Durability - MTBF value correspondance to TBW

@memrob wrote:

I bought a Extreme 240 GB with has according to SanDisk 2 million hours MTBF (that would be more than 200 years!?). 

 

Usually on SSD the durability / maximum life time is measured in TBW (TerraBytes Written) - a nice and easy to understand value. Additionally for customers it is easy to read out the current value via SMART - acording to my observations the value “Lifetime writes to host” shows in the RAW value the number of Gigybytes that has been written to the SSD in it’s life time.

 

Therefore I would like to know if and how the MTBF value can be translated into a more useful TBW value.

Hi memrob,

They are basically saying that they have a 2Million hour MTBF based upon Telcordia Stress Part testing… which is a rating of electronic component failures if kept within the enviromental specifications of the unit in question. 

Here is some info on the Telcordia MTBF rating…  http://www.reliabilityeducation.com/intro_bellcore.html

It looks like the Sandisk X100 MLC drive and the enterprise MLC SAS drives do have TBW ratings.   

I am currently assuming that it is not in Sandisk’s best interest to publish this information.

I have known about the difference between SLC / MLC ever since the industry started selling “consumer level” SSD’s…  My first SSD was a Samsung SLC SSD (Pre TRIM) and it was will over $1000…

Take a look at this review of the Intel 520 SSD (Which I consider to be a comparable drive to the Extreme)… pay attention to the pros and cons section… http://www.storagereview.com/intel_ssd_520_enterprise_review

The con’s state that the 520 MLC can’t compete with the endurance levels of eMLC… (or SLC of course)

I thought the whole idea of eMLC was that a drive of that classification was just a “Short Stroked / Overprovisioned” version of an MLC drive that was not overprovisioned by the factory.  The overprovisioned area is reallocated to account for failed cells, and when there is much more “Spare MLC” available, you could actually meet or exceed the reliability ratings of an SLC drive of the same amount of storage.

So … a 100GB eMLC drive would have 28GB of Spare Area… the 120GB Sandisk Extreme has 8GB Spare,  a 128GB MLC drive has little to no spare area…  but both have 128GB of comparable MLC NAND.     Also, when eMLC was introduced they were some of the first drives to have SuperCAP’s on them to prevent data failure from power loss.  Now most of the new SSD offerings have Super CAPs for power loss failure.

If you take a look at the OEM Sandisk X100 SSD, they are quoting 80TBW under 128GB/day workload, and the same 2,000,000 hour MTBF as the extreme.   (Does anyone know how close the X100 is to the Extreme, what controller, etc?)

I really would like to know whether or not the Extreme will allow you to reset the controller to a 20% overprovisioned mode.  I may have to dig up someof the low level HDD utils and do some experments soon, see what can be done… I would love to get an extra 20GB of spare in the Extreme and throw it into a high workload application.  My gut tells me it will hold up well… unfortunately the official specs for the drive are keeping their vow of silence.

My current opinion is that the Sandisk Extreme SSD line represents an amazing value as far as I am concerned.  If your application requires a drive with a manufacturer published TBW rating, then you may want to look at drives that are publishing this specification…  

If there are any guys out there who have poked around inside the 2281, or done some low level settings on this drive, or other SSD controllers, or have any comments… please chime in…  Just don’t say anything that would trigger a shortage on the Extreme…  :-)

Personally…  I love a bargain…  and I love this SSD… 

Rich