SATA/SAS vs plain SATA; Cost Per GB - What am I missing?

Posted on 2007-07-20
Last Modified: 2013-11-14
Hello all,

I am looking to upgrade our current storage systems in a few of our locations from about 300-500 Gb to 2-6 Tb.

Everyone that I talk to tells me I should go for a high end, but basic, SATA RAID array using a few 500Gb disks. If I start talking scalability, they start talking about fiber channel, but warn me against the high costs involved. Every time I mention SAS, I get warned against it, for it being either more expensive and complex than FC, or just that it's a "fancy new toy, but not as good as FC".

I feel like I'm really missing something here....

It looks to me like, assuming I am going simply for capacity and not extremely high availabity, that I should be able to host, on a single 8 port SAS card, an array easily scalable to over 60 Tb of storage for around $0.58 per Gb. (*) What's more, it appears that the overall cost will scale in a linear manner if I need more or less storage space...

(*) Assuming the following:
1 eight-port SAS card: ~$1000
8 12x SAS Edge Expanders: (1 host <-> 11 devices)  ~$8000
88 750Gb SATA disks (**):  ~$25000
11 basic 8-disk SATA ensclosures: ~$2500
Misc cabling and adapters: ~$2000
Total: $38500
Storage: 66000 Gb
Cost per Gb $0.5833

(**) I'm going with the assumption that you CAN use SATA disks within an SAS array... Everything I see in the specs indicates that you can, and since I'm only looking at capacity, I don't need the dual-port redundancy available to SAS drives.

Note, of course, that I only need 2-8 Tb in my arrays initially, something that can easily be taken care of with a standard SATA RAID controller. But I like the idea of being able to scale... The entry costs to SATA and SAS appear very similar when using SATA-only drives, and the overall costs appear to scale in a linear manner.

What am I missing? Why do OEM builders and other sysadmins shy away from SAS? Is it just because SAS equipment is so new? Is there something unusual about the way SAS cards interact with their arrays?

I can build the array I noted above, right?

Any input would be appreciated; thanks in advance.
Question by:matheweis
    LVL 3

    Assisted Solution

    You could read a bit more here on this link if you want:

    As sas storage (not too much writing, but more for reading), I could see that sas-drives is/was more expencive than sata storage. sasdrives would be more reliable while using them 24/7. SAS is most used for storage and a lot of writing and reading day and night.

    Hope you can get some idea of differences on that documentlink, and how to extend storage (specially on sas storage.

    Hope this would help a bit.
    LVL 55

    Accepted Solution

    These SATA disks that you assume to be equal to SAS in all but speed, did you check the duty-cycle figures offered by the manufacturer?

    Although there is nothing inherent in the SAS/SATA interface that implies reliability many of the SATA disks are offered as desktop / workstation drives designed to be running for 20% of the time whereas the SAS ones are designed to run full tilt 100% of the time. I think this is what you are missing in terms of info from the supplier, the MTBF against duty-cycle are pretty well hidden in the small print.

    We have had this question before as a SATA Vs [parallel] SCSI comparison and in general SCSI disks outperform SATA ones because the motor and actuator are designed to run all the time against the SATA ones being designed for the desktop where they spin down and cool off only to spin up and retrieve data when the single user requires it. An exception to the rule is the WD Raptor but you can see from the size / performance [and price] specs that it is a [parallel] SCSI disk assembly with a SATA interface board on it.

    You probably can build the array quoted above but after a year you will be replacing a couple of disks per week if it is used in a business environment, good for a personal video store though. I haven't heard of anyone shying away from SAS so I can't answer the last one, as far as I'm concerned SAS is the future unless solid state takes over.
    LVL 3

    Assisted Solution

    SAS is an acronym for Serial Attatched SCSI.  Meaning your using a device that houses SCSI drives attatched to a Serial controller.  The SCSI backplane attatches to the controller with a SATA cable, or cables, if the backplane is split somehow.  In any case the SCSI drives are used as andyalder stated due to their reliability and performance factors, and the Serial interface allows for increased bandwitch between the device and the controller.  Serial being considerably faster than SCSI's parallel architecture.  In addition, as you mentioned you can daisy chain many more devices on most SAS controllers than is possible on the average SCSI controller and the I/O contention is not and issue as it can be on the SCSI controller.  SAS is a very reliable and high performing solution for storage and as the cost of SCSI drives comes down it's becoming quite cost effective as well.

    Now, you mentioned SAS configurations with SATA drives, which I have heard of as well.  The reliability of current SATA drives does not compare to current SCSI drives and therefore it is recommended to stick with SAS that implements SCSI drives.  Now, if your storage implementation is mostly archival and will not require lots of I/O SAS that implements SATA drives is a perfect solution due to it's extreme cost efficiency over SCSI implementations.  

    Best of luck.
    LVL 3

    Author Comment

    I just want to thank you all for your input...

    I've seen and read the referenced whitepaper, which has helped me to come to the conclusions noted above. I see your point about SAS reliability, though I would point out that it has little to do with the SAS vs SATA protocol, and everything to do with the actual quality of the drives as manufactured. See my response to andyalder below:


    Yes, the drives are high end drives sold by Western Digital, advertised as 'server class' and/or 'enterprise class'. They appear to use components akin to the high end SCSI or SAS drives such as fluid dynamic bearings. They have an 5-year warranty, and an MTBF of 1 to 1.2 million hours. I am unable to find an AFR, but other SATA WD drives (Such as the Caviar SE series) have an AFR of < 0.5% I also read that all WD drives are manufactured to an AFR of < 0.8% The bottom line is that these cannot possibly be drives that will be failing at even a rate of even 1 out of 100 after 2 or 3 years; it would be disaster for Western Digital from a business standpoint.

    Take a look:

    Thanks for the added info.

    A few final comments:

    My main question was if there was any reason to choose between an SATA controller running SATA disks, or an SAS controller running SATA disks. Hearing such comments as SAS being only a fancy new toy scared me away a bit. However, as I saw it, SAS could do everything SATA could, with much more capability.

    My 66Tb disk array was only a theoretical demonstration of the capabilty of SAS. As noted before, my actual intent is only to build a few arrays (in different locations) of between 2 and 6 Tb. Using 500 Gb disks, that's a max of 12 disks in the array using full RAID 10. Therefore I don't see that I would practically be replacing a couple of disks per week, since replacing 12 disks would replace the entire array. (And in practice, I am looking at RAID 5EE, so that would only be 10 disks)

    The biggest point I was looking to verify is that the cost per Gb using the SATA infrastructure vs the SAS infrastructure are virtually the same, while the SAS infrastructure allows all the same capability of SATA with far greater expansion and capability in every aspect.

    Thus, it would be pointless to purchase of an enterprise level SATA RAID controller since the cost of an enterprise level SAS and SATA controller is more or less identical. (~$800)

    Thanks again for all your input.


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