# Serial SCSI (SAS)



## sidetone (Jan 15, 2018)

Serial Attached SCSI (SAS) has been around for a few years, but there hasn't been much news about them. There aren't that many consumer products out for it either.

I believe a SAS interface can run both Serial SCSI and SATA harddisks through the same type of cable. However, a SATA interface is incapable of operating a SAS harddrive.

SCSI has been known to have advantages over PATA, yet be more expensive. These advantages and cost for them are mentioned in SAS compared to SATA.

Serial Attached SCSI (SAS) seems like the way to go for reliable harddisk storage and speed.


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## SirDice (Jan 15, 2018)

sidetone said:


> There aren't that many consumer products out for it either.


LSI has HBA and RAID cards in all price categories. The reason you don't see it more often in consumer products is because SAS disks and controllers are relatively expensive compared to SATA. Consumers typically don't need the added features of SAS. So why put a more expensive controller onboard if most people don't need it anyway?

It's basically the same reason why consumer products used IDE and servers SCSI. On servers it's mostly SAS already nowadays.


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## usdmatt (Jan 15, 2018)

Is there a question here? SAS completely dominates the server market and pretty much has done since it replaced the old parallel SCSI drives.


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## ralphbsz (Jan 15, 2018)

sidetone said:


> I believe a SAS interface can run both Serial SCSI and SATA harddisks through the same type of cable.


Correct.  Physically, at the plug level, you can plug a SATA disk into a SAS connector (either cable or backplane), but not the other way around.  All SAS initiators (HBAs, the cards that you plug into your PCI bus or that are on your motherboard) are capable of using directly attached SATA drives.  Most SAS expanders (the chips in disk enclosures that turn a small number of SAS connections into a large number for connecting many disks) are capable of mixing SATA and SAS disks.  There are SAS enclosures that explicitly do not support SATA disks, but I think those are getting to be rare.



SirDice said:


> LSI has HBA and RAID cards in all price categories. The reason you don't see it more often in consumer products is because SAS disks and controllers are relatively expensive compared to SATA. Consumers typically don't need the added features of SAS. So why put a more expensive controller onboard if most people don't need it anyway?



For controllers, that's true: most consumer motherboards have SATA controllers built in; adding SAS capability requires buying a SAS card, and those cost extra.  Some server motherboards actually put SAS controllers right on the motherboard, and don't even bother with SATA; at that point, the extra cost of SAS is very small.

With disks, the story is more complicated.  People usually think that SAS disks are more expensive than SATA disks.  There is some truth to that: the cheapest disks are indeed SATA.  But if you pick a specific disk reliability and performance target (for example, a nearline 3.5" drive), you usually find that the SAS and SATA models of the same disk cost the same.  The reason for this discrepancy is: the lowest quality and reliability drives are only offered in SATA, not in SAS; once you go to the better disks (often sold as enterprise disks), SAS is at parity.


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## Snurg (Jan 15, 2018)

Not only servers have onboard SAS controllers, better workstations also have.
As ralphbsz correctly states, there are no low-quality sas drives.
What I find a bit sad is that the 15k sas drives have been phased out by the manufacturers. Their access times and latencies are very good and speed up compiler runs substantially. I prefer them over SSDs because they are (comparatively) cheap, reliable and durable.


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## sidetone (Jan 15, 2018)

I've wondered about compiling performance with SAS.

How well do SAS drives hold up to electrical interruptions, spikes?


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## MarcoB (Jan 15, 2018)

My workstation from 2004 has a U320 SCSI RAID controller. The drives are still original so I would choose SCSI any time again for reliablity.


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## SirDice (Jan 15, 2018)

ralphbsz said:


> With disks, the story is more complicated. People usually think that SAS disks are more expensive than SATA disks. There is some truth to that: the cheapest disks are indeed SATA. But if you pick a specific disk reliability and performance target (for example, a nearline 3.5" drive), you usually find that the SAS and SATA models of the same disk cost the same. The reason for this discrepancy is: the lowest quality and reliability drives are only offered in SATA, not in SAS; once you go to the better disks (often sold as enterprise disks), SAS is at parity.


