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.

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