Controlling Costs with no DRAM and Cheaper Flash

SandForce is a chip company. They don’t make flash, they don’t make PCBs and they definitely don’t make SSDs. As such, they want the bulk of the BOM (Bill Of Materials) cost in an SSD to go to their controllers. By writing less to the flash, there’s less data to track and smaller tables to manage on the fly. The end result is SF promises its partners that they don’t need to use any external DRAMs alongside the SF-1200 or SF-1500. It helps justify SandForce’s higher controller cost than a company like Indilinx.

By writing less to flash SandForce also believes its controllers allow SSD makers to use lower grade flash. Most MLC NAND flash on the market today is built for USB sticks or CF/SD cards. These applications have very minimal write cycle requirements. Toss some of this flash into an SSD and you’ll eventually start losing data.

Intel and other top tier SSD makers tackle this issue by using only the highest grade NAND available on the market. They take it seriously because most users don’t back up and losing your primary drive, especially when it’s supposed to be on more reliable storage, can be catastrophic.

SandForce attempts to internalize the problem in hardware, again driving up the cost/value of its controller. By simply writing less to the flash, a whole new category of cheaper MLC NAND flash can be used. In order to preserve data integrity the controller writes some redundant data to the flash. SandForce calls it similar to RAID-5, although the controller doesn’t generate parity data for every bit written. Instead there’s some element of redundancy, the extent of which SF isn’t interested in delving into at this point. The redundant data is striped across all of the flash in the SSD. SandForce believes it can correct errors at as large as the block level.

There’s ECC and CRC support in the controller as well. The controller has the ability to return correct data even if it comes back with errors from the flash. Presumably it can also mark those flash locations as bad and remember not to use them in the future.

I can’t help but believe the ability to recover corrupt data, DuraWrite technology and AES-128 encryption are somehow related. If SandForce is storing some sort of hash of the majority of data on the SSD, it’s probably not too difficult to duplicate that data, and it’s probably not all that difficult to encrypt it either. By doing the DuraWrite work up front, SandForce probably gets the rest for free (or close to it).

The Secret Sauce: 0.5x Write Amplification Capacities and Hella Overprovisioning
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  • Howard - Friday, January 01, 2010 - link

    Did you REALLY mean 90 millifarads (huge) or 90 uF, which is much more reasonable? Reply
  • korbendallas - Saturday, January 02, 2010 - link

    Yep, it's 0.09F 5.5V Supercapacitor.

    http://www.cap-xx.com/images/HZ202HiRes.jpg">http://www.cap-xx.com/images/HZ202HiRes.jpg
    Reply
  • iwodo - Friday, January 01, 2010 - link

    If, all things being equal, it just shows that the current SSD drives performance aren't really limited by Flash itself but the controller.

    So may be with a Die Shrink we could get even more Random RW performance?
    And i suspect these SSD aren't even using ONFI 2.1 chips either, so 600MB/s Seq Read is very feasible. Except SATA 3.0 is holding it all up.

    How far are we from using PCI-Express based SSD? I am sure booting problem could be easily solved with UEFI,
    Reply
  • ProDigit - Friday, January 01, 2010 - link

    One of the factors would be if this drive has a processor that does real life compression of files on the SSD,that would mean that it would use more power on notebooks.
    Sure it's performance is top, as well as it's length in time that it works, but how much power does it use?

    If it still is close to an HD it might be an interesting drive. But if it is more, it'd be interesting to see how much more!
    I'm not interested in equipping a netbook or notebook/laptop with a SSD that uses more than 5W TDP.
    Reply
  • chizow - Friday, January 01, 2010 - link

    I've always noticed the many similarities between SSD controller technology and RAID technology with the multiple channel modules determining reads/write speeds along with write differences between MLC and SLC. The differences in SandForce's controller seems to take this analogy a step further with what is essentially RAID 5 compared to previous MLC SSDs.

