Handling Disk Failures

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The best way to deal with disk failures it to assume they will happen and be prepared. See Reliable data storage for best practises.

If you have multiple disks in a mirror setup, and an additional off-site backup, you are fine.

The cause of action is to:

  1. Run a SMART test (if that hasn't been done) to find where the test fails.
  2. Write directly to the sector with dd. This forces the drive to relocate the sector to one of your extra sectors.
  3. Scrub the file system on the disk. If you have a mirrored copy, everything should be restored to normal.
  4. If you don't have a mirror, use your backup disk to restore the data.
  5. Decide if you want to replace the disk to prevent future failures.

SMART selftests

A bad block may be reported as follows:

Device: /dev/ada0, 1 Currently unreadable (pending) sectors
Device: /dev/ada0, Self-Test Log error count increased from 0 to 1


Bad block are best detected by running a SMART self-test, assuming that the disk has S.M.A.R.T. support.

To examine the status (including indication if a test is still running):

# smartctl -c /dev/ada0

which will include one of these results:

Self-test execution status:      (   0) The previous self-test routine completed
                                        without error or no self-test has ever
                                        been run.

or

Self-test execution status:      ( 249) Self-test routine in progress...
                                        90% of test remaining.

To get the log of the last completed test, run:

# smartctl -l selftest /dev/ada0

To start a test:

# smartctl -t long /dev/ada0

or

# smartctl -t short /dev/ada0

To abort a test:

# smartctl -X /dev/ada0



pakhuis# smartctl -a /dev/ada0

=== START OF INFORMATION SECTION ===
...
User Capacity:    3,000,592,982,016 bytes [3.00 TB]
Sector Sizes:     512 bytes logical, 4096 bytes physical
...
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


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     0x002f   200   200   051    Pre-fail  Always       -       0
  3 Spin_Up_Time            0x0027   155   150   021    Pre-fail  Always       -       9225
  4 Start_Stop_Count        0x0032   100   100   000    Old_age   Always       -       91
  5 Reallocated_Sector_Ct   0x0033   200   200   140    Pre-fail  Always       -       0
  7 Seek_Error_Rate         0x002e   200   200   000    Old_age   Always       -       0
  9 Power_On_Hours          0x0032   070   070   000    Old_age   Always       -       22013
 10 Spin_Retry_Count        0x0032   100   253   000    Old_age   Always       -       0
 11 Calibration_Retry_Count 0x0032   100   253   000    Old_age   Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   000    Old_age   Always       -       89
192 Power-Off_Retract_Count 0x0032   200   200   000    Old_age   Always       -       74
193 Load_Cycle_Count        0x0032   001   001   000    Old_age   Always       -       914156
194 Temperature_Celsius     0x0022   108   107   000    Old_age   Always       -       44
196 Reallocated_Event_Count 0x0032   200   200   000    Old_age   Always       -       0
197 Current_Pending_Sector  0x0032   200   200   000    Old_age   Always       -       1   <---- this ought to be 0
198 Offline_Uncorrectable   0x0030   200   200   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x0032   200   200   000    Old_age   Always       -       0
200 Multi_Zone_Error_Rate   0x0008   200   200   000    Old_age   Offline      -       5   <---- this is worrying.

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%     22010         -
# 2  Extended offline    Completed without error       00%     21999         -
# 3  Short offline       Completed without error       00%     21966         -
# 4  Short offline       Completed without error       00%     21965         -
# 5  Short offline       Completed without error       00%     21964         -
# 6  Short offline       Completed without error       00%     21963         -
# 7  Short offline       Completed without error       00%     21962         -
# 8  Short offline       Completed without error       00%     21961         -
# 9  Short offline       Completed without error       00%     21960         -
#10  Short offline       Completed without error       00%     21959         -
#11  Short offline       Completed without error       00%     21958         -
#12  Short offline       Completed: read failure       90%     21957         310949139
#13  Short offline       Completed without error       00%     21956         -
#14  Short offline       Completed without error       00%     21955         -
#15  Short offline       Completed without error       00%     21954         -
#16  Short offline       Completed without error       00%     21953         -
#17  Short offline       Completed without error       00%     21952         -
#18  Short offline       Completed without error       00%     21951         -
#19  Short offline       Completed without error       00%     21950         -
#20  Short offline       Completed without error       00%     21949         -
#21  Short offline       Completed without error       00%     21948         -
1 of 1 failed self-tests are outdated by newer successful extended offline self-test # 1

A few things to notice:

  • The disk has a sector size of 512 bytes logical, 4096 bytes physical. We need this info later.
  • SMART support is enabled. Good.
  • SMART still passes. So the disk is still usable. As long as it lasts, that is.
  • There is 1 Current_Pending_Sector. So there is 1 "bad block". This can be fixed, though for some people it is already enough to replace the disk.
  • The disk has 5 Multi_Zone_Error_Rate. Anything >0 is worrying, and an indication that the disk is starting to fail. This is a clear indication that it is time to buy a replacement disk.
  • The first sector that is failing is 310949139.