Don't be afraid to be SAS-sy ... a primer on basic SAS and SATA

[jgreco]

I agree with what you're saying about breakout cables. The issue is that in the first post in this thread, it states they can be used to connect directly to drives.

I don't see such a statement. However, it's definitely possible to connect SAS drives to a single-SAS breakout cable.

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So the eBay one is $2.37 per drive.

The Primer also says

SAS and SATA use different connectors on the drive. The SATA drive connector has a gap between the signal and power sections, which allows separate power and data cables to be easily connected.

So there's clearly a warning about this if you read the correct section.

What is pictured is the latter (SATA connectors) but the text calls it the former (SAS).

"SATA" and "SAS" refer to signalling, not connectors. It's the signalling that's important. To flip this around, there are plenty of examples to be had of SATA (only) signalling being run on top of connectors typically used for SAS, and this imposes issues such as length limitations and the inability to attach an SAS expander.

For anyone who has worked with either even in passing, it's no big deal. But in a topic that is calling itself a primer for people not familiar with SAS, it can be a problem. A new person, following what's currently posted, buying those cables and trying to connect them to SAS drives- which is what the text tells them they're for- is going to break hardware (if they force it) and/or have a non-functional setup (from a half-plugged-in connector).
Why is it a point of contention? I don't know why it should be. It seemed like a minor mistake, either in image choice or in the descriptions.

I guess I expect that people reading a primer actually read the primer in its entirety, and not just pick and choose a sentence here and an image there. Because the SAS/SATA interoperability thing is such a potential for trainwreck, and there are so many potential gotchas and caveats, I don't really think the primer is intended to cover stuff like the esoteric errata in SATP-via-expander differences between SAS1 and SAS2, and rather is intended to help the average FreeNAS user who has no idea what SAS is to get up to speed. I do not want to get into a situation where I am having to spell out every caveat and proviso at every possible point of misunderstanding; that would be terribly awful for a newbie to read and even worse to write.

So if I really wanted to be thorough, yes, I could go open up the bins in the shop here and I could write a SAS/SATA primer that includes all sorts of obscure and arcane bits of trivia that either zero or maybe one reader on the forum would ever need to know. I don't really care to do that and it would create a total TL : DR situation. It's a primer. It isn't everything you might ever need to know, and unfortunately due to design choices made by the designers of SAS and SATA to make these somewhat-interoperable on both the signalling and connector levels, there's quite a bit of arcana that you might possibly need to know that I do NOT cover.

See, I just don't really see a good fix here. I'm not willing to call them "SATA connectors" because they aren't SATA connectors (evidenced by SAS running over them). SATA has other similar boggles such as eSATA (which is yet another interesting divergence) and has been shown to opportunistically go over a variety of connectors. "SAS" is more-correct because a SAS lane can drive a SATA target, but a SATA channel cannot drive a SAS target. Because it's a SAS Primer, the possibility of a SATA-only controller and target setup is out of scope, so anytime something is downgraded to "SATA" when being discussed, it should be because it is "SATA-only." So both technically and contextually it is more appropriate to refer to the lane as a SAS lane, and because there are single lane SAS connectors on devices such as backplanes to accept them, it feels like it would be wrong to call them "SATA" as this would be an additional point for confusion.

It seemed like a minor mistake, either in image choice or in the descriptions.

I deem this to be a minor mistake in user interpretation, not in image choice or description. As an apparent victim of the circumstances here, can you tell me if it would have been clearer had there been a description of a 22-to-7-plus-15 adapter? When I wrote the text

SAS and SATA use different connectors on the drive. The SATA drive connector has a gap between the signal and power sections, which allows separate power and data cables to be easily connected.

I specifically picked the phrase "separate power and data cables to be easily" to indicate this type of issue. I concede that people sometimes miss the things I try hard to imply, but I also feel like what I wrote after it should be sufficient explanation, and that a reader just totally missed it, so further explanation is not meaningful. If you wish to argue that I should explicitly mention an adapter is necessary for dual-ported SAS HDD's to be able to accept a single-lane SAS cable, perhaps there's a point there, but it's implied by what I said. You could probably sell me on this argument though.

