Comparing DWPD to TBW

TBW-DWPDA couple of specifications for SSD endurance are in common use today: Terabytes Written (TBW) and Drive Writes Per Day (DWPD).  Both are different ways to express the same thing.  It seems that one vendor will specify endurance using TBW, while another will specify DWPD.  How do you compare the two?

First, some definitions.  “Terabytes Written” is the total amount of data that can be written into an SSD before it is likely to fail.  “Drive Writes Per Day” tells how many times you can overwrite the entire capacity of the SSD every single day of its usable life without failure during the warranty period.  Since both of these are guaranteed specifications, then your drive is most likely to last a lot longer than the number given by the SSD’s maker.

To convert between the two you must know the disk’s capacity and the warranty period.  If  drive maker gives you TBW but you want to know DWPD you would approach it this way:

TBW = DWPD * Warranty * 365 * Capacity/1,024

The constants are simply to convert years to days (365) and gigabytes to terabytes (1,024).  Some might argue that this number should be 1,000, and that may be correct, but the difference between the two is only 2.4%, and The SSD Guy highly doubts that you are planning resources so tightly that this will matter.

If you want to go the other way, and convert TBW to DWPD, you would use this formula:

DWPD = TBW * 1024/(Capacity * Warranty * 365)

Why are there two different specifications?  The TBW specification doesn’t really specify how long the drive will last in years.  An SSD with a TBW specification will fail either when it has exceeded its TBW goal or after its warranty period ends, whichever comes first.  The DWPD specification intertwines the number of writes with the warranty period in a way that should cause both to occur at the same time.  All in all, it’s just a matter of preference.  There is no one standard way that endurance is specified.

Before going to all this trouble, though, I would suggest for you to review the SMART attributes on an SSD that has been used in this application or a similar one for a number of months.  You are likely to find that that wear is so much smaller than the drive’s specification that you will never come close to exceeding the TBW or DWPD specifications.  If that’s the case you need not worry much about the SSD you select for this application.  On the other hand, if you are close to either limit then you would do well to choose an SSD that can handle your write requirements with room to spare.

16 Responses to Comparing DWPD to TBW

  • Danuiel says:

    This is not correct. Intel specifies 45TBW on it’s ARC page for the 80GB DC S3500. In their brochure “Why Choose a Data Center Class SSD” they state that this SSD is rated for 24.6 GB host writes per day (DWPD)
    45 * 1000 / 365 / 5 = 24,6575
    45 * 1024 / 365 / 5 = 25,2493 (so, it’s not 1024, “…Some might argue that this number should be 1,000…”)
    Nowhere in this calculation does the capacity come into play, as it is already considered in the specified TBW / DWPD

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  • rawraj says:

    Wow this is an old post but comes up first on google congratulations.
    I started googling as I was checking out warranty on drives and Samsung 750 evo has a 3 year warranty or 70TBW
    Which ever comes first.

    70 TBW not per day. So I decided to calculate its max live. That is if I kept writing at the 560Mbytes per second the fastest write speed of the drive.
    How soon will i write 70TBW
    My calculations showed 1.5 days!!!!
    Is my calculation wrong/

    I took 70TBW and converted it to 75161928 Megabytes. Divided it by 560 and that gave me how many seconds the drive would last. Divided it by 60 to get minutes, Then divided that by 60 to get hours and divided that by 24 to get number of days and I landed up with 1.5xxxx days!!!
    Is it true. I know it takes only 7 minutes to fill a 240 drive and yes using it as an operating system might not write that much of data per day.
    But I just wanted to know what is the actual life. And it seems that SSDs can never be used for incrememental backup of huge bandwidths of data. Nor can it be used as a download disk for an application like utorrent.
    But 1.5 days is too less.
    So if the drive is not writing 560mbps per second for every second of the day how much do you think it would write?
    Even for the drive to last 3 years of warranty how less do we have to use it.
    I think it would be an interesting article for you to write Please do so
    Please let me know your thoughts on this through comments first please.
    Thanks

    • Jim Handy says:

      Rawraj, Thanks for the congratulations and for a very insightful comment. Yes, the SSD would only last a day and a half at 560MB/s of constant writes, but it’s not as bad as this makes it seem.

      The 750 EVO SSD is not intended for high write workloads like you mention. It’s for general-purpose PC use. I have been surprised to hear from PC users who review their SMART attributes that they have discovered that, over the course of a full year, their SSDs have an average of fewer than ten writes per block. Since these blocks have endurances of 300-3,000 erase/write cycles, then just about ANY SSD should provide satisfactory endurance in a PC.

