[Info-vax] VMS and bad blocks, was: Re: analyze/disk errors

[Stephen Hoffman]

On 2013-12-06 18:34:13 +0000, Simon Clubley said:

> The bad block file has file ID 3,3. It is listed in the MFD as BADBLK.SYS;1.
> The bad block file is simply a file containing all the known blocks on the
> volume that cannot reliably store data. This file has the record format of
> 512-byte fixed-length records with no carriage control.

I'd expect most* of the BADBLK contents to be transient, and the bad 
block information will be migrated to the device-level lists, typically 
immediately after the HBVS block shuffling completes, or after the file 
with the bad cluster is deleted or rewritten with non-HBVS disks.

With SCSI, the factory-determined list of bad blocks goes into what's 
called the primary list (PLIST), the formatting and certification into 
the certification (CLIST), those manually specified on the format go 
into the defective list (DLIST), and then there's the grown list 
(GLIST) for the blocks that rot.  To get the details of the status of 
the device, toss a READ DEFECT DATA SCSI command at the disk.

> On disks containing a bad block descriptor, the last track of the 
> volume comprises the first several clusters of the bad block file. This 
> rule ensures that the bad block data is available to software in a 
> file-structured manner and is preserved when the volume is initialized 
> again.

The replacement and caching table (RCT) and some other details have 
been replaced by SCSI-level mechanisms.  See 
<http://h71000.www7.hp.com/wizard/wiz_6926.html> among other resources 
and postings.

This whole area was never particularly well documented IMO, as it 
inherently crossed multiple different layers and components; SCSI or 
SAS or SATA or the older DEC disk technologies, drivers, BAD, the file 
system, HBVS, etc.

*if the last partial cluster of the disk is here and not over in 
GPT.SYS or elsewhere, or there's some El Torito hackery lurking here, 
then that won't get moved around.


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