My goal with the Primer isn't to fill in every possible detail and bit of knowledge. It's to make sure that newcomers, hobbyists, and non-IT-pros who have no idea what all this "SAS" stuff is can get the clear idea that this is very usable technology.

 

[jgreco]

That is what I was trying to say, but as always you explain things with very good examples

Well, I understand the problem and the frustration. SAS/SATA is a trainwreck. If I can figure out how to make it clearer, I will.

 

[subhuman]

I don't see such a statement. However, it's definitely possible to connect SAS drives to a single-SAS breakout cable.

This will be the second time I quote this paragraph. It's the fifth paragraph under the 'cabling' heading in the first post in this thread:

A multilane connector may be broken into its four individual lanes using a breakout cable. For example, if you get an SAS HBA, it probably comes with one or two SFF8087's on it, but you may want to directly attach hard drives. A breakout cable allows this. This is a SFF8087-to-single-SAS breakout cable; this is a SFF8643-to-single-SAS breakout cable.

For emphasis: "For example, if you get an SAS HBA, it probably comes with one or two SFF8087's on it, but you may want to directly attach hard drives. A breakout cable allows this. This is a SFF8087-to-single-SAS breakout cable; this is a SFF8643-to-single-SAS breakout cable."
The adapters you just posted pictures of do not allow this. Note you say those breakouts can be used to directly attach to SAS drives. Directly means, by my understanding of the word, withOUT an adapter.
i.e. if you use the adapter, the breakout cable connects to the adapter, the adapter connects to the drive. Ergo, the breakout cable is not directly attached to the drive.
Those pictured breakouts do, however, allow direct attachment to SATA drives. No adapter required, a direct connection.


Let me flip this around, and try to make it as simple as possible:
Those two breakout cables, as pictured, what types of drives can they connect directly to, without needing modifications or extra hardware?

 

[Chris Moore]

Note you say those breakouts can be used to directly attach to SAS drives

That is not what was said and you are being far too argumentative. The vast majority of drives used to build home NAS systems are SATA drives and it is possible to connect SATA drives to these breakout cables and that is what was meant. Most people on the forum would not suggest using SAS drives for a home system. SATA drives are much easier to obtain at a good price. Typically, the only way you will get SAS drives at a competitive price is if they are used, which is probably not a good investment.
You need to give this a rest.

 

[Chris Moore]

Let me flip this around, and try to make it as simple as possible

The entire previous section of the guide is about connecting SATA drives to SAS controllers. This is what we are talking about doing. You are the one having problems understanding.

I don't think you are actually having a problem at this point. I think you are just trying to start some kind of war of words. It won’t be tolerated much longer.

 

[Ericloewe]

The entire previous section of the guide is about connecting SATA drives to SAS controllers. This is what we are talking about doing. You are the one having problems understanding.

I don't think you are actually having a problem at this point. I think you are just trying to start some kind of war of words. It won’t be tolerated much longer.

I think there's value in clearing things up. What's obvious to us might not be to everyone.

So both technically and contextually it is more appropriate to refer to the lane as a SAS lane, and because there are single lane SAS connectors on devices such as backplanes to accept them, it feels like it would be wrong to call them "SATA" as this would be an additional point for confusion.

I typically call them "SATA-style", which I think has a decent chance of getting the point across that they're carrying SAS but kook like SATA.

 

[jgreco]

Note you say those breakouts can be used to directly attach to SAS drives.

It does not say "directly attach to SAS drives" and you are being needlessly argumentative without trying to bother to understand the context or the goal of this Primer. The cable WILL directly attach to single-ported SAS drives, which are now a rarity. I also don't say that you need screws for mounting or a power supply to energize the drive, so my failures are legion if we wish to start looking at the bits of hardware I've failed to mention are necessary. I am fine with considering if there is some better way to clarify what's presented, but saying that I said something that I didn't is counterproductive.

For the purposes of the discussion, even though it does not say "directly attach to SAS drives," a single lane SAS cable *can* connect directly to a single-ported drive without anything further, and can also connect to a dual-ported SAS drive, where it simply needs the appropriate adapter hardware, as do many other things in the world of computers.

I am closing this thread for the time being.