    It seems like these drives use a lot of controller/processor power for redundancy/error checking code, which is very similar to a RAID 5 array. This allows them to do away with DRAM and gives them the flexibility to use cheaper NAND Flash, but at the expense of additional Flash capacity to store the parity/ECC data. I guess that begs the question, is 64MB of DRAM and the difference in NAND quality used more expensive than 30% more NAND Flash? Right now I'd say probably not until cheaper NAND becomes available, but if so it may make their technology more viable to widespread desktop adoption when that

    Last thing I'll say is I think its a bit scary how much impact Anand's SSD articles have on this emerging market. He's like the Paul Muad'dib of SSDs and is able to kill a controller-maker with a single word lol. Seriously, after he exposed the stuttering and random read/write problems on Jmicron controllers back when OCZ first started using them, the mere mention of their name combined with SSDs has been taboo. OCZ has clearly recovered since then, as their Vertex drives have been highly regarded. I expect SandForce-based controllers to be all the buzz now going forward, largely because of this article.
    Reply
  • pong - Friday, January 01, 2010 - link

    It seems to me that Anand may be misunderstanding the reason for the impressive write amplification. The example with Windows Vista install + Office 2007 install states that 25GB is written to the disk, but only 11GB is written to flash. I don't believe this implies compression. It just means that a lot of the data written to disk is shortlived because it lives in temporary files which are deleted soon after or because the data is overwritten with more recent information. The 11GB is what ends up being on the disk after installation whether it is an SSD or a normal hard-drive. If the controller has significantly more RAM than other SSD controllers it doesn't have to commit short-lived changes to flash as often. The controller may also have logic that enables it to detect hotspots, ie areas of the logical disk that is written to often to improve the efficiency of its caching scheme. This sort of stuff could probably be implemented mostly in an OS except the OS can't guarantee that the stuff in the cache will make it to the disk if the power is suddenly cut. The SSD controller can make this guarantee if it can store enough energy - say in a large capacitor - to commit everything it has cached to flash when power is removed. Reply
  • shawkie - Friday, January 01, 2010 - link

    Unless I misread it the article seems be claiming that the device actually has no cache at all. Reply
  • bji - Friday, January 01, 2010 - link

    I think the article said that the SSD has no RAM external to the controller chip, but that the controller chip itself likely has some number of megabytes of RAM, much of which is likely used for cache. It's not clear, but it's very, very hard to believe that the device could work without any kind of internal buffering; but that this device does it with less DRAM than other SSDs (i.e., the smaller amount of DRAM built into the controller chip versus a separate external tens-of-megabytes DRAM chip).
    Reply
  • gixxer - Friday, January 01, 2010 - link

    I thought the vertex supported Trim thru windows 7, yet in the article Anand says this:
    "With the original Vertex all you got was a command line wiper tool to manually TRIM the drive. While Vertex 2 Pro supports Windows 7 TRIM, you also get a nifty little toolbox crafted by SandForce and OCZ:"

    Does the Vertex drive support windows 7 trim or do you still have to use the manual tool?
    Reply
  • MrHorizontal - Friday, January 01, 2010 - link

    Very interesting controller, though they've seemed to have missed a couple of tricks...

    First why is an 'enterprise' controller like this not using SAS which is at 6GBps right now and we can see what effect a non-3GBps interface has on SSDs, and why when SATA 6GBps is being shipped in motherboards now, then in 2010, when these SandForce drives are going to be released will still be using 3GBps SATA...

    Second, the 'RAID' features of this drive seem to be like RAID5 distributing parity hashes across the spare area which is also distributed across the drive. However, all controllers have multiple channels and why they don't use RAID6 (the one where a dedicated drive holds parity data, not the 2-stripe RAID5) whereby they use 1 or 2 SLC NAND Flash chips to hold the more important data, and use really cheap MLC NAND to hold the actual data in a redundant manner?
    Reply

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