smartctl reports bad sectors, badblocks does not find any

[Juan R. de Silva]

On Sat, 25 Dec 2010 00:00:45 +0800, Ian Chapman wrote:

> 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.

Very much true. Bad wording from my part. What I wanted to say that the 
bad sector was reported by smartctl but was not detected by badsectors, 
when run on the drive.

> 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.

In my case I never was able to achieve this. I wrote to the sector in 
question directly with dd, I did it running badblocks in destructive 
(read/write) mode - the values of Current_Pending_Sector and 
Offline_Uncorrectable remained still.

> 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'.

This value for my disk is quite high sinc the drive is 4.5 years old. 
It's not a lot for a 3.5" drive but is much enough for a laptop drive 
(Dell Latitude d820) I think. Though I'm still disappointed. I expected 
it to last more.
 
> You can perform a disk test using smartctl (sounds like you have
> already) 

You bet. :-)

> 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.

As I've mentioned in one of my posts, this the only thing I did not do 
yet. I'll probably do it on the bad sector reported in 
Current_Pending_Sector (the value is 1) since I know it's starting LBA 
and I know the block size. But I have no idea what to do with those 
reported in Offline_Uncorrectable - the value here is 4. 

And it looks like actually Offline_Uncorrectable gives me more of 
warning. I looked through my log files and found that it was reported the 
first with the lower value a couple of months ago. Than a month later it 
increased, while Current_Pending_Sector remained 1. My bad, I've 
neglected my log files for a while.