Comparing DWPD to TBW
A 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.
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
Danuiel, you make a very interesting point – that Intel has come up with a third way of specifying the same thing.
While most SSD makers will specify TBW (as Intel does in its DC S3500 datasheet*) or DWPD (as Intel does in its DC S3700 datasheet**), the calculation in the Intel brochure*** you mention calculates GB of writes per day, rather than “Drive” writes per day (DWPD).
Most SSD makers that don’t specify TBW will specify DWPD, which means the entire drive, in which case the capacity is a key part of the specification.
You also are correct to say that Intel uses 1,000 in its calculation, but some companies choose 1,024. HDD makers gravitate towards using 1,000, and many SSD makers use 1,024, but this appears not to be the case with Intel, at least in the brochure you cited.
Thanks again,
Jim
* Intel DC S3500 Datasheet: http://www.intel.com/content/dam/www/public/us/en/documents/technology-briefs/data-center-class-solid-state-drive-brief.pdf
**Intel DC S3700 Datasheet: https://www-ssl.intel.com/content/dam/www/public/us/en/documents/product-specifications/ssd-dc-s3700-spec.pdf
*** Intel brochure: Why Choose a Data Center Class SSD?: http://www.intel.com/content/dam/www/public/us/en/documents/technology-briefs/data-center-class-solid-state-drive-brief.pdf
Pingback: = = 1 0 8 1 0 0 9 = = | Seagate เปิดตัว Nytro SSD จุสูงสุด 2TB เจาะกลุ่มตลาดเอนเทอร์ไพรส์ | ร้อยแปดพันเก้า.com 1081009