The New Indirection Table

While the binary tree structure was great for sequential IO performance and for keeping DRAM sizes low, it wasn't good for lowering random IO latency. The S3700 controller completely does away with the old indirection table.

The new controller ditches the binary tree entirely and moves to a completely flat structure with 1:1 mapping. What happens now is there's a giant array with each location in the array mapped to a specific portion of NAND. The array isn't dynamically created and, since it's a 1:1 mapping, searches, inserts and updates are all very fast.

The other benefit of being 1:1 mapped with physical NAND is that there's no need to defragment the table, which immediately cuts down the amount of work the controller has to do. Drives based on this new controller only have to keep the NAND defragmented.

The downside to all of this is the DRAM area required by the new flat indirection table. The old binary tree was very space efficient, while the new array is just huge. It requires a large amount of DRAM depending on the capacity of the drive. In its largest implementation (800GB), Intel needs a full 1GB of DRAM to store the indirection table. By my calculations, the table itself should require roughly 100MB of DRAM per 100GB of storage space on the drive itself. Intel appears to be using DDR3-1333 for its DRAM on-board S3700 drives.

There's a bit of space left over after you account for the new indirection table. That area is reserved for a cache of the controller's firmware so it doesn't have to read from slow flash to access it.

Once again, there's no user data stored in the external DRAM. The indirection table itself is physically stored in NAND (just cached in DRAM), and there are two large capacitors on-board to push any updates to non-volatile storage in the event of power loss.

It sounds like a simple change, but building this new architecture took quite a bit of work. The results, if they are anywhere close to what Intel is promising, are pretty awesome.

Final Words

The Intel SSD DC S3700 appears to be a very promising new architecture from Intel. If it ends up performing as Intel promised, the S3700 controller could be the beginning of a new era in SSD performance - one focused on consistency of performance, not just absolute performance. As soon as we run samples through our test suite you can expect a full review, putting Intel's claims to the test. Stay tuned.

A Brand New Architecture & The Old Indirection Table
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  • Impulses - Monday, November 5, 2012 - link

    I'd say Samsung is about on par with Intel if you look at the number of major bugs requiring immediate firmware updates etc. Intel's rep took a bit of a hit when even they couldn't release an entirely bug free Sandforce drive IMO (though it wasn't a surprise).
  • Death666Angel - Tuesday, November 6, 2012 - link

    Not to mention the 8MB bug with their own controller. No product is safe, but Samsung, Intel, Crucial and Plextor seem the safest, with Samsung and Crucial being also very price competitive. But that's just how I see it. :D I have had 2 OCZ drives and not a single problem with either.
  • Taft12 - Monday, November 5, 2012 - link

    Why are you complaining about scenarios that don't exist??

    "But with pricing like the S3700 is featuring, the days of Intel being competitive in the consumer space may be over..."

    "I'd rather see Intel take a two-tiered approach. By all means, keep putting out the enterprise drives for the high margins, but also keep a toe in the consumer market."
    THIS IS EXACTLY WHAT INTEL HAS RIGHT NOW! And no indication that will change. This doesn't just apply to the SSD space, they've had separate consumer and server CPU lines for decades.
  • chrnochime - Monday, November 5, 2012 - link

    Or just pay more for enterprise. Not like it isn't going to keep dropping in price anyway.
  • philipma1957 - Monday, November 5, 2012 - link

    My usage would take 10 or more years to kill this ssd. 800 gb would be pricey but a 400gb for 700 on a sale would be very tempting.
  • MrSpadge - Tuesday, November 6, 2012 - link

    Spending big on a drive with strong endurance, hoping it will last 10 years, doesn't sound like a good idea to me. Reasons:

    - other parts of the SSD may fail rather than NAND wear out
    - performance and price are still developing so rapidly that you probably won't want to use this drive in 5 years anymore anyway
    - see it lke this: if instead of paying 700$ now you'd go for a smaller drive with less endurance at 350$ you can use that 350$ (+interest) to buy a new drive in 5 years (if your SSD is really worn out by then). This one should be way faster and much bigger than the original drive, providing you much better value for the next 5 years. Plus if the old drive still works you could still use it in a less "enthusiastic" configuration.
  • mayankleoboy1 - Monday, November 5, 2012 - link

    So intel is proud that it keeps no user data on the DRAM.
    But what about /sandforce and Marvell controllers ? Do they use DRAM for caching userdata ?
    Is this configurable by the OEM ?
  • Death666Angel - Tuesday, November 6, 2012 - link

    As far as I know, everyone in the consumer space but Intel chaches user data in the DRAM and they aren't dodging that either. For normal consumer use, I don't see why that would be any issue either. If you are worried about that last bit of data integrity, get an enterprise solution or a UPS, which should solve the issue, too. :)
  • kaix2 - Monday, November 5, 2012 - link

    The new controller sounds very promising for all of us who have been waiting for a new Intel controller. I would expect Intel's consumer drives to eventually get the same controller and as far as the price concern, I bet most of the price premium is really from HET-MLC NAND vs regular MLC NAND. Regular consumers don't need 10 drive write per day and the drives should be much cheaper with just regular MLC NAND.
  • cdillon - Monday, November 5, 2012 - link

    Not caching user data writes in DRAM so that you can't lose them when the power goes out is all well and good, but what happens with indirection table updates which will have to happen AT THE SAME TIME and are inextricably linked? Losing an indirection table mapping to new user data that was just written is no less bad than losing the actual user data, because either way you're losing the data.

    Intel has two options here... They can either write indirection table updates directly to NAND at the same time as the user data, or they can cache the indirection table updates only in DRAM and then write them to flash later. Obviously the former is the safest option and I presume this is what Intel is doing, but I've never seen anybody mention how they handle protecting the mapping table updates on any SSD, since they can arguably be MORE important than a little bit of user data due to the risk of losing absolutely everything on the drive if the table gets completely out of whack.

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