Riddle me this: When is a SAS drive not a SAS drive?
anthonyv 2000004B9K Visits (37578)
Here are two common statement I often hear from clients:
So is this right? Is this the actual situation?
First up, if your storage uses a SAS based controller with a SAS backplane, then normally you can plug SAS drives into that enclosure, or you can plug SATA drives into that enclosure. This is great because when you plug SATA drives into a SAS backplane, you can actually send SCSI commands to the drive plus you can send native SATA commands t00 (which is handy when you are writing software for RAID array drivers).
But (and this is a big but) what we do know is that equivalent (size and RPM) SAS drives perform better than SATA drives. This is because:
So given a choice (and a very small price differential), why choose SATA over SAS? SAS is the clear winner. What we should instead differentiate on is speed (7.2K RPM vs 10K RPM vs 15K RPM vs SSD) and size (2.5" vs 3.5" form factor).
Which leads us to Nearline SAS
It is a common belief, that if you buy a Nearline SAS (or NL-SAS) drive it is really a SATA drive with a SAS connector (interposer) stuck on it. But this is confusion from the past.
What led to the confusion?
Most midrange and enterprise storage controllers and enclosures up until recent years, used disks that had fibre channel interfaces on them. We plugged those disks into fibre channel enclosures. Examples include the DS4700 or the DS8100. And yet these devices also offered SATA drives. How did they do this?
They took a SATA drive and added a SATA to Fibre Channel converter card to the disk. We call this extra piece of hardware an interposer or bridge card. So people start assuming that this is common practice in every product. In fact we are now seeing SAS drives being put into fibre channel disk enclosures by using a SAS to fibre channel interposer.
There are in indeed older products that did take a SATA drive and add a SATA to SAS interposer to achieve a similar thing. But that really is not necessary any more. The reason? The same hard drive can now be ordered from the factory as either a SAS drive or a SATA drive.
Lets look at an example. If you head over to the Seagate website and look at one of their ranges of 3.5" Enterprise Drives, you should hopefully make it to this URL
Seagate have a nice selector tool to let you see all their possible combinations. For instance you can order a 2 TB drive with a 6 Gbps SAS interface, which is a model ST32000444SS:
Or you can order a 2 TB drive with a 6 Gbps SATA interface, which is a model ST2000NM0011:
So what you get is very similar drive hardware (same spindles, heads, motors) but with different adapter hardware, built with the desired adapter at manufacture time. Meaning that if we install this drive into a SAS enclosure, there is no need to add an interposer or bridge card to the drive after you bought it.
This leads to the next question:
OK. So this is good, so Nearline SAS drives are MADE as SAS drives. Does that mean a driv
Now we are mixing up two different things. SAS as a standard is a combination of a connection technology (the
Nearline on the other hand is a statement about the disks rotational speed and it's mean time between failure (MBTF). A Nearline-SAS drive is
So we have now gone full circle. A Nearline-SAS drive can use the same physical disk hardware as a SATA drive, but with a superior adapter that uses a superior command set, built onto the drive at manufacture time.
Still confused or want to read some more? Check out these links: