Quantifying SSD endurance

Drive Writes Per Day (DWPD)

Drive Writes Per Day (DWPD) measures how many times you could overwrite the drive’s entire size each day of its life. For example, suppose your drive is 200 GB and its warranty period is 5 years. If its DWPD is 1, that means you can write 200 GB (its size, one time) into it every single day for the next five years.

If you multiply that out, that’s 200 GB per day × 365 days/year × 5 years = 365 TB of cumulative writes before you may need to replace it.

If its DWPD was 10 instead of 1, that would mean you can write 10 × 200 GB = 2 TB (its size, ten times) into it every day. Correspondingly, that’s 3,650 TB = 3.65 PB of cumulative writes over 5 years.

Terabytes Written (TBW)

Terabytes Written (TBW) directly measures how much you can write cumulatively into the drive over its lifetime. Essentially, it just includes the multiplication we did above in the measurement itself.

For example, if your drive is rated for 365 TBW, that means you can write 365 TB into it before you may need to replace it.

If its warranty period is 5 years, that works out to 365 TB ÷ (5 years × 365 days/year) = 200 GB of writes per day. If your drive was 200 GB in size, that’s equivalent to 1 DWPD. Correspondingly, if your drive was rated for 3.65 PBW = 3,650 TBW, that works out to 2 TB of writes per day, or 10 DWPD.

As you can see, if you know the drive’s size and warranty period, you can always get from DWPD to TBW or vice-versa with some simple multiplications or divisions. The two measurements are really very similar.

Summary formulas

DWPD or DW/D - Drive Writes Per Day.
GB/day - Gigabytes written per day.
TBW - Total Bytes Written, usually expressed in terabytes - TBW (TB) or petabytes - TBW (PB).
PBW - Petabytes Written.

Given S - disk capacity in GB and T - warranty / useful life period in years, conversion between different SSD endurance specifications can be performed as follows:

DWPD = GB/day ÷ S
DWPD = (TBW_(TB) × 1000) ÷ (S × T × 365)
GB/day = DWPD × S
GB/day = (TBW_(TB) × 1000) ÷ (T × 365)
TBW_(TB) = (DWPD × S × T × 365) ÷ 1000
TBW_(TB) = (GB/day × T × 365) ÷ 1000
PBW = TBW(PB) = TBW_(TB) ÷ 1000

В Enterprise решениях необходимо учитывать следующие показатели DWPD:

• Большая интенсивность записи, нужно придерживаться >= 10 DWPD
• В смешанном режиме > 1 и < 10 DWPD
• При высоком чтении <= 1 DWPD