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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  • semo - Saturday, January 02, 2010 - link

    Anand,

    After reading your very informative SSD articles, I still found something new from GullLars. I think it would be useful to include the queue length when stating IOPS figures as it will give us more technical insight of the inner workings of the different SSD models and give hints to performance for future uses.

    When dial up was the most common way of connecting to the internet, most sites were small with static content. As connection and CPU speeds grew, so did the websites. Try going to a big ugly site like cnet with a 7-8 year old pc with even the fastest internet connection. I'm sure that all this supposed untapped performance in SSDs will be quickly utilized in future (probably because of inefficient software in most cases rather than for legit reasons). With virtualization slowly entering the consumer space (XP mode, VM unity and so on) as giant sandboxes and legacy platforms, surely disk queue lengths can only grow...
    Reply
  • shawkie - Saturday, January 02, 2010 - link

    Anand,

    I agree that its also helpful to know what the hardware can really do. It seems to me that longer queue depths are becoming important for high performance on all storage devices (even hard disks have NCQ and can be put in RAID arrays). At some point software manufacturers are going to wake up to that fact. This is just like the situation with multi-core CPUs. I'm fortunate because in my work I not only select the hardware platform but also develop the software to run on it.
    Reply
  • DominionSeraph - Monday, January 04, 2010 - link

    A jumble of numbers that don't apply to the scenario at hand is nothing but misleading.

    Savvio 15K.1 SAS: 416 IOPS
    1TB Caviar Black: 181.

    Ooooh... the 15k SAS is waaaay faster!! Sure, in a file server access pattern at a queue depth of 64. Try benchmarking desktop use and you'll find the 7200RPM SATA is generally faster.
    Reply
  • BrightCandle - Friday, January 01, 2010 - link

    With which software and parameters did you achieve the results you are talking about? Everything I've thrown at my X25-M has shown results in the same park as Anand's figures so I'm interested to see how you got to those numbers. Reply
  • GullLars - Friday, January 01, 2010 - link

    These numbers have been generated by several testing methods.
    *AS SSD benchmark shows 4KB random read and random write at Queue Depth (QD) 64, and x25-M gets in the area of 120-160MB/s on read and 65-85MB/s on write.
    *Crystal Disk Mark 3.0 (beta) tests 4KB random at both QD1 and QD32. At QD32 4KB random read, Intel x25-M gets 120-160MB/s, and at random write it gets 65-85MB/s here too.
    Here's to a screenshot of CDM 2.2 and 3.0 of x25-M 80GB on 750SB with AHCI in fresh state. http://www.diskusjon.no/index.php?act=attach&t...">http://www.diskusjon.no/index.php?act=attach&t...
    *Testing with IOmeter, parameters 2GB length, 30 sec runtime, 1 worker, 32 outstanding IO's (QD), 100% read, 100% random, 4KB blocks, burst lenght 1. On a forum i frequent most users with x25-M get between 30-40.000 IOPS with theese parameters. For the same parameters only 100% write the norm is around 15K IOPS on a fresh drive, and a bit closer to 10K in used state with OS running from the drive. x25-E has been benched to 43K random write 4KB IOPS.

    Regarding the practical difference 4KB IOPS makes, the biggest difference can be seen in the PCmark vantage test Application Launching. Such workloads involve reading a massive amount of small files and database listings, pluss logging all file access this creates. Prefetch and superfetch may help storage units with less than a few thousand IOPS, but x25-M in many cases actually get worse launch times with these activated. Using a RAM disk for known targets of small random writes make sense, and i've put my browser cache and temp files on a RAM disk even though i have an SSD.
    With x25-M's insane IOPS performance, the random part of most workloads is done whitin a second and what you are left waiting for is the loading of larger files and the CPU. Attempting to lower the load time of small random reads during an application launch from say 0,5 sec by running a superfetch script or read-caching with a RAMdisk makes little sense.
    Reply
  • Zool - Friday, January 01, 2010 - link

    For a average user 4KB random performance are the most useless results out there. If a user encounters that much random 4KB read/writes than he need to change the operating system asap.
    And if something realy needs to randomly read/write 4KB files than your best bet is to cache it to Ram or make Ram disk i think.
    Reply
  • LTG - Thursday, December 31, 2009 - link

    This statement seems really dubious - Isn't it in fact the opposite?

    The majority of storage space is taken up by things that don't compress well: Music, Videos, Photos, Zip style archives...

    Everything else is smaller.


    Anand Says:
    ==========================
    That means compressed images, videos or file archives will most likely exhibit higher write amplification than SandForce’s claimed 0.5x. Presumably that’s not the majority of writes your SSD will see on a day to day basis, but it’s going to be some portion of it.
    Reply
  • DominionSeraph - Friday, January 01, 2010 - link

    That stuff just gets written once.
    Day-to-day operations sees a whole lot of transient data.
    Reply
  • Shining Arcanine - Thursday, December 31, 2009 - link

    As someone else suggested, I imagine that the SATA driver could take all of the data written/read to the drive and transparently implement the algorithms on the much more powerful CPU.

    Is there anything to stop people from reverse engineering the firmware to figure out exactly what the drive in terms of compression is doing and then externalizing it to the SATA driver, so other SSDs can benefit from it as well? i.e. Are there any legal issues with this?
    Reply
  • Anand Lal Shimpi - Friday, January 01, 2010 - link

    Patents :) SandForce holds a few of them with regards to this technology.

    Obviously that's up to the courts to determine if they are enforceable or not, SandForce believes they are. Other companies could license the technology though...

    Take care,
    Anand
    Reply

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