Serial Attached SCSI (SAS) is the newest flavour of SCSI, the ubiquitous, reliable and user-friendly technology found in most IT infrastructures today. Techworld has published a Western Digital view of why enterprise-class SATA drives are as good as SAS drives. Here are the top ten reasons you should favour SAS, courtesy of Andrew Batty (AB: in the text), sales and marketing director at Fujitsu Europe. Interspersed with them are comments from Western Digital's enterprise marketing director, Hubbert Smith (HS: in the text), arguing the case for enterprise SATA.

AB: Peace of mind: parallel SCSI is a proven technology but it has reached the end of the line; Ultra 320 SCSI is out of steam and there will be no Ultra 640 to replace it. SAS is an industry standard, and with its combination of high performance, ease-of-use, reliability and low cost, it is the ideal next step.
HS: I think everyone is OK with this

AB:. Highest performance: with more than four times the performance of SATA and Parallel SCSI SAS rivals the performance of Fibre Channel while offering all the traditional SCSI performance features such as Deep Command Queuing and Asynchronous reporting. In addition SAS performance scales with each drive that is added to the RAID set, making it a sound investment.
HS: Oh really? Is Mr Beatty speaking of usable performance or just wire speed? Would Mr. Batty be willing to provide a system benchmark that demonstrates that SAS has four times the performance of SATA? Our performance tests indicate that rotational speed, seek and queuing determine performance, not wire speed.

AB: Lowest cost: the cost of an infrastructure based on SAS is similar to that of Parallel SCSI and SATA; which means ROI is higher.
HS: We agree with Mr. Batty here, SAS as an infrastructure or system-to-storage interconnect solves interesting problems. There is lots of bandwidth to transfer data from box to box, global naming, fan-out technologies. SAS as a disk drive interface is a completely different and separate subject; we're talking about disk drives aren't we? So is Mr. Batty saying that SAS drives have a higher ROI than other drives?

AB: Best reliability: SAS is designed for enterprise-class drives and thus offers extremely high reliability in spite of its lowest cost of ownership. Complete fault tolerance with dual porting and full SCSI failure prediction features add to this reliability.
HS: Right you are; SAS is designed for enterprise applications. SAS servers and SAS storage systems are designed to accept SAS drives AND SATA drives. So the industry consensus is SATA is viable in the enterprise (if SATA weren't viable for enterprise systems, then the industry would not have collaborated to create the SAS-SATA compatibility as an industry standard). Western Digital has an established track record of producing enterprise-class disk drives (that happen to have a SATA interface). And these drives have been widely accepted by the market. So in the future we expect SAS systems to deploy; they will accommodate both SAS drives and enterprise SATA drives. And the market will determine how many of each are manufactured.

AB: Ultimate flexibility and scalability: SAS infrastructure accommodates both SAS and SATA devices, enabling users to mix and match SAS (for performance and reliability) and SATA drives (non-mission critical applications) and to change the disks ratio according to the organisation’s needs over time.
HS: Enabling users to mix/match SAS and SATA solves important end-user problems. There are some applications that require performance (usually addressed with high priced 15K RPM drives), and whenever additional capacity is required, more drives are applied.

There is a very different market that requires high capacity with reliability and affordability. Characterising SATA drives as non-mission critical is an un-informed statement. WD Enterprise SATA drives have a 1.2 million hours MTTF rating, exactly the same rating as Fujitsu’s latest SAS enterprise drive, the MAV2073. And addressing high capacity markets with 10K and 15K drives is about as satisfactory as eating with a SPORK.

AB: Fully compatible: SAS drives are fully compatible with existing SCSI software and Operating Systems thus making the deployment quick and painless.
HS: That's an important issue and the folks working on SAS have that right. It's important to understand the deployment of Enterprise SATA is virtually complete, shipping today, works today.

AB: Available now: hard disk drives, RAID controllers, and other SAS components and subsystems are already available; this means that it is really not a question of if but a matter of when to deploy SAS. However, organisations do not have to start with a heavy investment in SAS; the new technology can be gradually brought into the network as the demand for performance and reliability increases.
HS: Having the entire recipe for SAS is an important issue and the folks working on SAS have that right. It's important to understand the deployment of Enterprise SATA is virtually complete, shipping today, works today.

