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SATA vs SAS - What is the difference?

 

Deciding between SATA vs SAS is important when considering the right storage infrastructure for such key projects as server virtualization for small business.  SATA or Serial ATA stands for Serial Advanced Technology Attachment and is the next generation computer bus interface from the parallel interface ATA.  SAS, or Serial Attached SCSI, is a faster and historically more expensive interface.

sata vs sas

Because SAS drives are able to rotate so much faster (up to 15K RPM) than SATA drives (typically 7.2K RPM), seek times may be substantially faster by more than 2 times.  The figure above shows different capacities and list prices of SAS and SATA drives available for a particular flavor of HP DL380 G7.  While SATA is the least expensive drive available, for servers, it is typically much better to go with SAS for low capacity use cases.  For example, a 10K 146GB SAS drive is not much more expensive than the substantially slower 146GB 7.2K SATA drive.

At higher capacities, high performance SAS drives climb in price steeply.  It may be better to take advantage of dedicated storage infrastructure such as Network Attached Storage (NAS) or a Storage Area Network (SAN), rather than populate a server with expensive direct attached storage.

As discussed in an almost identically titled blog entry - SAS vs SATA – what’s the difference? - SATA drives tend to be better solutions for desktops and SAS drives for servers. 

Key takeaways:

  • The performance benefits of SAS far outweigh the price difference at low capacities
  • There is a point of diminishing returns for the performance of SAS at high capacities
  • SATA tends to be a better solution for desktops and SAS for servers
  • Consider a dedicated storage infrastructure for high capacity needs

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Comments

Good, crisp explanation. Thanks
Posted @ Monday, July 16, 2012 12:12 AM by Vijay
Welcome and thank you for the kind words!
Posted @ Monday, July 16, 2012 8:59 AM by Sean Kline
simple and good explanation
Posted @ Tuesday, August 14, 2012 7:17 PM by jayachandran
Thank you Jayachandran!
Posted @ Tuesday, August 14, 2012 8:11 PM by Sean Kline
In the consumer class there is SATA, and in the enterprise class there is SAS. In comparison, SATA is like Windows 3.1 and SAS is like Windows Server 2008. Although the gap is much, much wider than that as SATA is such a dated technology and SAS is so much better.  
Of course, SAS is overkill for a home user since they will never use more than 1% of the drive’s features. However, SAS does have features that a home user will love, and SAS drives and controllers have gotten cheaper over the years and are now usable by PC enthusiasts like gamers, servers, video editing and CAD users.  
To keep things as simple as possible, let’s look at the most basic features that a home user will use.  
As with all PATA and SATA devices, they are half-duplex. Most people don’t know that for some reason. Meaning that they can only read or write data at any one time. This of course includes SSD drives as they are also SATA. SAS drives are full-duplex and can read and write at the same time which greatly increases performance.  
SAS drives have higher transfer speeds, even when only reading or writing. In comparison, let’s look at a WD VelociRaptor 10,000 RPM SATA drive and a WD XE SAS 10,000 RPM SAS drive.  
WD VelociRaptor SATA: Transfer Rate (Buffer To Disk) 145 MB/s (Sustained) 
WD XE SAS: Transfer Rate (Buffer To Disk) 204 MB/s (Sustained) 
The SAS drive performs much better, but the SAS drive is also full-duplex which will allow it to perform even better than the SATA drive. Also, SAS drives use ECC for less errors and better data integrity.  
SAS drives and controllers now compare in price to the more expensive VelociRaptor type drives. SAS drives come in 7,200, 10,000, and 15,000 RPM spindle speeds and in capacities of up to 4TB. You can of course RAID these drives together for even higher performance.  
So if you’re building a new high-end system or are looking for an upgrade, I highly recommend considering the SAS drive option.
Posted @ Monday, October 01, 2012 3:07 PM by Stormprobe
Which tech is better really depends on what you are using the storage for. 
 
If your storage is primarily archival in nature - that is, you write a lot of data but don't read a lot of data - then there isn't much difference between SATA and SAS until you begin saturating the SATA3 bus, which caps out at 600 MB/s. Before that you're just wasting money with SAS. The SAS3 bus has 50% higher bandwidth and the SAS protocol has a 20% lower overhead, so it caps out at about 1200 MB/s. Bear in mind that you will not achieve these speeds regardless of which path you choose without RAID, period. Spinning disks are right at their physical limits for speed, so you likely won't see a spinning disk drive with a much higher throughput than about 200 MB/s.  
 
