Storage

How SAS, Near Line (NL) SAS, and SATA disks compare

Scott Lowe breaks down the differences in reliability and performance between SAS, Near-Line SAS, and SATA drives.

When you buy a server or storage array these days, you often have the choice between three different kinds of hard drives: Serial Attached SCSI (SAS), Near Line SAS (NL-SAS) and Serial ATA (SATA). Yes, there are other kinds of drives, such as Fibre Channel, but I'm focusing this article on the SAS/SATA question. Further, even though solid-state disks (SSD) can have a SAS or SATA interface, I'm not focused here on SSDs. I'm focusing solely on the devices that spin really, really fast and on which most of the world's data resides.

So, what is the real difference between SAS, NL-SAS and SATA disks? Well, to be cryptic, there are a lot of differences, but I think you'll find some surprising similarities, too. With that, let's dig in!

SAS

SAS disks have replaced older SCSI disks to become the standard in enterprise-grade storage. Of the three kinds of disks, they are the most reliable, maintain their performance under more difficult conditions, and perform much better than either NL-SAS or SATA disks.

In reliability, SAS disks are an order of magnitude safer than either NL-SAS or SATA disks. This metric is measured in bit error rate (BER), or how often bit errors may occur on the media. With SAS disks, the BER is generally 1 in 10^16 bits. Read differently, that means you may see one bit error out of every 10,000,000,000,000,000 (10 quadrillion) bits. By comparison, SATA disks have a BER of 1 in 10^15 (1,000,000,000,000,000 or 1 quadrillion).  Although this does make it seem that SATA disks are pretty reliable, when it comes to absolute data protection, that factor of 10 can be a big deal.

SAS disks are also built to more exacting standards than other types of disks. SAS disks have a mean time between failure of 1.6 million hours compared to 1.2 million hours for SATA. Now, these are also big numbers - 1.2 million hours is about 136 years and 1.6 million hours is about 182 years. However, bear in mind that this is a mean. There will be outliers and that's where SAS's increased reliability makes it much more palatable.

SAS disks/controller pairs also have a multitude of additional commands that control the disks and that make SAS a more efficient choice than SATA. I'm not going to go into great detail about these commands, but will do so in a future article.

NL-SAS

NL-SAS is a relative newcomer to the storage game, but if you understand SATA and SAS, you already know everything you need to know about NL-SAS. You see, NL-SAS is basically a merging of a SATA disk with a SAS connector. From Wikipedia: "NL-SAS drives are enterprise SATA drives with a SAS interface, head, media, and rotational speed of traditional enterprise-class SATA drives with the fully capable SAS interface typical for classic SAS drives."

There are two items of import in that sentence: "enterprise SATA drives" and "fully capable SAS interface". In short, an NL-SAS disk is a bunch of spinning SATA platters with the native command set of SAS. While these disks will never perform as well as SAS thanks to their lower rotational rate, they do provide all of the enterprise features that come with SAS, including enterprise command queuing, concurrent data channels, and multiple host support.

  • Enterprise/tagged command queuing. Simultaneously coordinates multiple sets of storage instructions by reordering them at the storage controller level so that they're delivered to the disk in an efficient way.
  • Concurrent data channels. SAS includes multiple full-duplex data channels, which provides for faster throughout of data.
  • Multiple host support. A single SAS disk can be controlled by multiple hosts without need of an expander.

However, on the reliability spectrum, don't be fooled by the acronym "SAS" appearing in the product name. NL-SAS disks have the same reliability metrics as SATA disks - BER of 1 in 10^15 and MTBF of 1.2 million hours. So, if you're thinking of buying NL-SAS disks because SAS disks have better reliability than SATA disks, rethink. If reliability is job #1, then NL-SAS is not your answer.

On the performance scale, NL-SAS won't be much better than SATA, either. Given their SATA underpinning, NL-SAS disks rotate at speeds of 7200 RPM... the same as most SATA disks, although there are some SATA drives that operate at 10K RPM.

It seems like there's not much benefit to the NL-SAS story. However, bear in mind that this is a SATA disk with a SAS interface and, with that interface comes a number of benefits, some of which I briefly mentioned earlier. These features allow manufacturers to significantly simplify their products.

SATA

Lowest on the spectrum is the SATA disk. Although it doesn't perform as well as SAS and doesn't have some of the enterprise benefits of NL-SAS, SATA disks remain a vital component in any organization's storage system, particularly for common low-tier, mass storage needs.

When you're buying SATA storage, your primary metric is more than likely to be cost per TB and that's as it should be. SAS disks are designed for performance, which is why they're available in 10K and 15K RPM speeds and provide significant IOPS per physical disk. With SAS, although space is important, the cost per IOPS is generally just as, if not more, important. This is why many organizations are willing to buy speedier SAS disks even though it means buying many more disks (than SATA or NL-SAS) to hit capacity needs.

