RAID pair of IDE drives, if I were building a desktop without a ton of drives.

Re:Is this a trend?

[curious.corn]

Right, and the setup/hold deltas become irrelevant. As the switching speeds go up the time wasted for all the pin signals to be good becomes a hard limit for the bus; after all parallel is just a workaround to slow inverters. As long as the cables aren't too long for signal distortion to arise, serial can really push the limit and when that'll come I expect Si optoelectronic frontends will be mature enough to substitute electrical transmission lines in consumer electronics ;-)

this is not the merge of scsi and ata

[jkorty]

Didn't anybody read the pdf whitepaper? The only thing common between serial SCSI and SATA is the connector and the power and ground pins on the connector. The two protocols use entirely different signal waveforms and higher level protocols on the signaling pins. The article specifically states that to plug the wrong device into the connector results in a nonfunctional unit.

Re:Does SCSI now compete with firewire2 ?

[sconeu]

And how about 10Gbit Ethernet ? What's stopping you from using this as a drive interface ?

Is iSCSI a standard yet?