How to Monitor Disk Health with smartd and smartctl (S.M.A.R.T.)

Updated: 28 September 2026 · Applies to: smartmontools 7.2 to 7.4 on AlmaLinux and Rocky Linux 9 and 10, RHEL 9 and 10, Ubuntu 24.04 and 26.04, Debian 13 (dedicated servers with SATA, SAS or NVMe drives)

Hard disks and SSDs track their own health with S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology). The smartd service from the smartmontools package reads these values regularly, runs self-tests, and warns you before a failing drive loses data. It needs a physical drive, so it is for dedicated servers: a VPS disk is virtual and has no S.M.A.R.T. data.

1. Install smartmontools

dnf install smartmontools     # AlmaLinux, Rocky Linux, RHEL
apt install smartmontools     # Ubuntu, Debian

2. Check a drive by hand

1. List the drives: smartctl --scan
2. Show the drive and whether S.M.A.R.T. is available and enabled:

   smartctl -i /dev/sda

   If it says "SMART support is: Disabled", switch it on with smartctl -s on /dev/sda.
3. Overall health: smartctl -H /dev/sda. "PASSED" is good, but read the attributes too, because a drive can pass and still be failing.
4. All values: smartctl -a /dev/sda (for NVMe drives, smartctl -a /dev/nvme0).

Values to watch on SATA and SAS drives: Reallocated_Sector_Ct, Current_Pending_Sector and Offline_Uncorrectable should be 0; if they grow, plan a replacement. A rising UDMA_CRC_Error_Count usually points to a cable problem. On NVMe drives, check Critical Warning (should be 0x00), Percentage Used (wear) and Media and Data Integrity Errors.

3. Run a self-test

smartctl -t short /dev/sda     # about 2 minutes
smartctl -t long /dev/sda      # an hour or more on large disks
smartctl -l selftest /dev/sda  # results

Tests run inside the drive while the server keeps working.

4. Configure automatic monitoring

1. Open the configuration file: /etc/smartmontools/smartd.conf on AlmaLinux, Rocky Linux and RHEL, or /etc/smartd.conf on Ubuntu and Debian.
2. Comment out the existing DEVICESCAN line with # and add:

   DEVICESCAN -a -o on -S on -n standby,q -s (S/../.././02|L/../../6/03) -W 4,45,55 -m admin@example.com

3. Restart the service and enable it at boot:

   systemctl enable --now smartd
   systemctl restart smartd

   On Ubuntu and Debian, smartd is another name for the smartmontools service, so the same commands work.
4. Check that it found your drives: journalctl -u smartd -b

What the line does: DEVICESCAN monitors every drive found; -a checks health, attributes and error logs; -o on and -S on switch on the drive's background data collection; -n standby,q skips drives that are asleep; -s runs a short test every day at 02:00 and a long test every Saturday at 03:00; -W 4,45,55 reports temperature changes of 4 °C and warns at 45 °C and 55 °C; -m is where warnings are emailed.

5. Make sure warnings reach you

smartd sends email with the system's mail command, so the server needs a working mail setup (for example the s-nail package and a mail relay). Test it by adding -M test to the line and restarting smartd: you should get a test message for each drive. Remove -M test afterwards.

Common problems

• "Unable to detect device type" or no drives found: the drives sit behind a hardware RAID controller. Name them with the controller type, for example /dev/sda -d megaraid,0 -a -m admin@example.com, one line per drive.
• No email arrives: test the mail command itself with echo test | mail -s test admin@example.com.
• On a VPS: smartctl reports no S.M.A.R.T. support. The host's drives are monitored by the provider.

On a Ucartz dedicated server, if smartctl shows reallocated or pending sectors, open a support ticket with the smartctl -a output so the drive can be checked.

Official documentation: smartmontools project (smartctl and smartd.conf manual pages).