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Re: unreadable sectors, what to do?



On Thu, Oct 11, 2007 at 07:07:47 -0400,
  Neal Becker <ndbecker2 gmail com> wrote:
> I have a raid0 array, and I'm getting this:
> 
>  Currently unreadable (pending) sectors detected:
>         /dev/sda - 48 Time(s)
>         2 unreadable sectors detected
>  
>  Offline uncorrectable sectors detected:
>         /dev/sda - 48 Time(s)
>         2 offline uncorrectable sectors detected
> 
> What should I do?  Run badblocks?

Not initially, the drive will handle this. It won't do anything until it gets a
good read or the sectors are overwritten. Once that happens it may reallocate
the sector based on whether or not the drive firmware thinks the sector is
permanently hosed or not.

You can get the sector numbers by running self tests with smartctl. You
can start with short self tests, but sometimes you need to run the long
ones.

The big problem is figuring out what you lost. If those blocks are part of
a file currently you have lost some data. I don't know if there is a howto
anywhere for figuring out which sectors are in which files when using raid 0.
If you are using propietary raid, it is probably going to be even harder to
get this information.

So you will need to make a determination about your recovery path. If there
isn't anything valuable on the machine you might just overwrite the disks
(using dd running from a rescue cd is one way to do this), test the drives
using badblocks and convince yourself they are OK or replace the drives,
reinstall and reload from backups. If you have valuable data on the machine,
then you will want to start by making a new back up and then you will have
to figure out what you want from the new back up and what you want from your
old backups when you do the reinstall.

You may also want to reconsider which raid you are using. Unless you are doing
a lot of file I/O that really needs to be fast, raid 0 is not going to give
you a lot of benefit and does make recovery harder. raid 1 allows you to repair
the kind of damage you have without too much difficult, but at the cost of
half of your disk space, which may be too high for you.


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