bad sector in gmirror HDD

[Kevin Oberman]

On Fri, Aug 19, 2011 at 4:57 PM, Diane Bruce <db at db.net> wrote:
> On Fri, Aug 19, 2011 at 04:50:01PM -0400, Dan Langille wrote:
>> System in question: FreeBSD 8.2-STABLE #3: Thu Mar  3 04:52:04 GMT 2011
>>
>> After a recent power failure, I'm seeing this in my logs:
>>
>> Aug 19 20:36:34 bast smartd[1575]: Device: /dev/ad2, 2 Currently unreadable (pending) sectors
>>
>
> Personally, I'd replace that drive now.
>
>> Searching on that error message, I was led to believe that identifying the bad sector and
>> running dd to read it would cause the HDD to reallocate that bad block.
>
> No, as otherwise mentioned (Hi Jeremy!) you need to read and write the
> block. This could buy you a few more days or a few more weeks. Personally,
> I would not wait. Your call.
>

While I largely agree, it depends on several factors as to whether I'd
replace the drive.

First, what does SMART show other then these errors?  If the reported
statistics look generally good, and considering that you a mirror with
one "good" copy of the blocks in question, the impact is zero unless
the other drive fails. That is why the blocks need to be re-written so
that they will be re-located on the drive.

Second, how critical is the data? The mirror gives good integrity, but
you also need good backups. If the data MUST be on-line with high
reliability, buy a replacement drive. You need to look at cost-benefit
(or really the cost of replacement vs. cost of failure).

It's worth mentioning that all drives have bad blocks. Most are hard
bad blocks and are re-mapped before the drive is shipped, but marginal
bad blocks can and do slip through to customers and it is entirely
possible that the drive is just fine for the most part and replacing
it is really a waste of money.

Only you can make the call, but if further bad blocks show up in the
near term, I'll go along with recommending replacement.

-- 
R. Kevin Oberman, Network Engineer - Retired
E-mail: kob6558 at gmail.com