SAS and SATA in the same enclosure – Say it ain’t so Joe!
Posted in General, Storage Interconnects & RAID, Advisor - Joe by Joe DisherOne of the promises and major advantages of SAS is that the drive connection is compatible with both the newer SAS drives as well as existing SATA drives. This has long been the overwhelming topic discussed when the promoters of SAS (See TT’s previous blog) are out talking about the greatest thing since sliced bread. (I’m not sure if anyone has actually compared SAS to sliced bread – but in the world of storage geeks, it could certainly happen!) To add to this amazing miracle we call SAS we have the astounding capability of being able to utilize both SAS and SATA drives within the same enclosure!
No way - you say?
It’s true I tell you – you can absolutely deploy SAS and SATA drives within the same physical enclosure… but be careful! In a discussion I had today with some colleagues about this lovely side effect of having a common interconnect between the two drive types, it became clear that although this feature is quite wonderful because you can install crazy-fast 15,000 rpm SAS drives right next to super-high-capacity 7200 rpm SATA drives, there are definitely some best practices to keep in mind before going crazy plugging those drives in.
As it turns out, the slower spinning SATA drives performance can be negatively affected when seated next to their blazing fast SAS counterparts. The cause of this is calculated and described by something called RVI (Rotational Vibration Index). Through a quick web search I found an old 2003 article on EEtimes.com that talks about RVI and SATA drives. Although the article focused on a solid chassis design the specifics of RVI are relevant to SAS and SATA in the same enclosure. The amount of performance degradation will depend on a number of factors (too detailed for me to want to talk about), but to some degree the affects can not be avoided. For Adaptec SAS JBOD enclosures and RAID boxes we are recommending that in our 12 drive 2U chassis, drives are installed with the same type/speed drive in vertical columns of 3. It’s further recommended that the drives are grouped either in two or three columns side-by-side. The one big no-no is to not “sandwich” one column of SATA drives between two columns of SAS drives. Following the above guidelines will minimize the affects of performance loss due to rotational vibration.
So, as you are going out and buying that fancy new SAS/SATA box – when the vendor tells you that you can put both SAS and SATA drives in the same enclosure, look closer and make sure you understand any potential gotchas. It’s great to be able having your primary and secondary storage in the same box saving on power and rack space, but make sure you can handle any potential degraded performance on the secondary, slower spinning storage.
Even though the concept gets a big thumbs-up on the cool technology feature scale, IMHO the best approach (assuming power and rack space aren’t a real problem) would be to buy two enclosures with SAS in one and SATA in the other. BTW, you could make the primary enclosure a RAID box with SAS drives and the secondary enclosure an expansion JBOD with SATA drives connected to the RAID box to keep your storage management tasks simplified. Plus, if you don’t fill up both boxes with drives you’ll have plenty of room to grow!
Blog ya later!
Joe
November 2nd, 2005 at 7:58 am
Hey everyone - me again. I am planning on inviting a fellow Adaptec-ite to comment further on this entry to give more details on RVI and SAS and SATA in the same enclosure - so stay tuned for more!
Blog ya later!
Joe
December 8th, 2005 at 9:28 am
SATA and SAS drives can be mixed in the same enclsoure, as long as some basic rules are followed. First, only the same interface of drives (SAS or SATA) should be present in the same column. Columns of like drives (SAS and SATA) should be placed adjacent to each other. It is important not to randomly mix and match the drives within the enclsoure. The SAS drives operate at a higher rpm and they will induce additional RVI components within the SATA drive.
It is also important to note that SATA drives have a longer response time than SAS drives, so they will behave differently. This difference in behaviour should be kept in mind when mixing drive types in the same enclsoure. Given these differences, I don’t recommend mixing SAS and SATA in the same RAIDset.
October 12th, 2006 at 10:48 pm
Odie said that he/she doesn’t recommend mixing SAS and SATA in the same RAIDset. I want o know if it is at all possible.
October 16th, 2006 at 8:15 am
As Odie pointed out - the response time for SATA drives versus SAS drives will be slower. So much so, that you would lose the main performance benefit that you would gain by having SAS drives.
SAS drives have three main characteristic differences when compared to SATA drives:
(1) Capacity - SAS drives have smaller capacities available to them that SATA (300GB versus 500GB respectively - with 750GB SATA drives due out soon)
(2) Reliability - Generally speaking, higher quality components are used on SAS drives. A good explanation of this can be found from Tom here.
(3) Performance - SAS drives run at 10,000 RPM or 15,000 RPM. SATA drives run typically at 7,200 RPM and as slow as 5,400 RPM. There are some 10,000 RPM SATA drives - but they are more expensive than the slower SATA drives. Stumbled across this interesting ZDNET blog that has more thoughts on drive performance.
To answer the question specifically - it IS “possible” to mix SATA and SAS in the same RAID set, but due to the vast differences in performance characteristics it IS NOT practical. Why pay the additional money for a higher performance SAS drive, if you’re going to slow down the overall RAID performance by grouping it with slower SATA drives? If performance is not a key concern, then you’d be better off with more capacity for less money - don’t you think?
Blog ya later!
Joe
November 28th, 2006 at 8:37 am
Please look at tests performed by Seagate:
http://www.seagate.com/docs/pdf/whitepaper/TP-543_SAS-15K.pdf
They say that mixture of 15k SAS/7.2k SATA is faster than enlosure of 10K SAS drives.
December 6th, 2006 at 9:28 am
Slava - thanks for the link to the Seagate Technology Paper.
What they are talking about specifically is utilizing 15K/SAS and 7200/SATA drives in the same enclosure - but not in the same RAID sets. This is an important distinction. If you tried to combine 15K and 7200 RPM drives in the same RAID set, two undesirable things would occur. (1) The capacity used on each drive for the RAID set would be limited to the smallest drive or partition - most likely on the 15K drives, so that extra capacity on the large 7200/SATA drives would not be utilized. (2) The performance of the RAID set would suffer because of the timing differences between the two drive types. If you’re lucky it would be no slower then using all 7200 RPM drives!
The most optimal configuration would create a RAID set with all 15K RPM drives for your applications with strict performance requirements, and a RAID set with your large SATA drives for your nearline storage needs.
Combining these drives in the same enclosure is efficient from a rack space percpective, but combining them in the same RAID set will not give the outcome you might expect. So go ahead and put your mixed speed drives in the same enclosure (keeping in mind RVI talked about in original blog) - just keep those drives separated into different RAID sets.
Blog ya later!
Joe
January 9th, 2007 at 2:59 am
I have to comment to correct a misunderstanding regarding RVI. 15K kicks out a lot more RV than 7.2K but this has nothing to do with RPM. Its becasue a 15K drive has a much larger actuator motor that can accelerate and decelerate the arm to achieve 3.0ms seeks. Mr Newton’s 3rd law says that as the arm accelerates towards the spindle, the spindle will jerk towards the arm and when the actuator sticks on the brakes the spindle will actually move away from the arm. This will happen at about 100-300Hz in a busy environment - the same as IOPS rate in fact. Its called RV because its a moment about the centre of gravity of the drive (not about the actuator bearing - not that it matters!) A 7.2k is many times slower and so kicks out much less.
Joe