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Serial ATA Technology Explained
Mike Parsons writes "Explosive Labs has an interesting article on Serial ATA . Here is a quote: 'In the rapidly moving computer industry, there are rarely the kinds of revolutionary changes like what is about to take place in secondary storage segment. Soon the hard drives and configuration methods that have existed since the origins of the personal computer will change forever. The basic IDE technology has been around for nearly twenty years. When the lifetimes of other computer components like CPUs and video are measured in months, twenty years ago seems like prehistory.'"
Think about this - how long has RS232 been defined? How long has the PC's parallel (i.e., LPT1) pin-out been defined? How long has the VGA pin-out been defined? How long has the PC keyboard pin-out and protocol been defined? A lot of things change pretty fast; a lot of things stay around forever. It all depends on whether upgrading them is worth the cost in the long run.
-Hentai [in vita non pacem est]
All of IDE's shortcomings are fixed by SCSI (except for a small degree of added complexity). SCSI hardware is more expensive, and rarely does it come built-in to motherboards.
If more people used it, it would be a cheaper solution, and would fix all of IDE's problems without re-inventing the wheel--it's a solution that, right now, works.
15k rpm scsi drives get seek times in the low three range--that's three times faster than your average 5400 rpm ide hdd.
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Couldn't the same arguments be made for SCSI? And where is it now? Relegated to obscure servers and macs. It has to do with the price -- when will anyone ever realise that?
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Serial ATA will only take off if it is not more expensive than parallel ATA. If we (meant users) want to spend more we would buy SCSI. What I want is a low cost way to stipe 4+ drives at home.
...'cause that means prices will drop on hard disks that I can use.
I can't wait to get my hands on some SATA devices. However, we're still stuck with PCI, here on the desktop end. WHEN will we finally start seeing the old original PCI spec phased out on the desktop end? Not until then will new technologies like SATA be able to shine. Bus bandwidth is everything these days.
>Forgive me if I sound a bit naive but wouldn't parallel be faster than serial?
Yes, but just like with memory, serial is cheap, parallel costs. Those extra wires just ain't free.
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What on earth does the type of interface have to do with heat or noise?
4 of them in RAID 0?
This won't be an issue since SATA is strictly point-to-point, every drive gets it's own 150MB/s link.
I don't get it ... I quite agree that, as a serial bus, it'll be clocked a lot faster than IDE ... but a simple back-of-the-envelope calculation tells us that it has to be at least 8 times as fast as the current devices (it'd have to be 533 MHz to be on par with ATA-66)
It looks like a technology whose main purpose is to make things incompatible, and thus require people upgrade more stuff. And anyway, it's not the speed of the bus the limiting factor (for the vast majority of users), but the mechanics of the harddrive (SCSI hardrives are faster than IDEs because they almost always are top-of-the-line products with higher rotational speeds - anybody saw a 15000 RPM IDE ?)
The Raven
The Raven
Eh, parallel is on the way out for good. You've obviously never heard of a little thing called clock skew. It's what makes your DS3 example impractical since the bits won't all arive at the same time, with the run lengths you're talking about with a DS3 the potential difference would be too large to effectivly compensate for.
Not to mention the #1 stumbling block for most people: price. SCSI costs significantly more for comparatively little actual performance benefit. On a server, which is hard-drive intensive, this performance difference is big enough to make SCSI worthwhile. On the desktop, on the other hand, it costs more, allows smaller sizes, and apparently makes more noise and heat. No wonder IDE is here to stay...
For starters, the PCI spec isn't limited to 133 MB/s. PCI 2.1 specs allow for 66 MHz 64-bit transfers, which equals 528 MB/s. PCI 2.2 specs allow for PCI-X mode, which adds 133 MHz 64-bit transfers: 1056 MB/s.
:)
That being said, it is entirely possible to reach throughputs in excess of 133 MB/s using a PCI bus... though currently most desktop motherboards do not support anything faster than 133 MB/s. In time this will change as NICs, hard disks, and other gear requires it.
