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Endurance: Comparing MLC, eMLC, And SLC

Toshiba's $7000+ 400 GB SSD: SAS 6Gb/s, SLC Flash, And Big Endurance
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Endurance is a term thrown around a lot in discussions of solid-state storage because we all worry about that point where an SSD is no longer able to reliably store our data. If you have an SSD in your notebook or mainstream desktop, endurance shouldn't be much of a concern. It's unlikely that you'll ever write enough data per day, every day, to exhaust the useable life of the NAND flash cells that make up your drive. Far more likely is a firmware-related issue that results in problematic operation. But even those are fairly rare.

IMFT's 25 nm NANDIMFT's 25 nm NAND

Endurance is a much more important discussion in the enterprise world, though. Demanding workloads force many machines to read or write data continuously, day in and day out. On a conventional hard drive, other issues contribute to eventual failures. But when it comes to SSDs, those business-oriented tasks gradually chip away at the rated number of program/erase cycles that each NAND vendor affixes to its memory products. Because eMLC and SLC flash offer the highest endurance ratings, they're particularly attractive for enterprise-oriented products.

That's not to say multi-level cell NAND is out of place in professional applications. Based on our discussions with data center managers, we know there are plenty of original X25-M and SSD 320s used in mission-critical environments. They are used in such a way that a failure won't result in data loss, though, and they aren't bombarded with writes in the same way one of these Toshiba drives might be.

Evaluating SSD Endurance

Before we take a stab at quantifying the endurance of different flash technologies, we want to discuss our methodology. Our estimates come from monitoring each drive's media wear indicator (referred to as the MWI), which counts down from 100 to 1. Because the number of program-erase cycles a NAND cell can withstand is finite, the MWI is designed to facilitate a rough estimate of endurance.

In theory, once you reach the end of the counter, all of the memory's rated P/E cycles are exhausted. That's not to say something bad happens when you hit the bottom, but nobody wants to entrust irreplaceable data to a drive living on borrowed time, either. Naturally, enterprise customers place a lot of importance into the MWI, then, because it represents “the safe zone.”

Endurance Rating (Sequential Workload, QD=1, 2 MB)
Intel SSD 320
Intel SSD 710
Toshiba MK4001GRZB
NAND Type
Intel 25 nm MLC
Intel 25 nm eMLC (HET)
Toshiba 32 nm SLC
RAW NAND Capacity
320 GB
320 GB512 GB
IDEMA Capacity (User Accessible)
300 GB
200 GB
400 GB
Overprovisioning
7%
60%
28%
P/E Cycles Observed (IDEMA)
5460
36 600
225 064
P/E Cycles Observed (Raw)
511922 875175 831
Host Writes per 1% of MWI
16.38 TB
73.20 TB
900.2 TB


According to Toshiba's spec sheet, the 100 GB MK100GRZB comes with an endurance rating of 8.2 PB. Each vendor uses its own method of estimating longevity, which is why it’s difficult to compare endurance across different SSD brands and models. Our numbers assume a purely sequential workload, which means we’re ignoring random access. However, this allows us to take a step back and look at SSD and NAND endurance academically.

Look at the numbers. It’s really clear to see why SLC flash remains the crème of the crop. While it continues to fetch a high premium, SLC is also capable of withstanding many more writes than MLC technology. If you remove the effects of overprovisioning, the Toshiba’s SLC NAND has a rating close to 175 000 P/E cycles. That’s 58 times higher than Intel’s 25 nm MLC NAND, which clocks in at ~5000 P/E cycles.

Remember that P/E-cycle ratings apply to each flash cell. But because larger SSDs employ more NAND (and consequently, a lot more flash cells), it takes longer to write across all of them. As a result, larger drives enjoy a higher endurance rating. If we do the math, our 400 GB MK4001GRZB should be capable of writing 88 PB of data sequentially. That’s insanely high. And perhaps it explains why Toshiba doesn’t provide endurance ratings on its higher-capacity SSDs. Instead, the 200 GB and 400 GB models come with a guarantee that you won’t have to worry about endurance during the company's five-year warranty period (a telling promise, indeed).

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Top Comments
  • 23 Hide
    bennaye , February 24, 2012 4:41 AM
    nebun$7000 any company willing to pay this much for an SSD is fullish


    ...fullish of cash? Definitely. Foolish? Probably not.
  • 15 Hide
    spazoid , February 24, 2012 9:36 AM
    amdfreakIt is too expensive for the performance it offers. You can get a RAID array of many Intel SSDs beating Toshiba in every segment.


    You've clearly not understood the purpose of this article. Stick to commenting the desktop drive reviews in the future, please.


    Thank you for this review, and especially your estimations on the endurance of the drive. It's something that's damn near impossible for us IT professionals to get accurate estimations of in the real world. For some reason, bosses tend to want the expensive hardware to be put to use instead of being thoroughly tested.

    More of these types of articles please! :]
Other Comments
  • 9 Hide
    compton , February 24, 2012 4:09 AM
    Good job, Mr. Ku.

    Perhaps the Enterprise SSD Fairy will bring you a Hitatchi UltraStar with Intel's 6gbps controller. I'd be eager to see how it compares.