      It would take four complete overwrites of a 250GB SSD to make a terabyte, so the 70TB of endurance is 4*70 = 280 full-SSD writes. This is very close to the endurance rating that I gave above of 300 erase/writes, so the numbers agree.

      I would not expect for anyone to want to use a client SSD this way. Still, it’s an interesting way to look at the numbers.

  • Paul Simek says:

    Jim Handy says:
    December 29, 2016 at 5:12 pm
    Rawraj, Thanks for the congratulations and for a very insightful comment. You are correct in your concern, but you math is in error.

    One terabyte is 1,000 gigabytes to some people, and 1,024 to others. Let’s not worry about small differences like that. The highest write bandwidth of 560GB/s is then about one terabyte every two seconds. Since the drive is rated at 70TBW, and since it only takes two seconds to write a terabyte, then you should reach the guaranteed endurance in 70*2 = 140 seconds. (I double-checked the Samsung site which showed 520MB/s, but that’s not much different. The 70TBW specification was for the 250GB drive. See http://www.samsung.com/semiconductor/minisite/ssd/product/consumer/750evo.html).

    As far as I know there is 1000x mistake! The highest write bandwidth of 560GB/s is actually 560 MB/s(or 520 MB/s). That means in 2 seconds you can write only around 1 GB.
    Paul.

  • J Rad says:

    VERY FEW users write more than 5GB DWPD, and the Intel Devel forum presentation PPT indicates the top 1% of users perform 50GB writes in a day. 5GB _per_day_ (written) is just under 2TBW _per_year_. The *burst data* may be peaking at 500+ MB/s per sec but that is atypical /non-sustained in typical HDD nor SSD use. It is doubtful that a typical user could exceed 150TBW in 10 years (Samsung V-NAND warranty), and if you hit this level, you have likely deployed the drive in HyperV Virtualization server ; in a recent look at SMART in one of the busiest Alarm collection servers at a Mexican CLARO Telco server, I observed the equivalent of 10 TBW in all of 2015.

    • Jim Handy says:

      J Rad.

      You are very right. PC users I have spoken with have looked at their SMART attributes and found that their writes are far below what they expected.

      About three years ago everyone was obsessing about the maximum number of drive writes. Today it appears that more sophisticated users understand their workloads enough to purchase SSDs with lower DWPD since they know that they don’t need the highest number.

      Jim

      • Timbo says:

        I think we should preface this with regard to VM users. In my experience with VM PC clients on Macs (ie unix Darwin kernel) running OS X as host, hard drives and SSDs are hit fairly hard. If you have a mission critical application and might lose lots of money with downtime, it’s better to go with the more reliable SSDs and harddrives in situations like that. For the prosumer, that means that generally Samsung Pro series drives are the best way to go.

        Similarly, if you are going to use an SSD as a constantly and heavily used backup/archiving drive, you want to get one with waaay better than 70 TBW rated SSDs.

        By RAID 5, 5+1, or 6-ing SSDs, you can not only increase throughput performance while lessening the chance of a sudden loss of operations. Thus, in a RAID 5 configuration of 3 SSDS, each with a TBW 70, the effective TBW is approximately 200TBW. Note also that this is still less than a single SSD with a 300 TBW rating (eg modern Samsung Pros in the larger sizes). Thus, you could set up a RAID 5+1 configuration and have a fairly reliable system and great performance. (And assuming that you weren’t trying to always max out write to drive per day, etc…since, yeah, even this setup would only give you approximately 5 days of reliable drive use under constant max write if the TBWs are accurate, as per rawraj’s theory above!)

        • Jim Handy says:

          Timbo

          Thanks for a very useful comment from “Down in the trenches”. I am sure that a lot of folks will find it extremely useful.

          Since I don’t have experience with VM PC clients I find this to be more than puzzling. I would imagine that the client would have extremely little local write traffic, with everything going to the server. Since PCs without virtualization already have very light write loads it would make sense for virtualized PCs to have even less!

          Can you shed any light on why the opposite is true?

          Thanks again,

          Jim

        • Bryan DeLaney says:

          I would suggest you go RAID 1 using two Micron 5100 MAX Drives. They are not cheap, a 960GB SATA runs around $550 each. However, the endurance rating is 8,300 TBW or 8.3 PBW. These are true Enterprise drives with a 5 year warranty.
          Mfg. Part: MTFDDAK960TCC-1AR1ZABYY CDW sells them and so does Amazon. You could also probably use the 5100 PRO which is the mixed use version, or the 5100 ECO which are the read intensive version which are only 800 TBW.

          There are three general categories of Enterprise drives. Read intensive, Mixed Use, and Write Intensive.

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