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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  • Wwhat - Wednesday, January 06, 2010 - link

    You make a good point, and anand seems to deliberately deflect thinking about it, now you must wonder why.
    Anyway don't be disheartened, your point is good regardless of this support of 'magic' that anad seems to prefer over an intellectual approach.
    Reply
  • Shining Arcanine - Thursday, December 31, 2009 - link

    As far as I can tell from Anand's description of the technology, it seems that this is being done transparently to the operating system, so while the operating system thinks that 25GB have been written, the SSD knows that it only wrote 11GB. Think of it of having two balancing sheets, one that other people see that has nice figures and the other that you see which has the real figures, sort of like what Enron did, except instead of showing the better figures to everyone else when the actual figures are worse, you show the worse figures to everyone else when the actual figures are better. Reply
  • Anand Lal Shimpi - Thursday, December 31, 2009 - link

    Data compression, deduplication, etc... are all apparently picked and used on the fly. SandForce says it's not any one algorithm but a combination of optimizations.

    Take care,
    Anand
    Reply
  • AbRASiON - Friday, January 01, 2010 - link

    What about data reliability, compressed data can normally be a bit of an issue recovering it - any thoughts? Reply
  • Jenoin - Thursday, December 31, 2009 - link

    Could you please post actual disk capacity used for the windows 7 and office install?
    The "size" vs "size on disk" of all the folders/files on the drive, (listed by windows in the properties context tab) would be interesting, to see what level of compression there is.

    Thanks
    Reply
  • Anand Lal Shimpi - Thursday, December 31, 2009 - link

    Reported capacity does not change. You don't physically get more space with DuraWrite, you just avoid wasting flash erase cycles.

    The only way to see that 25GB of installs results in 11GB of writes is to query the controller or flash memory directly. To the end user, it looks like you just wrote 25GB of data to the drive.

    Take care,
    Anand
    Reply
  • notty22 - Thursday, December 31, 2009 - link


    It would be nice for the customer if OCZ did not produce multiple models with varying degrees of quality . Whether its the controller or memory , or combination thereof.
    Go to Newegg glance at OCZ 60 gig ssd and greeted with this.

    OCZ Agility Series OCZSSD2-1AGT60G

    OCZ Core Series V2 OCZSSD2-2C60G

    OCZ Vertex Series OCZSSD2-1VTX60G

    OCZ Vertex OCZSSD2-1VTXA60G

    OCZ Vertex Turbo OCZSSD2-1VTXT60G

    OCZ Vertex EX OCZSSD2-1VTXEX60G

    OCZ Solid Series OCZSSD2-1SLD60G

    OCZ Summit OCZSSD2-1SUM60G

    OCZ Agility EX Series OCZSSD2-1AGTEX60G

    219.00 - 409.00
    Low to high the way I listed them.
    I can understand when some say they will wait until the
    manufactures work out all the various bugs/negatives that must
    be inherent in all these model/name changes.
    Which model gets future technical upgrades/support ?
    Reply
  • jpiszcz - Thursday, December 31, 2009 - link

    I agree with you on that one.

    What we need is an SSD that beats the X25-E, so far, there is none.

    BTW-- is anyone here running X25-E on enterprise severs with > 100GB/day? If so, what kind of failure rates are seen?



    Reply
  • Lonyo - Thursday, December 31, 2009 - link

    I like the idea.
    Given the current state of the market, their product is pretty suitable when it comes to end user patterns.
    SSDs are just too expensive for mass storage, so traditional large capacity mechanical drives make more sense for your film or TV or music collection (all of which are likely to be compressed), which all the non-compressed stuff goes on your SSD for fast access.

    It's god sound thinking behind it for a performance drive, although in the long run I'm not so sure the approach would always be particularly useful in a consumer oriented drive.
    Reply
  • dagamer34 - Thursday, December 31, 2009 - link

    At least for now, consumer-oriented drives aren't where the money is. Until you get 160GB drives down to $100, most consumers will call SSDs too expensive for laptop use.

    The nice thing about desktops though is multiple slots. 80GB is all what most people need to install an OS, a few programs, and games. Media should be stored on a separate platter-based drive anyway (or even a centralized server).
    Reply

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