And your hard disk performance is barely par by today's standards. IDE drives are currently topping 50MB/s, while SCSI gear is hittin > 70MB/s. Though I am a SCSI man, i can see the future need for SATA. Right now it may be mainly a marketing ploy... But in a couple years it will be a necessity. Parallel cabling is nearing the end of the road.. all those wires in a cable allow for too much signal interference. Serial is the answer. Though it has less wires, the dramatic increase in signal strength allows for insane transfer rates.
Anyhoo.. personally I don't see any reason to go out and buy a new system just to have SATA. At the current it offers few advantages.. but in the not so distant future it will be a necessity for desktop systems. As for me, i plan on going Fibre-Channel SCSI
/dev/random
Absolutely. But this has nothing to do with SCSI, it has to do with the high spindle speeds at the bleeding edge. The card on the underside of the drive is not making that ear shattering racket. They even acknowledge that in your quote.
SCSI is better than ATA. Even SATA. ATA has been trying to catch up by stealing some of the best parts of SCSI (like TCQ). But it just isn't quite as good yet. Quite frankly, I agree with the majority of SCSI zealots: if the damn PC makers would embrace SCSI, then the cost of SCSI would come down to near parity from the volume of sales.
Now, is SCSI better for your average Joe? Maybe not significantly. Neither is 7200 vs 5400, 2MB vs 8MB buffers, or 8.9 vs 9.1 ms access times.
However, if they could use one cable to connect 15 devices in their tower, they'd be alot happier than having the 8 cables they'd need to do it with current IDE tech (let alone IDE's relative inability to be used externally).
Note that almost all the Firewire hard drives that you see on the market are ATA hard drives with FirewireATA hardware in the enclosures. As far as I'm aware, the only disks that you can readily get your hands on will have interfaces of IDE/ATA, SATA, SCSI (of various connectors), and FC-AL. That's why you can't use Firewire inside a PC. Using SATA makes far more sense, especially for migrating to a new standard, as it's most likely easier to make a SATAATA adapter since the protocol is very similar.
It is an impressive jump ahead, and it's not going to be much more expensive. It's estimated that Serial ATA interfaces will be less than $10 more expensive than their parallel counterparts, and that will go down once it becomes more popular. Also, it's not 150 M-bits/sec, it's 150 M-Bytes/sec. Another big advantage is that Serial ATA is that the drives will be hot-swapable. The new power connectors have extended ground plugs so that the grounds contact before power... also drives will get to use a new voltage line (+3.3V) that they didn't get before.
Another interesting thing about the technology is that drives that are currently using the parallel SCSI interface will be moving to either SAS (Serial-Attached-SCSI) or Fibre Channel. SAS will use the SCSI protocol over the Serial ATA cables, so you can get rid of those nice giant ribbons.
I really love how the Maxtor paper compares SATA to parallel ATA, USB (?!) and Firewire...but not to SCSI or FC. I wonder why that is. Actually, no I don't. ;-)
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Is IDE appropriate for the desktop? absolutely.
Will retards continue using IDE in applications where SCSI is far more appropriate? definitely.
Does your post make any fucking sense at all? nope.
I think I am. Really.
The article seems immensely biased and lacking in technical detail. It also raises some "dubious" points IMHO. Let's see:
- P-ATA cables cannot be longer than 40cm. S-ATA cables can be up to 1m long:
Granted, those cables are annoying. But really, how many times have you felt the need for a cable much longer than 40cm? People with full-sized cases may benefit, but then the author says that the current trend is "small footprint machines". So, why do I need a cable that is bigger than my server?
Also, if you dislike flat cables, buy "rounded" P-ATA cables (available today, just google for it).
- P-ATA connectors are big!
Yes, they are! But you'll require at least twice as many S-ATA connectors, as only one device is supported... In the end, the real state on the mobo is going to be similar.
