[svlug] scsi troubleshooting

Karsten M. Self kmself at ix.netcom.com
Mon Dec 3 00:33:02 PST 2001


on Mon, Dec 03, 2001 at 12:20:07AM -0800, Seth David Schoen (schoen at loyalty.org) wrote:
> J C Lawrence writes:
> 
> > If you have neither, then you're in for a job of some hours if not
> > small order days (depending on how bad it is) working thru the disk
> > with a sector editor piecing the bits back together (actually most
> > of your time will be spend finding the bits to piece together).  
> 
> There are also some good tools to guess the boundaries of partition
> tables; I've heard people say that some of them even tend to guess
> correctly. :-)
> 
> I don't remember which one is the best; gpart is the only one I
> remember, but I think there's another which may be more sophisticated.

I've had an email exchange with someone on one of the lists I monitor
(likely deb-user) who had very good results with gpart.

> Another priority might be determining whether the disk has suffered
> some kind of physical failure, making parts of it unreadable or
> scrambled, or whether it's just that particular data (like the
> partition table) has been corrupted.
> 
> One approach to this is to do
> 
> dd if=/dev/sda of=/dev/null
> 
> and watch for physical hardware errors.  (This could take a while.)

Two comments.

  - "A while" could be most or all of a day or two, particularly for a
    larger drive.  dd isn't particularly fast, and you're literally
    readeing *every bit on the drive*.

  - The very act of this might damage the disk further, depending on
    what's already happened to it.  Might make more sense to dump it to
    some suitably large aggregate storage device.

I saw one instance of a fried device last year resulting from poor cable
seating (as in, not, when I looked at it).  The drive was utterly
unrecoverable.

JC's comments on keeping, verifying, and maintaining, suitable backups,
are well taken.

Peace.

-- 
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