Despite its name offline is a short foreground test of less than 2 minutes that all devices should support. The default self test makes no entry into the self test log. Most devices perform a default self test when they are being powered up.
Note that the SMART automatic offline test command is listed as Obsolete in every version of the ATA and ATA/ATAPI Specifications. It was originally part of the SFF-8035i Revision 2.0 specification, but was never part of any ATA specification. However it is implemented and used by many vendors. (1)
You can tell if automatic offline testing is supported, if the command smartctl -o VALUE or smartctl --offlineauto=VALUE enables and disables it, as indicated by the Auto Offline Data Collection part of the SMART capabilities report (that is displayed with smarctl -c). Valid VALUES are on and off.
This type of test can, in principle, degrade the device performance. The -o on option causes this offline testing to be carried out, automatically, on a regular scheduled basis. Normally, the disk will suspend offline testing while disk accesses are taking place, and then automatically resume it when the disk would otherwise be idle, so in practice it has little effect. Note that a one-time offline test can also be carried out immediately upon receipt of a user command. See the -t offline option, which causes a one-time offline test to be carried out immediately.
If the -c option to smartctl shows that the device has the Suspend Offline collection upon new command capability then you can track the progress of the Immediate Offline test using the -c option to smartctl. If the -c option show that the device has the Abort Offline collection upon new command capability then most commands will abort the Immediate Offline Test, so you should not try to track the progress of the test with -c, as it will abort the test.
The choice (made by the SFF-8035i and ATA specification authors) of the word testing for this test is unfortunate, and often leads to confusion. In fact offline testing could have been more accurately described as offline data collection.
The results of this automatic or immediate offline testing (data collection) are reflected in the values of the SMART Attributes. Thus, if problems or errors are detected, the values of these Attributes will go below their failure thresholds; some types of errors may also appear in the SMART error log. These are visible with the -A and -l error options respectively.
Some SMART attribute values are updated only during off-line data collection activities; the rest are updated during normal operation of the device or during both normal operation and off-line testing. The Attribute value table produced by the -A option indicates this in the UPDATED column. Attributes of the first type are labeled Offline and Attributes of the second type are labeled Always.
(1) Good documentation can be found in IBM's Official Published Disk Specifications. For example the IBM Travelstar 40GNX Hard Disk Drive Specifications (Revision 1.1, 22 April 2002, Publication # 1541, Document S07N-7715-02) page 164. You can also read the SFF-8035i Specification.
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