Understanding SSD Life

Understanding SSD Life
Zoey

This is to get an idea about the life remaining of the SSD on my laptop to make sure that I am prepared enough if I were to be hosting something that does not have enough backups

So, firstly, we will look at the SMART data that the drive spits out:

$ sudo smartctl -a /dev/sda
smartctl 7.2 2020-12-30 r5155 [x86_64-linux-5.14.15-arch1-1] (local build)
Copyright (C) 2002-20, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Model Family:     Samsung based SSDs
Device Model:     Samsung SSD 860 EVO 250GB
Serial Number:    SXXXXXXXXXXXXXL
LU WWN Device Id: 5 0xxxx8 exxxxxxxc
Firmware Version: RVT02B6Q
User Capacity:    250,059,350,016 bytes [250 GB]
Sector Size:      512 bytes logical/physical
Rotation Rate:    Solid State Device
Form Factor:      2.5 inches
TRIM Command:     Available, deterministic, zeroed
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   ACS-4 T13/BSR INCITS 529 revision 5
SATA Version is:  SATA 3.1, 6.0 Gb/s (current: 6.0 Gb/s)
Local Time is:    Sat Nov  6 21:41:05 2021 IST
SMART support is: Available - device has SMART capability.
SMART support is: Enabled

=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED

General SMART Values:
Offline data collection status:  (0x00)	Offline data collection activity
					was never started.
					Auto Offline Data Collection: Disabled.
Self-test execution status:      (   0)	The previous self-test routine completed
					without error or no self-test has ever 
					been run.
Total time to complete Offline 
data collection: 		(    0) seconds.
Offline data collection
capabilities: 			 (0x53) SMART execute Offline immediate.
					Auto Offline data collection on/off support.
					Suspend Offline collection upon new
					command.
					No Offline surface scan supported.
					Self-test supported.
					No Conveyance Self-test supported.
					Selective Self-test supported.
SMART capabilities:            (0x0003)	Saves SMART data before entering
					power-saving mode.
					Supports SMART auto save timer.
Error logging capability:        (0x01)	Error logging supported.
					General Purpose Logging supported.
Short self-test routine 
recommended polling time: 	 (   2) minutes.
Extended self-test routine
recommended polling time: 	 (  85) minutes.
SCT capabilities: 	       (0x003d)	SCT Status supported.
					SCT Error Recovery Control supported.
					SCT Feature Control supported.
					SCT Data Table supported.

SMART Attributes Data Structure revision number: 1
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  5 Reallocated_Sector_Ct   0x0033   100   100   010    Pre-fail  Always       -       0
  9 Power_On_Hours          0x0032   097   097   000    Old_age   Always       -       14016
 12 Power_Cycle_Count       0x0032   099   099   000    Old_age   Always       -       691
177 Wear_Leveling_Count     0x0013   098   098   000    Pre-fail  Always       -       34
179 Used_Rsvd_Blk_Cnt_Tot   0x0013   100   100   010    Pre-fail  Always       -       0
181 Program_Fail_Cnt_Total  0x0032   100   100   010    Old_age   Always       -       0
182 Erase_Fail_Count_Total  0x0032   100   100   010    Old_age   Always       -       0
183 Runtime_Bad_Block       0x0013   100   100   010    Pre-fail  Always       -       0
187 Uncorrectable_Error_Cnt 0x0032   100   100   000    Old_age   Always       -       0
190 Airflow_Temperature_Cel 0x0032   061   038   000    Old_age   Always       -       39
195 ECC_Error_Rate          0x001a   200   200   000    Old_age   Always       -       0
199 CRC_Error_Count         0x003e   100   100   000    Old_age   Always       -       0
235 POR_Recovery_Count      0x0012   099   099   000    Old_age   Always       -       623
241 Total_LBAs_Written      0x0032   099   099   000    Old_age   Always       -       9752441688

SMART Error Log Version: 1
No Errors Logged

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Extended offline    Completed without error       00%        69         -
# 2  Short offline       Completed without error       00%        67         -

SMART Selective self-test log data structure revision number 1
 SPAN  MIN_LBA  MAX_LBA  CURRENT_TEST_STATUS
    1        0        0  Not_testing
    2        0        0  Not_testing
    3        0        0  Not_testing
    4        0        0  Not_testing
    5        0        0  Not_testing
Selective self-test flags (0x0):
  After scanning selected spans, do NOT read-scan remainder of disk.
If Selective self-test is pending on power-up, resume after 0 minute delay.

The important things to note in this are:

Understanding Wear Leveling

ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
177 Wear_Leveling_Count     0x0013   098   098   000    Pre-fail  Always       -       34

Every Cell in an SSD is rated for a limited number of write/erase cycles. This value tracks the average number of writes/erase cycles across the SSD.

The value will start at 100 and will slowly decrease to 0. The RAW_VALUE is the real count or write/erase cycles.

We can see in the above data that the present VALUE is at 98, which means we have used up 2% of the max cycles.

We can estimate the maximum number of cycles from this by assuming the RAW_VALUE corresponds to 2%.

2   corresponds to 34
100 corresponds to x
x = 34 / 2 * 100 = 1700

This assumes a linear relationship between the formula used to calculate Wear_Leveling_Count and RAW_VALUE which might not always be the case.
Ref:[2]

Understanding TBW

ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
241 Total_LBAs_Written      0x0032   099   099   000    Old_age   Always       -       9752441688

TBW (Tera Bytes Written) is another metric that can be used to gauge the health of your SSD by comparing it with what the manufacturer warrants it against.

For this drive, from [1], we can easily find that Samsung warrants this drive for 5 Years or 150 TBW. What that means that, if you have written 150TB to the drive, even if you might have done it in under 5 years, the drive will not be covered under warranty.

To make sense of that number, let us first understand where our drive stands based on the Total_LBAs_Written value.

All SSD operations happen on a block of fixed size. For our SSD, the block size is also output as part of the SMART data

Sector Size:      512 bytes logical/physical

So, each block equals 512 bytes. Which means 2 blocks equal 1kB. With this info, it will be easy to find the amount of bytes that have been written to our drive by simple math

9752441688 blocks = 9752441688 / 2 kB = 9752441688 / 2 / 1024 MB =  9752441688 / 2 / 1024 / 1024 GB
9752441688 / 2 / 1024 / 1024 GB = 9752441688 / 2 / 1024 / 1024 / 1024 TB = 4.54 TB

So, we have written 4.54 TB of data to our drive. Can we check if this data also aligns with the Wear_Leveling_Count attribute that we checked in the previous section?

150 TB correspond to 100
4.54 TB correspond to x
x = 4.54 / 150 * 100
x = 3.03

So, we are pretty close to the 2% that is reported by Wear_Leveling_Count and the 1% can easily be due to rounding errors or other tolerances in the formula being used internally to calculate Wear_Leveling_Count

References

[1] https://www.samsung.com/semiconductor/minisite/ssd/product/consumer/860evo/

[2] https://superuser.com/questions/1037644/samsung-ssd-wear-leveling-count-meaning