Bad sectors: how bad can it be

[Michael Powell]

Michaël Grünewald wrote:

[snip]
> 
> I have backups of the data contained in the broken, so the data on this
> disc are not a concern. I have however a question: How do I verify that
> a hard-drive is accurately working if its firmware will hide the bad
> sectors as long as possible?
> 
[snip]

As Polytropon indicated the smartctl commands for testing contained within 
the smartmontools port will extract the error logs from within the drive's 
firmware. There are two modes you can select from (basically a long and a 
short) that you can execute "now" at a command prompt. It can also be run as 
a daemon for continual monitoring. The data returned is somewhat arcane and 
can be semi difficult to interpret.

There are various levels of usability which can vary by hardware. Some RAID 
controllers may get in the way of direct communication to some hard drives. 
Other controllers, as you go up the 'expensive high dollar' ladder will 
often do built-in SMART monitoring and will beep and/or send emails when it 
detects error conditions from a drive. Some even either contain, or have an 
external utility which provide a web based browser accessible view in real 
time. The purpose is to attempt to detect a drive that is about to fail.

As far as the most basic level goes, you would look for numbers which 
indicate that the bad sector remap area has filled. Once this space gets 
filled any new bad sectors that develop can no longer be mapped out. This 
usually shows up in the operating system as some generic form of 
"unrecoverable read/write error" message and Bad Things begin to happen.

I have not used Spinright in a very long time, but it may buy some life on 
such a drive. If it can clear the bad sector remap area after adjusting the 
remap table it can give new life to a drive. The same thing used to be 
possible on SCSI drives by running the low level format utility usually 
contained within the controller firmware. 

Such "fixes" should only be viewed as extremely temporary in nature, as the 
general pattern with regard to magnetic media failure is that once it starts 
to get bad spots it will keep on getting bad spots on a fairly regular basis 
afterwords.

Interesting reading:

http://www.usenix.org/publications/login/2008-06/openpdfs/bairavasundaram.pdf

-Mike