- One device per controller is an "Advantage".
C'mon... This guy must be joking. I couldn't believe my eyes when I read it! One device per controller is an *advantage*???? Why??? I wish I could add more devices (like SCSI and Firewire) to my curreny P-ATA technology. And then he says ONE is good for me? Don't think so...
- High transfer rates are useful for multi-disk RAIDS.
What kind of RAID? RAID 5 is slow in writes due to the computational power needed to calculate the XOR. Adding bandwidth won't help. And I can't see why or how only RAIDs will benefit from higher throughput.
- Speed:
Granted. It may be faster than P-ATA. But what about established technologies like SCSI and Firewire? I *think* (not sure) Firewire can go much faster than S-ATA in its initial version.
I'm disappointed...
Of course, no discussion of Serial ATA would be complete without mentioning the answer from the SCSI camp - Serial Attached SCSI. SAS will use the same connector as SATA, but will support longer cable lengths, multiple initiators (if you don't know what an initiator is you don't even belong in this discussion), full SCSI semantics instead of lame-o ATA semantics, etc. Even so, the SAS folks are still ceding the high end to Fibre Channel and talking about three coexisting technologies for the low-end/midrange/enterprise market segments. Sorry, kiddies, but SATA is still low-end.
If there's one mistake you should try not to make more than once in this business, it's that competitors have been standing still since their previous generation. Announcing something brand new and having it be less than half a generation ahead of the competitor's last version is a failure.
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Firewire (1394) was killed by Apple's licensing fees and Intel's sudden backstabbing policy change on building it into south-bridge, along with their NIH attitude. There existed working 1394 Device Bay drives over 6 years ago, with OS support from m-soft. 1394 was an attempt to keep the good parts of SCSI protocol, while leaving out as much of the useless stuff as possible (MODE SELECT).
Fibre-channel is still Real Pricey, for the same reason that SCSI is -- "just because". Or, as the hardware vendors say "harrumph, well, it's all about volume".
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This article is very pro the S-ATA standard. However later on in the article it states that S-ATA hard disks will be of the same speed and size as current ATA. The article says that the only way to get real performance increases in disks is to make them faster - so then we get to the same problems as SCSI, namely noise and heat.
As far as I can see, a couple of pages of this article are denoted to the new smaller cable size and connector footprint - who really cares ? I run an overclocked PC and as such use readily-available rounded IDE cables to afford better air flow. What other tangible advantages is S-ATA going to offer ?
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First, this is IDE (just as ATA-66 or ATA-133 are IDE).
Second, the reason why Betamax died (well, didn't actually die, but didn't take off, either) was Sony kept it a proprietary format, while JVC let pretty much everyone make VHS products.
Serial ATA is one of the most unrevolutionary evolutions ever made. Basically it just changes the cables. The drives stay pretty much the same, the controllers stay pretty much the same, the drivers can stay exactly the same. Instead of wide, flat cables and two disks per channel you now get thin round cables and just one disk per channel (but since the connectors are so much smaller, you can have many on the same board). It's a good thing.
There are basically three reasons for having multiple standards. The first is a purely commercial one. Brand A invents the A-link and patents it, and brand B decides to create B-link so they don't have to pay a fee to Brand A. The second is evolution. Sometimes, a standard needs to be replaced or updated to cope with new demands (ex., ATA-33 becomes ATA-66). The third is that some standards are specifically suited to some situations (ex., SCSI lets you connect a lot of drives, and has support for other kinds of peripherals, but IDE is cheaper to make, and enough for most people).
RMN
~~~
I don't think Serial ATA is the monstrous revolution they'd like for it to be. I think solid state storage will represent that...when we no longer have to rely on precision mechanical components that become royally fscked with the introduction of 1 speck of dust...THAT will be a revolution.
Well, I'm not the only one that calls it 1.5 Gb/s. The serial ATA website often refers to it as 1.5 Gb/s.
