Estimating the Life of the 850 Pro

As mentioned in previous posts, Samsung rate the whole 850 Pro range at 150TBW, regardless of size. As our disk reaches this point, what can we learn about its health and life expectancy? After 152 test cycles, the SSD’s SMART stats are:

ID# ATTRIBUTE_NAME          VALUE WORST THRESH TYPE      RAW_VALUE
  5 Reallocated_Sector_Ct   100   100   010    Pre-fail  0
  9 Power_On_Hours          099   099   000    Old_age   707
 12 Power_Cycle_Count       099   099   000    Old_age   158
177 Wear_Leveling_Count     097   097   000    Pre-fail  138
179 Used_Rsvd_Blk_Cnt_Tot   100   100   010    Pre-fail  0
181 Program_Fail_Cnt_Total  100   100   010    Old_age   0
182 Erase_Fail_Count_Total  100   100   010    Old_age   0
183 Runtime_Bad_Block       100   100   010    Pre-fail  0
187 Reported_Uncorrect      100   100   000    Old_age   0
190 Airflow_Temperature_Cel 068   060   000    Old_age   32
195 Hardware_ECC_Recovered  200   200   000    Old_age   0
199 UDMA_CRC_Error_Count    100   100   000    Old_age   0
235 Unknown_Attribute       099   099   000    Old_age   26
241 Total_LBAs_Written      099   099   000    Old_age   293504697147

If we start by looking at the total LBAs written: 293504697147 sectors equates to 150.27TBW, just over the warranty threshold. We might reasonably expect the disk to be starting to show some signs of wear at this stage, perhaps with the odd sector fail here and there? But looking through the numbers, not a single error has been reported of any kind. In fact, according to the wear levelling count, we have hardly got started - the SSD still has 97% of its life remaining!

Let's just think about wear levelling for a moment. It's a feature specific to SSDs; in order to ensure the flash wears evenly, the disk controller will try to ensure an equal number of writes to all parts of the memory. It will even move data around within the SSD so that less-used areas are freed up for new data to be stored in. The wear levelling count indicates how many times on average each part of the flash has been erased and written to.

We can use this information to help us estimate the life of the disk, and assess its health. Take a look at the two sets of SMART stats below - the first set is from just after the wear levelling count dropped to 98%, and the second from just after it dropped to 97%:

ID# ATTRIBUTE_NAME          VALUE WORST THRESH TYPE      RAW_VALUE
  5 Reallocated_Sector_Ct   100   100   010    Pre-fail  0
  9 Power_On_Hours          099   099   000    Old_age   542
 12 Power_Cycle_Count       099   099   000    Old_age   158
177 Wear_Leveling_Count     098   098   000    Pre-fail  61
179 Used_Rsvd_Blk_Cnt_Tot   100   100   010    Pre-fail  0
181 Program_Fail_Cnt_Total  100   100   010    Old_age   0
182 Erase_Fail_Count_Total  100   100   010    Old_age   0
183 Runtime_Bad_Block       100   100   010    Pre-fail  0
187 Reported_Uncorrect      100   100   000    Old_age   0
190 Airflow_Temperature_Cel 065   060   000    Old_age   35
195 Hardware_ECC_Recovered  200   200   000    Old_age   0
199 UDMA_CRC_Error_Count    100   100   000    Old_age   0
235 Unknown_Attribute       099   099   000    Old_age   26
241 Total_LBAs_Written      099   099   000    Old_age   131308515644
ID# ATTRIBUTE_NAME          VALUE WORST THRESH TYPE      RAW_VALUE
  5 Reallocated_Sector_Ct   100   100   010    Pre-fail  0
  9 Power_On_Hours          099   099   000    Old_age   673
 12 Power_Cycle_Count       099   099   000    Old_age   158
177 Wear_Leveling_Count     097   097   000    Pre-fail  122
179 Used_Rsvd_Blk_Cnt_Tot   100   100   010    Pre-fail  0
181 Program_Fail_Cnt_Total  100   100   010    Old_age   0
182 Erase_Fail_Count_Total  100   100   010    Old_age   0
183 Runtime_Bad_Block       100   100   010    Pre-fail  0
187 Reported_Uncorrect      100   100   000    Old_age   0
190 Airflow_Temperature_Cel 066   060   000    Old_age   34
195 Hardware_ECC_Recovered  200   200   000    Old_age   0
199 UDMA_CRC_Error_Count    100   100   000    Old_age   0
235 Unknown_Attribute       099   099   000    Old_age   26
241 Total_LBAs_Written      099   099   000    Old_age   260700975068

First of all, let's look at data written. The volume of data required to drop the wear levelling count by 1% is (260700975068 - 131308515644) * 512 (sector size) / (1000 * 1000 * 1000 * 1000) = 66.25TB. That's a lot - a significant proportion of the warranty threshold - and by this measure, the disk is estimating its actual lifespan at >6.5PB!

We can also calculate the number of program / erase cycles that the disk can support by looking at the raw value for the wear levelling count. Here, 1% lifespan equates to (122 - 61) = 61 P/E cycles. So the flash should be able to sustain 6000+ erase cycles. Now that’s going to take quite some time to test!