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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  • Anand Lal Shimpi - Friday, January 01, 2010 - link

    Correct. Highly Random and highly compressed data will not work well with SandForce's current algorithm. Less than 25% of the writes you'll see on a typical desktop machine are random writes, even then they aren't random over 100% of the LBA space. I'm not sure how well the technology works for highly random server workloads (SF claims it's great), but for the desktop user it appears to be perfect.

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

    Thinking about this further I've come to the conclusion that the files must be divided into small blocks that are compressed independently. Firstly because the disk doesn't know about files (only sectors) and secondly because its the only way you could modify a small part of a compressed file quickly. I don't think 512 bytes would be big enough to acheive respectable compression ratios so I think 4KB is more likely. This might explain why Seagate are pushing to make 4KB the smallest addressable unit for storage devices. So then they take each 4KB block, compress it, and write it to the next available space in flash. If they use 64 bit pointers to store the location of each 4KB block they could easily address the entire space with single-bit granularity. Of course, every overwrite will result in a bit of irregularly sized free space. They could then just wait for a bit of compressed data that happens to fit perfectly or implement some kind of free space consolidation or a combination. I'm starting to come around to the idea. Reply
  • shawkie - Friday, January 01, 2010 - link

    Apologies to Anand, I completely missed the page titled "SandForce's Achilles' Heel". I do think there are some scenarios that still need testing though. What happens when a small modification has to be made to a large file that the drive has decided to compress? Not an easy thing to benchmark but something I can imagine might apply when editing uncompressed audio files or some video files. The other question is what happens when the disk is made dirty by overwriting several times using a random write pattern and random data. What is the sequential write speed like after that? Reply
  • lesherm - Friday, January 01, 2010 - link

    with a Seinfeld reference. Reply
  • LTG - Friday, January 01, 2010 - link

    Definitely the only one with a Seinfeld and a Metallica and a StarWars reference :).


    Sponge Worthy
    Enter the Sandforce
    Use the Sandforce
    Reply
  • GullLars - Thursday, December 31, 2009 - link

    It seems anand has a problem with identifying the 4KB random performance of the drives.

    Intel x25-M has time and time again been shown to deliver 120MB/s or more 4KB random read bandwidth. x25-E delivers in the area of 150MB/s random read and 200MB/s of random write at 4KB packet sizes for queue depth of 10 and above.

    I do not know if the problem is due to testing not being done in AHCI/RAID mode, or if it is because of a queue depth lower than number of internal flash channels, but these numbers are purely WRONG and misrepresentative. I probably shouldn't post while drunk :P but this upsets me enough to disregard that.

    Anandtech is IMO a site too good to post nonsensical data like this, pleese fix it ASAP. If you choose to sensor my post after fixing it, pleese mail me notifying me of it in case i don't remmeber posting.
    Reply
  • Anand Lal Shimpi - Friday, January 01, 2010 - link

    My 4KB read/write tests are run with a queue depth of 3 to represent a desktop usage scenario. I can get much higher numbers out of the X25-M at higher queue depths but then these tests stop being useful for desktop/notebook users. I may add server-like iometer workloads in the future though.

    All of our testing is done in non-member RAID mode.

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

    Thank you for the response, but i still feel the need to point out that posting 4KB random numbers for queue depth 3 should be explicitly pointed out, as this only utilizes less than 1/3 of the flash channels in the x25-M. Here is a graph i made of the 4KB random read IOPS numbers of an x25-M by queue depth: http://www.diskusjon.no/index.php?act=attach&t...">http://www.diskusjon.no/index.php?act=attach&t...
    As shown in this graph, the performance scales well up to a queue depth of about 12, where the 10 internal channels get saturated with requests.

    A queue depth of 3 may be representative for average light load running windows, but during operations like launching programs, booting windows, or certain operations whitin programs that read database listings, the momentary queue depths often spike to 16-64, and it is in theese circumstances you really feel the IOPS performance of a drive. This is one of the reasons why x25-M beats the competition in the application launch test in PCmark vantage despite having the same IOPS performance at queue depths 1-4 and about the same sequential performance.

    The sandforce SF-1500 controller is rated for 30.000 4KB random IOPS, 120MB/s. In order to reach these read performance numbers with MLC flash, you need at least 6 channels, with corresponding outstanding IO's to make use of them. Then you also need to take into account controller overhead. The SF-1500 controller has 16 channels, and the SF-1200 controller has 8 channels.
    To test IOPS performance of a drive (not enterpreted for usage but raw numbers), outstanding IOs should be at least equal to number of channels.
    Reply
  • Anand Lal Shimpi - Friday, January 01, 2010 - link

    I'm not sure I agree with you here:

    "A queue depth of 3 may be representative for average light load running windows, but during operations like launching programs, booting windows, or certain operations whitin programs that read database listings, the momentary queue depths often spike to 16-64,"

    I did a lot of tests before arriving at the queue depth of 3 and found that even in the most ridiculous desktop usage scenarios we never saw anything in the double digits. It didn't matter whether you were launching programs in parallel or doing a lot of file copies while you were interacting with apps. Even our heavy storage bench test had an average queue depth below 4.

    Take care,
    Anand
    Reply
  • GullLars - Saturday, January 02, 2010 - link

    I'm not out to be difficult here, so i will let it be after this, but what i and a few others who have been doing SSD benchmarking for about a year now have found using the windows performance monitor indicates Queue Depth spikes in the area of 16-64 outstanding IO's when launching apps, and certain other interactions with apps that cause reading of many database entries.

    Copying files will only create 1 outstanding sequential IO-queue, and does not contribute significantly to the momentary queue depth during short high loads.

    Scanning for viruses may contribute more to the queue depth, but i have not tested it this far.

    At a queue depth of 1-4 for purely reads, there is little difference between JMicron, Indilinx, Samsung, Mtron, and Intel based SSDs, and the difference seen in PCmark Vantage applauch test and real world tests of "launch scripts" (a script launching all programs installed on the computer simultaneously) also indicate there is a notable difference. Some of this may be caused by different random write performance and sequential read, but queue depths above 4 in bursts help explain why x25-M with the 10-channel design beats the competing 4-channel controllers in this type of workload even when sequential read is about the same.

    I also like to think Intel didn't make a complex 10-channel "M" drive optimized for 4KB random IOPS targeted at consumers only to win in benchmarks. If the queue depth truly never went above 3-5, even when counting bursts, there would have been wasted a ridiculus amount of effort and resources in making the x25-M, as a 4-channel drive would be a lot cheaper to develop and produce.


    Thanks for taking the time to reply to my posts, and i hope you know i value the SSD articles posted on this site. My only concern has been the queue depths used for performance rating, and a concern for the future is that the current setup does not forward TRIM to drives supporting it.
    Reply

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