AB: Small Form Factor enterprise drives: 2.5 inch drives offer many advantages over their 3.5 inch rivals including higher IOPS per unit volume, higher reliability, and lower power consumption, enabling users to achieve the lowest cost of ownership per transaction.
HS: We're convinced the majority of the market cares about capacity, reliability, price, and $/GB, and that's the focus of WD. We're not so convinced the majority of the market sees significant value in 2.5 inch with lower capacity, and a higher price tag. The market may prove me wrong, but I don't think so.

AB: Option to add more disks into each RAID set: the more spindles in each RAID set the higher the transaction rate. While parallel SCSI and Fibre Channel Arbitrated Loop become a bottleneck with as few as four drives, SAS’s scalability enables users to add tens of drives into a RAID set. Furthermore up to eight Small Form Factor (SFF) enterprise drives can fit in the space of just two standard 5.25 inch server bays or up to ten in a 1U high enclosure versus four 3.5 inch drives.
HS: Since all RAID systems have the option to add more disks, and more spindles, and therefore higher performance, I don't see anything new or special. Regarding Enterprise SATA and bottlenecking, there is no production drive regardless of interface that can saturate Gen 1 SATA 1.5 Gbit/s. So, making next generation faster wire and connecting it to a disk drive, which does not use all the available bandwidth of the first generation wire, will not produce any user-visible performance improvement. Regarding the SFF drives, please refer to comment immediately above.

AB: Safe investment: Choosing a SAS-ready environment offers customers the option to deploy a technology that can grow with their storage requirements, even allowing them to easily switch from SATA to SAS drives without a costly infrastructure upgrade.
HS: Here we completely agree, SAS drives and SATA drives will be compatible with SAS systems allowing users to easily choose Enterprise SATA or SAS drives. Let the market decide.

What does Andrew Batty think about Western Digital's enterprise SATA drives?

"A first glance of paper specifications may make SATA look similar in performance to SAS but do not be fooled by first impressions as when it comes to enterprise class server performance SAS drives will considerably outperform any SATA drives due to the fact that the interface is Full Duplex rather than Half Duplex (as is the case for SATA), the SAS drives have lower latency, lower command overheads, faster access times, deeper command queue depths and also offer the possibility of configuring dual-ported solutions which enables a further doubling of data rates."

"Even if the SAS drives are only used in single-ported mode the Full Duplex nature of SAS could bring about a doubling in performance from the interface alone. The important thing is to measure transactions rather than data rates as it is transactions per second performance that normally matters in most server applications. If data streaming only is required then SATA interfaced drives may prove adequate and the user may as well choose the lowest cost per GB in these instances provided they have a secure backup of their data"

Hubbert Smith has a detailed response to this:-

"Perhaps just the benchmarks might be more relevant. Mr Batty speaks of wire speed as if it's the only metric that matters, in fact wire speed has very little to do with actual perceived performance. Case in point - when your PC went from 10baseT to 100baseT did you see a 10x improvement in your network response. Again, wire speed does not equal performance, am I right?"

"FYI, SAS full duplex means that one channel can be used for data traffic and the other channel can be simultaneously used for command traffic. Both channels cannot be simultaneously used for data. So when Mr Batty says 6Gb/s is available and that's 4x SATA I, he is technically correct, but end users will not see 4x performance."

"Also, Mr Batty claims SAS drives offer lower latency, lower command overhead, faster access times, deeper queue depths and dual port."

Here is the Hubbert Smith examination of these points:

1) Avg Latency:
FJ MAV2073 - 2.99 ms
WD Raptor - 2.99 ms

2) Lower command overhead:
FJ MAV2073 - not specified (actually SCSI has significantly higher overhead than ATA).

3) Access times
FJ MAV2073 - not specified, perhaps he means seek times.

3.1) Avg Seek Time (ms):
FJ MAV2073 - 4.0 read, 4.5 write
WD Raptor - 4.5 read, 5.9 write -

OK, FJ 2.5 wins this because the platter is smaller, hope it's worth the price-tag.

4) Queue depth:
FJ MAV2073 - 264 (SCSI) WD Raptor - 32 (ATA)

IT managers consistently say real world queue depths rarely exceed 8.

5) Dual port:
FJ MAV2073 - SAS
WD Raptor - SATA dual port shipping on interposer card now

Here is the source for this data.

Hubbert Smith's concluding remark is: "We see SAS systems as a fine thing, and we believe all of the customer-visible problems are addressed today with Enterprise SATA drives, we believe tomorrow Enterprise SATA drives will have a large share within SAS systems, and we are certain that either way the market will speak."