    There is no substitute for SLC though.
  • 23 Hide
    bennaye , February 24, 2012 4:41 AM
    nebun$7000 any company willing to pay this much for an SSD is fullish


    ...fullish of cash? Definitely. Foolish? Probably not.
  • 6 Hide
    nebun , February 24, 2012 5:57 AM
    bennaye...fullish of cash? Definitely. Foolish? Probably not.

    damn the english language.....there are way to many words that sound alike
  • -7 Hide
    confish21 , February 24, 2012 6:00 AM
    How is this $7000 drive profitable over it's competition again?
  • -4 Hide
    nitrium , February 24, 2012 8:03 AM
    Why is the 4KB Random read/write performance shown as IOPS, but 128KB and 2MB performance is in MB/sec? What speed (in MB/sec) does this drive achieve in 4KB? I guess I could calculate it from (IOPS * 4KB) / 1024 (I think that's right), but why should I have to?
  • 15 Hide
    spazoid , February 24, 2012 9:36 AM
    amdfreakIt is too expensive for the performance it offers. You can get a RAID array of many Intel SSDs beating Toshiba in every segment.


    You've clearly not understood the purpose of this article. Stick to commenting the desktop drive reviews in the future, please.


    Thank you for this review, and especially your estimations on the endurance of the drive. It's something that's damn near impossible for us IT professionals to get accurate estimations of in the real world. For some reason, bosses tend to want the expensive hardware to be put to use instead of being thoroughly tested.

    More of these types of articles please! :]
  • -4 Hide
    Anonymous , February 24, 2012 10:57 AM
    @spazoid, so you are telling me that you are willing to pay 10x for an endurance of 3x over the INTEL 520 SSD?
    Even when the INTEL SSD already has an endurance longer than your refresh cycle for your tech stack?
  • 6 Hide
    EJ257 , February 24, 2012 2:17 PM
    frozonicLOL, i can just imagine myself in ten years telling my kids that we had to pay 7000$ for a 400gb ssd...by that time we are gonna have 400+ TB ssds


    "Back in my days storage drives used to have moving parts. Now its all solid state."
  • 2 Hide
    jaquith , February 24, 2012 2:50 PM
    I own a small data center and thankfully have access to a 'major' financial institutions test data, and I agree with your conclusions especially regarding deployment into production. $7K SSD is a tough call with a 5-year, but if it were 7~10-year then probably an easy call.

    Unlike super-sized enterprise which I am not, the cost/benefit calculations would be difficult for myself. I know firsthand the money that i.e. financial institutions push into their data centers, and for those folks $7K isn't out of the question.

    Interesting SSD and if the prices come down and warranty extended then IMO it would be something to consider and compare against Intel's products.
  • 8 Hide
    willard , February 24, 2012 3:12 PM
    I came into this article expecting people to bitch about prices, compare to consumer products and just misunderstand enterprise class hardware in general.

    I was not disappointed.
  • 3 Hide
    therabiddeer , February 24, 2012 3:36 PM
    nebun$7000 any company willing to pay this much for an SSD is fullish

    I refer you to the ~$20,000 1.2TB fusion-io SSD's.
  • 2 Hide
    Anonymous , February 24, 2012 4:47 PM
    I've got one of those $20,000 fusion IO drives... and it stomps all over my $130,000 storage san...
  • -2 Hide
    andywork78 , February 24, 2012 5:30 PM
    Good review and test.

    but wow... $7000...

    I go with 10 of 128GB SSD....
  • 6 Hide
    nforce4max , February 24, 2012 5:31 PM
    nebun$7000 any company willing to pay this much for an SSD is fullish


    Hell I'll gladly pay that much because drives like this save money in the long run. They are cheaper and much easier to set up and maintain vs hundred of mechanical drives in a raid setup. In power alone over the live of the drive vs mechanical drives adds up. So $7k isn't that bad and this isn't the most expensive SSD that I have seen.
  • 6 Hide
    holyprof , February 24, 2012 7:26 PM
    amdfreakIt is too expensive for the performance it offers. You can get a RAID array of many Intel SSDs beating Toshiba in every segment.


    Throw 50TB daily writes on that Intel SDD array of yours and it will last you only 3 months until full failure.
  • 9 Hide
    A Bad Day , February 24, 2012 8:01 PM
    nebun$7000 any company willing to pay this much for an SSD is fullish


    "Hey uh, our entire rack of $50 SSDs simply died on us, along with all of our business files."
  • -3 Hide
    garciam , February 24, 2012 9:05 PM
    Anyone thinking this can last longer than a few SSD's raided obviously does not know *** about how NAND works and how much it lasts.
    Throw 3 Intel MLC 480 GB SSD's in RAID-5 (1k each), make an agressive overprovisioning...and they will both last MUCH longer and also run circles to this expensive piece of hardware being reviewed.

    Heck, it's pretty much touching Fusion-IO pricing without even coming close on speed.

    This will only work for people needing plug & play replacement for their SAS drives AND with very deep pockets. Since i suspect the replacement should be made in batches...it will be VERY expensive.

    Anyone else with brains can find a lot of cheaper, faster AND more reliable solutions.

    I'd wait for a Velodrive, raid a couple of them and just have regular backups on a storage with regular HDD's (that is, read GB/s from a couple SSD's...write GB/s sequentially to a storage).

    I do understand though that there are out there companies that can't risk innovation and smart choices and have to recur to handwritten promises and warranties of the big guys.

    Reason why buying a Dell costs a hell lot more than building it yourself.
    Reason why building your own storage is a fraction of the price of an EMC solution.

    And so on...
  • 1 Hide
    Reynod , February 25, 2012 12:57 AM
    Anybody checked to see if it is worth it's weight in gold or platinum ?


    For $7000 that is the first thing I would have done Andrew.

    :) 
  • 2 Hide
    peevee , February 25, 2012 9:45 AM
    EJ257"Back in my days storage drives used to have moving parts. Now its all solid state."


    "Why are they called drives, granpa?"
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