[TUHS] Disk data layout (was: /dev/drum)

Paul Winalski paul.winalski at gmail.com
Thu Apr 26 07:17:49 AEST 2018

On 4/24/18, Greg 'groggy' Lehey <grog at lemis.com> wrote:
> "LBA" is newer than the time we're talking of.  In those days, disk
> data was addressed physically, by cylinder, head and sector, terms
> that only died out round the turn of the century.

IBM DASD--Direct Access Storage Device, a term that encompassed drums,
disks and the data cell drive--addressed data on the media physically
by bin, cylinder, head, and record, as a hexadecimal number BBCCHHR.
Bin number was zero except on the IBM 2321 data cell drive.  CKD
drives supported a variable number of records on each track, hence the
term "record" rather than "sector".

Logical block addressing (LBA) for sector-oriented disks allowed the
OS (or later, the disk controller) to hide bad block replacement from

VAX/VMS used a protected system file ([sysexe]badblock.sys) to keep
track of the bad blocks on each disk volume.  DEC once got a customer
bug report complaining that if a privileged user gave the command
"TYPE SYS$SYSTEM:BADBLOCK.SYS", the console logged bad block errors.

How does/did Unix handle bad block replacement?

-Paul W.

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