You might be right about that. The only SAS disks I see are typically of a much higher quality, and are more expensive compared to the low(er) quality SATA disks I come across.  



Snurg said:


> Not only servers have onboard SAS controllers, better workstations also have.


I'm almost finished with "Frankensteining"[*] a server at home. I needed a mainboard for it. I've looked at a lot of older, higher-end, workstation boards. All of them had a 6-10 port SATA controller. But some of them did indeed have an additional SAS/SATA controller. In the end I settled on a server board because it had 4 Gigabit ports, IPMI _and_ an onboard LSI  AS1068. 

[*] Cobble together a working machine from parts of other defunct machines and some spares.


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## ralphbsz (Jan 15, 2018)

Snurg said:


> What I find a bit sad is that the 15k sas drives have been phased out by the manufacturers.


Are they completely gone?  About a year or two ago, they were still available (in large quantities to OEMs, and still supported models).  The problem with 10K and 15K RPM drives is that there is no sweet spot for them: For $/GB and $/(GB/sec), they are beaten by slow-spinning drives.  For $/IOps, they are beaten by SSDs.  Once you take performance/W or performance/liter into account, they are even less efficient. 
People with large systems can usually segregate their workload to put them on a mix of slow-spinning drives and SSDs.  The only time the fast-spinning drives are still sensible is for people who need to make that compromise, but only need a very small number of drives.  And the industry isn't interested in folks who need a very small number of drives.



sidetone said:


> How well do SAS drives hold up to electrical interruptions, spikes?


The bigger problem you didn't mention is mechanical vibration.  But the answer is the same: No better than comparable enterprise-grade (quality and price) SATA drives.  Usually way better than cheap drives.

About 10 years ago, Erik Riedel and Dave Anderson (at that time of Seagate, they have since moved on) wrote a real good paper, called "SCSI versus ATA", which explained the difference between consumer and enterprise drives.  While many of the technical details have changed, and the world of drives has become more complex, their conclusion is still correct: you get what you pay for.



MarcoB said:


> My workstation from 2004 ... The drives are still original so I would choose SCSI any time again for reliablity.


I would choose ... drives of the same manufacturer, model, and assembly run any time.  All disk drive manufacturers have "bad days", and I've had to deal with disastrous events (where we ended up replacing thousands of drives at a single customer location, because quality control for one particular batch of drives had failed to find a serious problem).  Obviously, not all manufacturers are the same in terms of quality and reliability, nor are all models.  But the differences are complex, and can't be summarized in a single statement like "Vendor X good, vendor Y bad" or "SATA bad, SCSI good".  Of the companies that actually have accurate statistics, Backblaze is the only one willing to publish their numbers, and they are not a very large user of disks, nor do they use a wide variety of disks.  But in general, going with enterprise-grade disks will cost more, and give a better long-term experience.


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## phoenix (Jan 18, 2018)

Snurg said:


> Not only servers have onboard SAS controllers, better workstations also have.
> As ralphbsz correctly states, there are no low-quality sas drives.
> What I find a bit sad is that the 15k sas drives have been phased out by the manufacturers. Their access times and latencies are very good and speed up compiler runs substantially. I prefer them over SSDs because they are (comparatively) cheap, reliable and durable.



Seagate and Toshiba both have new models of 10,000 RPM SAS drives.  It's true, though, that 15,000 RPM drives appear to be discontinued by everyone.


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## Snurg (Jan 18, 2018)

phoenix said:


> It's true, though, that 15,000 RPM drives appear to be discontinued by everyone.


The last new ones have been 2.5" 600gb drives introduced 4 years ago.
I have read a background article about why the 15k drives were being phased out, but I cannot find it anymore. But the reasons have basically these which ralphbsz summarized.
Thing seems that trend is to use SSD for fast access and HDDs for long term storage.

But it's still easily possible to get used 15k drives.
And I like them, as they are really fast (~2ms average access instead of the ~4ms plus half the latency of common consumer drives) and ZFS makes it easy to swap the occasional crashed drive.


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