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Toshiba's $7000+ 400 GB SSD: SAS 6Gb/s, SLC Flash, And Big Endurance

Endurance: Comparing MLC, eMLC, And SLC

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 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 320Intel SSD 710Toshiba MK4001GRZB
NAND TypeIntel 25 nm MLCIntel 25 nm eMLC (HET)Toshiba 32 nm SLC
RAW NAND Capacity320 GB320 GB512 GB
IDEMA Capacity (User Accessible)300 GB200 GB400 GB
Overprovisioning7%60%28%
P/E Cycles Observed (IDEMA)546036 600225 064
P/E Cycles Observed (Raw)511922 875175 831
Host Writes per 1% of MWI16.38 TB73.20 TB900.2 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).

  • compton
    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.
    Reply
  • nebun
    $7000 any company willing to pay this much for an SSD is fullish
    Reply
  • bennaye
    nebun$7000 any company willing to pay this much for an SSD is fullish
    ...fullish of cash? Definitely. Foolish? Probably not.
    Reply
  • nebun
    bennaye...fullish of cash? Definitely. Foolish? Probably not.damn the english language.....there are way to many words that sound alike
    Reply
  • confish21
    How is this $7000 drive profitable over it's competition again?
    Reply
  • nitrium
    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?
    Reply
  • spazoid
    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! :]
    Reply
  • @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?
    Reply
  • EJ257
    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."
    Reply
  • jaquith
    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.
    Reply