The drive firmware runs the tests.
The details of the tests can be read in eg www.t13.org/Documents/UploadedDocuments/technical/e01137r0.pdf, which summarises the elements of the short and long tests thus:
an electrical segment wherein the drive tests its own electronics. The particular tests in this segment
are vendor specific, but as examples: this segment might include such tests as a buffer RAM test, a
read/write circuitry test, and/or a test of the read/write head elements.
a seek/servo segment wherein the drive tests it capability to find and servo on data tracks. The
particular methodology used in this test is also vendor specific.
a read/verify scan segment wherein the drive performs read scanning of some portion of the disk
surface. The amount and location of the surface scanned are dependent on the completion time
constraint and are vendor specific.
The criteria for the extended self-test are the same as the short self-test with two exceptions: segment
(3) of the extended self-test shall be a read/verify scan of all of the user data area, and there is no
maximum time limit for the drive to perform the test.
It is safe to perform non-destructive testing while the OS is running, though some performance impact is likely. As the
smartctl man page says for both
-t short and
This command can be given in normal system operation (unless run in captive mode)
If you invoke captive mode with
smartctl assumes the drive can be busied-out to unavailability. This should not be done on a drive the OS is using.
As the man page also suggests, the offline testing (which simply means periodic background testing) is not reliable, and never officially became part of the ATA specifications. I run mine from cron, instead; that way I know when they should happen, and I can stop it if I need to.
- The results can be seen in the
smartctl output. Here's one with a test running:
[root@risby images]# smartctl -a /dev/sdb
smartctl 6.4 2015-06-04 r4109 [x86_64-linux-4.1.6-201.fc22.x86_64] (local build)
Copyright (C) 2002-15, Bruce Allen, Christian Franke, www.smartmontools.org
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% 20567 -
# 2 Extended offline Completed without error 00% 486 -
SMART Selective self-test log data structure revision number 0
Note: revision number not 1 implies that no selective self-test has ever been run
SPAN MIN_LBA MAX_LBA CURRENT_TEST_STATUS
1 0 0 Self_test_in_progress [90% left] (0-65535)
2 0 0 Not_testing
3 0 0 Not_testing
4 0 0 Not_testing
5 0 0 Not_testing
Note two previous completed tests (at 486 and 20567 hours power-on, respectively) and the current running one (10% complete).