smartctl reports bad sectors, badblocks does not find any

[Ian Chapman]

On 24/12/10 11:46, Juan R. de Silva wrote:
>
>> I haven't used badblocks, but suspect that the hard disk drive
>> electronics is clever enough to hide damage and relocate bad sectors.
>> The S.M.A.R.T. electronics will still know they are there, though.
>
> OK. This might be the answer to a question I had: why badblocks never
> found them but smartctl did.

AFAIK smartctl doesn't actually "detect" bad sectors on a disk, it just 
reports what it sees in the SMART counters. The particular counter is 
Current_Pending_Sector I believe. Each time the disk encounters a sector 
it can't read, the firmware increments this counter. Only when that bad 
sector is written to, does the disk remap the sector and usually 
Current_Pending_Sector decreases by 1. I have however seen on occasions, 
the counter increase but no bad sector appears to be on the disk. I just 
put this down to a temporary glitch, ie the disk at some short point in 
time could not read that sector but now it's fine, however the increment 
remains. Some drives I believe also increment Reallocated_Sector_Ct 
every time a remap is done, but certainly many do it 'silently'.

You can perform a disk test using smartctl (sounds like you have 
already) to find the LBA of a bad sector. The output of smartctl -a, 
once the test completes, provides test results which look a little like:

Num  Test_Description    Status                  Remaining 
LifeTime(hours)  LBA_of_first_error
# 1  Extended offline    Completed without error       00%     13397 
      -
# 2  Extended offline    Completed without error       00%     13293 
      -
# 3  Selective offline   Completed without error       00%     13290 
      -
# 4  Selective offline   Completed without error       00%     13290 
      -
# 5  Selective offline   Completed: read failure       70%     13290 
      379731309
# 6  Selective offline   Completed: read failure       70%     13290 
      379729059
# 7  Extended offline    Completed: read failure       50%     13290 
      379718855

The last value is the LBA which can used with hdparm --write-sector to 
write to that specific sector triggering a remap and usually 
decrementing the counter. Be extremely careful when using this command 
though as a slight mistake can trash data.


-- 
Ian Chapman.