Also, relating archival storage, it is entirely possible for a large, 7200 RPM drive to have a faster sequential read rate than a small 10,000 or 15,000 RPM drive. This is just physics. The majority of a disks data is in the outer half of the disk. At the edge of the disk, a 2TB drive is going to read 3 times the data in a single rotation than a 500GB drive will. So a 2TB 7200 RPM drive is going to be 50% faster at the edge of the disk than a 500GB 15,000 RPM drive, and nearly double a 10,000 RPM drive. Obviously a 2TB 15,000 RPM will be twice as fast (roughly) as the 7200 RPM drive. 
 
This advantage gets destroyed if the data is not archival in nature, and is instead written, deleted, and re-written constantly. In this case, absolutely nothing beats an SSD, though they tend to be extremely expensive even compared to SAS. SAS is a big step up from SATA in this regard, so if you need lots of storage and decent seek times and can't spend $100,000 on your storage solution, SAS is the way to go. 
 
SAS is pretty much the default for servers these days as it generally provides the best balance between cost, speed, and space. However if you aren't putting in a RAID you probably don't need SAS anyway.
Posted @ Sunday, October 07, 2012 6:01 PM by bigjeff5
There are allot of misconceptions when it comes to hard drives. One is that having a faster interface makes the drive faster. As long as the drive's interface speed is faster than the drive's transfer speed, it won't be a bottleneck. If a drive can only transfer speeds at 200MB/s, it is useless having a SATA3 interface speed of 600MB/s. It's like driving a car down a 10 lane highway made for driving at 300 MPH but your car's max speed is only 100 MPH. So having a drive interface speed of 1,000,000 MB/s doesn't mean your drives is going to perform at that speed. The only thing a faster interface will help with is the burst speed, and that only lasts for a fraction of a second. Although, a faster bus is needed when you RAID drives. Since you will have multiple drives on the same bus, you will need a faster bus to support the combined speeds of all of the drives in the array.  
Another misconception is physical drive platter size, writing at the edge of the platter, and having more platters and heads in a drive so the platters act as RAID 0 arrays for faster transfer speeds. It sounds logical in theory, but it doesn't work that way in practice, and it can even be counter intuitive. There are allot of variables involved in drive mechanics and performance, and you may get slightly faster speeds when writing at an edge of a platter, but the difference is negligible and irrelevant as you cannot tell your drive to save your game on the outer edge of the platter and your Office suite at the inner edge. Also, lower platter count drives actually perform slightly better than higher platter count drives. So if you have two 1TB drives and one has one platter and the other has two, the one platter drive will actually perform slightly better. Also, a one platter 1TB drive and a and a four platter 4TB drive will perform about the same as well, even when physics suggest that the 4TB drive would be four times faster, regardless of interface speed.
Posted @ Monday, October 08, 2012 8:05 AM by Stormprobe
Everyone always talks about speed speed speed. But, what about longevity? I hear that SAS drives last must longer than SATA drives.  
 
I have an Ultra3 Scsi that is 13 years old and still running just fine. (probably on it's last leg, so to speak) but still 13 years ! Of continuous use in a production server. 
 
Of course I tried to utilize tmpfs (that is using system RAM as a small 500meg hard drive ) and write temporary fast write/read/delete files there. Like PHP sessions and log files I do not care about if the system is reset. 
 
Posted @ Sunday, September 01, 2013 6:45 PM by Matt Kukowski
Thank you for the comment Matt! You are right regarding better longevity of SAS drives as compared to SATA. According to the following, SAS has a 1.6M hour Mean Time Between Failure (MTBF) compared to 1.2M hour MTBF for SATA. 
 
http://www.techrepublic.com/blog/networking/how-sas-near-line-nl-sas-and-sata-disks-compare/5323
Posted @ Tuesday, September 03, 2013 10:28 AM by Sean Kline
So nice comparison & nice comments, thanks
Posted @ Friday, February 21, 2014 8:14 AM by emeckei
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