Summary

At a high level, SAS and SATA are two sides of the storage coin and serve different needs -- SAS for performance and SATA for capacity. Straddling the two is NL-SAS, which brings some SAS capability to SATA disks, but doesn't bring the additional reliability found with SAS. NL-SAS helps manufacturers streamline production, and can help end users from a controller perspective, but they are not a replacement for SAS.

In an upcoming post, I'll talk about SAS commands and why they help cement SAS's enterprise credibility.

About

Since 1994, Scott Lowe has been providing technology solutions to a variety of organizations. After spending 10 years in multiple CIO roles, Scott is now an independent consultant, blogger, author, owner of The 1610 Group, and a Senior IT Executive w...

9 comments
Adam_hearts_IT
Adam_hearts_IT

Too bad this article is high up in the google organic results as it is misleading.  There is no such thing as a "SATA platter" and NL-SAS drives are not SATA drives with SAS connectivity simply added as this article implies.  Manufacturers could build 15k SATA drives if the really wanted too using the exact same disk and platters as an SAS 15k if they really wanted to (which would be weird, yes, but possible).  The MTBF numbers provided are misleading.  Consumer SATA drives may have a 1.2m MTBF, but this is not a 100% duty cycle.  NL-SAS and NL-SATA drives are designed for data center use and 100% duty cycle at 1.2m MTBF.  NL drives are significantly more reliable than consumer drives and almost as reliable as SAS 10k and 15k drives (and the 10k, 15k drives are more reliable mostly because of physical reasons).

The SAS protocol is superior for many reasons, but don't confuse drive mechanics with interface.  Previously, certain drive mechanics came with SAS, and inferior drive mechanics came with SATA.  NL drivers do not follow this rule and seem to confuse many experienced rackers.


Miss Jennifer
Miss Jennifer

So SAS is SCSI? Does it have the same read write abilities as SCSI 255 Read and Write at any given time simultaneously? Where SATA / ATA / IDE has the ability to read or write 1 time at any time? The SCSI bus was at one time the BEST and most reliable until IDE came along for the Mass Market.

SCIS Bus had up to 15 Connections on one Card with the use of one IRQ where adding more than 1 drive to an IDE / SATA system was 2 per channel and with SATA one per channel.

If SCSI was proliferated instead of IDE then everyone would have had stability in the home, where personal data is important, and business where customer data is paramount for business. Don't we all want "Stability" in our lives when it comes to the Data Server or our Gaming Rig to have a system that just works without question?

DexterF
DexterF

I'd like to point out that NL-SAS still gives you a full SAS electronics set including superior error correction and certified media plus a BER of 10^15 as pointed out in the article, but this still exceeds today's consumer sATA disks by a factor of 10 as consumer disks usually have a BER of 1 in 10^*14*. Do the math on the probability of an unrecoverable error during a raid resync on let's say 6x2TB raid5/z1. While 10^15 is available on sATA if you search for it with the price charged for such drives it simply does not make sense to go for NL right away. Different story with raid6/z2. Still the better option for reliable data tombs is consumer disks on raid 5 and regular backups to a jbod made of TB range disks.

Mohr Cowbell
Mohr Cowbell

I wish I had known more about NL-SAS before I decided to purchase a new Dell server with NL-SAS drives -- to be used as a SQL server. I regret that decision -- even with only a dozen users hitting this server, the disk I/O is a real bottleneck. I will be sticking to SAS again from now on.

SumeshN
SumeshN

Wouldnt an expander be a necessity to connect multiple disks to an initiator.I am confused From wikipedia An expander is not necessary to interface a SAS initiator and target but allows a single initiator to communicate with more SAS/SATA targets. A useful analogy: one can regard an expander as akin to a network switch in a network which allows multiple systems to be connected using a single switch port.

Marc Erickson
Marc Erickson

I'm looking at drives to buy for my first home server (it's a bare bones unit). I was thinking that if I'm going to spend this kind of money on it, I should go the extra mile and go for the best reliability I could get - my data is important to me. I hadn't heard of NL-SAS before and this article told me that's not the way I want to go.

Adam_hearts_IT
Adam_hearts_IT

I take back my statement that the article is misleading.  I am seeing different information about duty cycles all over the place.  Drive manufactures are advertising high duty cycles for nearline drives these days (which seems to contradict the name nearline, but oh well).

Scott Lowe
Scott Lowe

Marc, Absolutely right. If you're after reliability and that's your primary metric, NL-SAS isn't the answer. Scott

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