Ignore:
Timestamp:
02/15/2013 09:14:37 PM (11 years ago)
Author:
Krejzi <krejzi@…>
Branches:
7.5-systemd, 7.6-systemd, 7.7-systemd, 7.8-systemd, 7.9-systemd
Children:
b588d62
Parents:
4a4e017
Message:

Move Chapter 7 in place for Systemd.

git-svn-id: ​http://svn.linuxfromscratch.org/LFS/branches/systemd/BOOK@10159 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

File:
1 edited

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  • chapter07/symlinks.xml

    r4a4e017 rf3317d4  
    1010
    1111  <title>Creating Custom Symlinks to Devices</title>
    12 
    13   <sect2>
    14 
    15     <title>CD-ROM symlinks</title>
    16 
    17     <para>Some software that you may want to install later (e.g., various
    18     media players) expect the <filename class="symlink">/dev/cdrom</filename>
    19     and <filename class="symlink">/dev/dvd</filename> symlinks to exist, and
    20     to point to a CD-ROM or DVD-ROM device. Also, it may be convenient to put
    21     references to those symlinks into <filename>/etc/fstab</filename>. Udev
    22     comes with a script that will generate rules files to create these symlinks
    23     for you, depending on the capabilities of each device, but you need to
    24     decide which of two modes of operation you wish to have the script use.</para>
    25 
    26     <para>First, the script can operate in <quote>by-path</quote> mode (used by
    27     default for USB and FireWire devices), where the rules it creates depend on
    28     the physical path to the CD or DVD device. Second, it can operate in
    29     <quote>by-id</quote> mode (default for IDE and SCSI devices), where the
    30     rules it creates depend on identification strings stored in the CD or DVD
    31     device itself. The path is determined by Udev's <command>path_id</command>
    32     script, and the identification strings are read from the hardware by its
    33     <command>ata_id</command> or <command>scsi_id</command> programs, depending
    34     on which type of device you have.</para>
    35 
    36     <para>There are advantages to each approach; the correct approach to use
    37     will depend on what kinds of device changes may happen. If you expect the
    38     physical path to the device (that is, the ports and/or slots that it plugs
    39     into) to change, for example because you plan on moving the drive to a
    40     different IDE port or a different USB connector, then you should use the
    41     <quote>by-id</quote> mode. On the other hand, if you expect the device's
    42     identification to change, for example because it may die, and you would
    43     replace it with a different device with the same capabilities and which
    44     is plugged into the same connectors, then you should use the
    45     <quote>by-path</quote> mode.</para>
    46 
    47     <para>If either type of change is possible with your drive, then choose a
    48     mode based on the type of change you expect to happen more often.</para>
    49 
    50 <!-- If you use by-id mode, the symlinks will survive even the transition
    51      to libata for IDE drives, but that is not for the book. -->
    52 
    53     <important><para>External devices (for example, a USB-connected CD drive)
    54     should not use by-path persistence, because each time the device is plugged
    55     into a new external port, its physical path will change. All
    56     externally-connected devices will have this problem if you write Udev rules
    57     to recognize them by their physical path; the problem is not limited to CD
    58     and DVD drives.</para></important>
    59 
    60     <para>If you wish to see the values that the Udev scripts will use, then
    61     for the appropriate CD-ROM device, find the corresponding directory under
    62     <filename class="directory">/sys</filename> (e.g., this can be
    63     <filename class="directory">/sys/block/hdd</filename>) and
    64     run a command similar to the following:</para>
    65 
    66 <screen role="nodump"><userinput>udevadm test /sys/block/hdd</userinput></screen>
    67 
    68     <para>Look at the lines containing the output of various *_id programs.
    69     The <quote>by-id</quote> mode will use the ID_SERIAL value if it exists and
    70     is not empty, otherwise it will use a combination of ID_MODEL and
    71     ID_REVISION. The <quote>by-path</quote> mode will use the ID_PATH value.</para>
    72 
    73     <para>If the default mode is not suitable for your situation, then the
    74     following modification can be made to the
    75     <filename>/lib/udev/rules.d/75-cd-aliases-generator.rules</filename> file,
    76     as follows (where <replaceable>mode</replaceable> is one of
    77     <quote>by-id</quote> or <quote>by-path</quote>):</para>
    78 
    79 <screen role="nodump"><userinput>sed -i -e 's/"write_cd_rules"/"write_cd_rules <replaceable>mode</replaceable>"/' \
    80     /lib/udev/rules.d/75-cd-aliases-generator.rules</userinput></screen>
    81 
    82     <para>Note that it is not necessary to create the rules files or symlinks
    83     at this time, because you have bind-mounted the host's
    84     <filename class="directory">/dev</filename> directory into the LFS system,
    85     and we assume the symlinks exist on the host. The rules and symlinks will
    86     be created the first time you boot your LFS system.</para>
    87 
    88     <para>However, if you have multiple CD-ROM devices, then the symlinks
    89     generated at that time may point to different devices than they point to on
    90     your host, because devices are not discovered in a predictable order. The
    91     assignments created when you first boot the LFS system will be stable, so
    92     this is only an issue if you need the symlinks on both systems to point to
    93     the same device. If you need that, then inspect (and possibly edit) the
    94     generated <filename>/etc/udev/rules.d/70-persistent-cd.rules</filename>
    95     file after booting, to make sure the assigned symlinks match what you need.</para>
    96 
    97   </sect2>
    9812
    9913  <sect2>
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