Re: kern/48808

["Thomas Schmitt" <scdbackup%gmx.net@localhost>]

The following reply was made to PR kern/48808; it has been noted by GNATS.

From: "Thomas Schmitt" <scdbackup%gmx.net@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: 
Subject: Re: kern/48808
Date: Thu, 15 May 2014 09:37:24 +0200

 I overhauled the man page description of the new option.
 
 The main problem is that several device models meet at this
 topic.
 
 There is NetBSD CD partitioning, which might surprise even
 experienced users, when they encounter it for the first time.
   http://www.netbsd.org/docs/guide/en/chap-rmmedia.html#multisession
 
 It reflects the device model of SCSI/MMC which distinguishes
 two large families of media capabilities:
 Sequential, which get shown partitioned by NetBSD.
 Overwritable, which get shown like a hard disk with a single partition.
 Both under the device name cd.
 These are not really media families, because DVD-RW and BD-R media
 can play on both fields.
 I may not even distinguish the families by "formatted" and
 "unformatted", because one of the three BD-R personalities is
 formatted and sequential at the same time.
 
 This has to be explained to a person who might assume the normal
 VFS device model of partitions and filesystems.
 
 Well, did i succeed with the following text ?
 --------------------------------------------------------------------
   -s block_offset
      Read the superblock from block address block_offset rather than
      from the default position implied by the choice of the device
      file.  The block size is 2048 bytes.
 
      Optical drives /dev/cd* with multi-session media are handled
      differently than other storage partitions.  /dev/cd?d points to
      the start of the first and oldest logical track on the loaded
      medium. /dev/cd?a points to the last and youngest logical track.
      Nevertheless all block addressing is done relative to /dev/cd?d.
      Only the default offset is taken from /dev/cd?a, if that one is
      used. This matches the needs of ISO-9660 multisession on such
      optical media.
 
      Optical media, which are formatted to support random read-write
      access, as well as other kinds of storage partitions show no such
      default superblock offsets.  Nevertheless they may bear several
      ISO-9660 sessions. The default superblock is at offset 0 and
      usually leads to the youngest session.
 
      Other ISO-9660 sessions can be mounted by help of option -s with
      a block_offset which can be obtained from burn software for
      optical media.
 ----------------------------------------------------------------------