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From: Bela Lubkin <belal@sco.com>
Subject: Re: opensever cmd=0x000000C8 on fixed disk
Date: Tue, 14 Oct 2003 18:09:24 GMT
References: <563ac914.0310140728.32e6d772@posting.google.com> 

tully wrote:

> i keep getting errors on this 80 gig drive, the blocks are usually
> different and badtrk does not seem to find anything.  What does the
> cmd C8 used for?  do we need a new one here?
> 
> example below:
> wd0: Error on fixed disk dev 1/42, block=8049847, cmd=0x000000C8
> status=0x00000040, LBA sector=16792639...

C8 is the "READ using DMA" command.  These are read errors on the disk.

When you say the blocks are "usually" different, what do you mean?  Have
you actually seen some where the block numbers were the same?

Modern drives do auto-sparing; that is, if they find a bad sector, they
replace it for you without having to bother the operating system (most
of the time).  There are a few different scenarios:

- Bad sector detected on a write: the drive can silently spare it out,
OS never needs to know a thing.

- Bad sector detected on a read: the drive will attempt many times to
read the data, seeking from different sides, trying different head
current strengths, whatever the particular drive is designed to do.  If
it eventually reads successfully, it can assign a spare sector, write
the data there, pass it back to the OS without informing it about the
problem.

- Bad sector detected on a read, and data was unrecoverable: then the
drive _has_ to tell the OS about the problem.  It will probably also
replace that sector with a spare (but the spare will contain garbage, or
[hopefully] all 0's or some other such intentional "blank" pattern).

You're seeing this problem on reads, which makes sense.  The fact that
it's actually coming through to the OS means that the sectors it's
finding are totally unreadable.

If a `badtrk` scan finds nothing, either there are no other bad sectors
at the time, or (if there are any) the drive is retrying and eventually
getting a good read.

And then if you get more bad sectors later, whatever is wrong is an
ongoing deterioration; as if, for instance, something has come loose
inside the drive and is bouncing around in there, scratching more and
more places on the media.

There's one thing to check before replacing the drive.  It's using Ultra
DMA.  Make sure that you have it attached with a proper 80-pin UDMA IDE
cable.  IDE connectors are 40 pins wide.  UDMA drives require an 80-pin
cable where the separating pins are ground lines.  Operating a UDMA
drive with a 40-pin cable can cause all sorts of bizarre behavior.

If that isn't the problem, you need to replace the drive.  Do not
overwrite your last few backups.  Make a new full backup.  Replace the
drive, restore onto it.  The older backups are important because you may
later notice that some files are corrupt; you will want several backups
from various time ranges to have the best possible chance of being able
to find a good copy.

>Bela<

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