:)
Also, signalling bits that are thrown away are often counted. Otherwise we would have 98 Mb/s ethernet instead of 100
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Aside from the fact that this article was very poorly written, it's difficult to understand what the author is so excited about. Between phrases like "dawn of a new era" and so on, the author makes a terrible, rotten case for why anybody should be excited about this at all. Basically, all I got out of it that was of seemingly immediate importance was the fact that the author seems to think thinner, longer drive cabling is revolutionary and heralds a new day in personal computing.
The article was extremely misleading in that it said next to nothing about the kind of drive technology to be used in SATA drives. Is this because the author doesn't know anything about these drives, or is it because the author knows there's nothing new whatever in the current SATA technology as far as the actual drives go except for the interface and cabling?
From what little I've read the first SATA drives are standard parallel IDE drives with serial interfaces. Is this approach supposed to make them run faster, or something? *chuckle* (facetious question)
Without some interesting new drive technology to make the interface change worthwhile, what's the point, here? *IS* there some point aside from thinner/longer cables???
A couple of days ago I saw a 200GB WD SE with 8 megs of cache. I'm already enjoying the benefits of RAID 0 + 1--the current IDE subsystems are *already* much faster than the drives they host. What's the problem? Cables? These days you can buy well-made rounded IDE cables (that are not simple ribbon cables folded, spindled, and mutilated.)
Maybe I'll become impressed when I can read more about the drive technology planned here and how it differs (if it does) from current IDE drive technology, but right now I'm not impressed at all. It's articles like these that definitely give SATA a "bad name"--if it in fact deserves something better.
This is all fine and good, but why not just treat the wires in a parallel cable as individual serial wires? Sure, if you increase the signal frequency, it becomes next to impossible to guarantee that all the signals arrive at exactly the same time, but I don't see the need for bit-level synchronization. If each wire has its own protocol, its own synchronization, and its own buffers, then as long as there is synchronization at the packet level, there should be no need to worry about synchronizing at the bit level. This would allow both high frequencies, and lots of wires.
ATA wil debut at 150 mb/s, not really an improvment at all.
Does it matter? At all? No.
Frankly it could be 150 GB/s and it wouldn't matter in the least.
Go look at the manufacturer specs. Read the line that says "drive to host, sustained throughput". Note that no manufacturer claims more than 52 MB/s. Reality is closer to 48 MB/s for the fastest IDE drive. That's right! We're not even exceeding ATA/66 bandwidth yet. And still people are talking about 600 MB/s in a few years. Who cares? You can't reach that throughput anyway. Not to mention that the PCI bus is limited to 133 MB/s.
Ok, the bus speed does make a bit of difference. If the data you need is in cache then you can use the maximum theoretical bandwidth while reading from cache. So dumping a 8 MB cache via ATA-133 saves you about half a millisecond over ATA-66. You noticed that, right?
The advantages of SATA have nothing to do with the bus speed. The longer cable is useful in a select few tower cases. The hot swap will be nice for a small percentage of enthusiasts and idiot admins (I'm not a SCSI fan boy, but if you're running a server you really should be running SCSI). The small, thin cable is useful for everyone though -- the air traps created by ribbon cables are causing more and more problems as everything runs hotter. Most drives fail due to poor cooling. Want to bet that SATA drives have a significantly lower failure rate?
The best thing Apple did was put SCSI in the mac plus. Suddenly you could add hard drives, cd roms, scanners, tape drives, ethernet adapters, serial port adapters.
I used to put my Syquest and DAT tape drive on a mac, PC, and Sun. I still use the DAT drive I bought in 1993. How many 10 year old peripherals do you use?
If PCs had gone to SCSI instead of IDE (remember MFM?) we'd all have cheap SCSI drives we could use on any platform. There'd be more innovation in SCSI.
Instead, I get IDE in my Ultra 5, Macintosh, and PC. Only 2 devices unless you want speed issues. It can only really be internal devices.
And I still need SCSI for my tape drive or other external media.