Smartctl

This guide is the most complete I've seen on how to interpret SMART HDD info, by the author of the tool: https://www.linuxjournal.com/article/6983?page=0%2C1

Documentation

wikipedia SMART attributes

https://en.wikipedia.org/wiki/S.M.A.R.T.#ATA_S.M.A.R.T._attributes

Tutorials

Watching a hard drive die (in smartctl)	http://notemagnet.blogspot.com/2009/10/watching-hard-drive-die.html
Dan Langville monitoring HDD health	https://www.freebsddiary.org/smart.php

Install

sudo pacman -S smartmontools    # ArchLinux
sudo pkg install smartmontools  # FreeBSD

Usage

TL;DR - familiarize yourself with your drive's smartctl info early,
determine whether to use value/raw,
and create monitors specific to each drive.

If error rates are suspicious, view the smartctl error logs.

Types of Info

Error Logs

sudo smartctl -l error /dev/sda  # list logs of type 'error' (see man page to help read result)

...
Error 2 occurred at disk power-on lifetime: 153 hours (6 days + 9 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  84 51 00 00 00 00 a0

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  ec 00 00 00 00 00 a0 08      00:00:17.300  IDENTIFY DEVICE

Error 1 occurred at disk power-on lifetime: 154 hours (6 days + 10 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  40 51 01 01 00 00 a0  Error: UNC 1 sectors at LBA = 0x00000001 = 1

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  25 03 01 01 00 00 a0 ff      11:17:17.000  READ DMA EXT
  25 03 01 01 00 00 a0 ff      11:17:17.000  READ DMA EXT
  25 03 30 5e 00 d4 48 04      11:17:05.800  READ DMA EXT
  25 03 40 4f 00 d4 40 00      11:16:56.600  READ DMA EXT
  35 03 08 4f 00 9c 40 00      11:16:56.600  WRITE DMA EXT

Auto Health Assessment (not always accurate)

sudo smartctl -H /dev/sda        # overall health assessment

SMART overall-health self-assessment test result: PASSED

SmartCtl Attributes and values

sudo smartctl -a /dev/sda        # list smartctl values

• check raw_value of Reallocated_Sector_Ct and Reallocated_Event_Count If you see a value over 0, the drive is starting to die.
• check Power_On_Hours to verify your error log entries pertain to the current drive issues.
• check Total_LBAs_written on SSDs. SSDs have a max number of writes. LBAs may refer to sectors, or for example with INTEL, raw_value is incremented by 1 every 32MB. Generally, refers to logical sector size. Find your sector-size using fdisk -l. http://www.dewassoc.com/kbase/hard_drives/lba.htm

...
SMART Attributes Data Structure revision number: 16
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x000b   092   092   016    Pre-fail  Always       -       2621443
  2 Throughput_Performance  0x0005   100   100   050    Pre-fail  Offline      -       0
  3 Spin_Up_Time            0x0007   111   111   024    Pre-fail  Always       -       600 (Average 660)
  4 Start_Stop_Count        0x0012   100   100   000    Old_age   Always       -       74
  5 Reallocated_Sector_Ct   0x0033   100   100   005    Pre-fail  Always       -       7
  7 Seek_Error_Rate         0x000b   100   100   067    Pre-fail  Always       -       0
  8 Seek_Time_Performance   0x0005   100   100   020    Pre-fail  Offline      -       0
  9 Power_On_Hours          0x0012   100   100   000    Old_age   Always       -       154
 10 Spin_Retry_Count        0x0013   100   100   060    Pre-fail  Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   000    Old_age   Always       -       69
192 Power-Off_Retract_Count 0x0032   100   100   050    Old_age   Always       -       77
193 Load_Cycle_Count        0x0012   100   100   050    Old_age   Always       -       77
194 Temperature_Celsius     0x0002   114   114   000    Old_age   Always       -       48 (Lifetime Min/Max 18/56)
196 Reallocated_Event_Count 0x0032   100   100   000    Old_age   Always       -       12
197 Current_Pending_Sector  0x0022   100   100   000    Old_age   Always       -       4
198 Offline_Uncorrectable   0x0008   100   100   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x000a   200   253   000    Old_age   Always       -       4
...

Self Testing

sudo smartctl -t short /dev/sda   # run a short test
sudo smartctl -t long /dev/sda    # run a long test

sudo smartctl -l selftest /dev/sda  # print self test report

JSON Format

# list smart attributes as JSON
sudo smartctl -a /dev/ada2 -j \
  | jq '.ata_smart_attributes.table[] |  with_entries(select(.key | in({"id": 1, "name": 1, "raw": 1, "value": 1})))'

Misc

Logical Block Address Size (LBA)

LBAs are an abstraction overtop of Sectors.
You can for example have a 512b sector-size on your disk,
but your driver may abstract this as 1024b LBAs.

Since SSD drive wear is often measured in TBW, but writes are measured in LBAs it is important to be able to convert between them.
Obtain using smartctl

sudo systemctl -a /dev/ada0 | grep "Sector Size"
# Sector Size:    512 bytes logical/physical
#
# this means:
#   Sectors: 512bytes  (physical)
#   LBAs:    512bytes  (logical)

Operating Temperatures

Most HDDs can safely operate between 0-70*C,
but higher temperatures wear faster.

According to harddrivegeek, 30-50*C is ideal.

facebook study

https://www.digitaltrends.com/computing/facebook-study-finds-no-correlation-between-ssd-reads-and-failure-rate/