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A Linux User Reference

Search tips
  • search ignores words that are less than 4 characters in length
  • searches are case insensitve
  • if a search does not return anything try it in Boolean mode then Query expansion mode by checking the appropriate radio button e.g. searching for 'cron' in just the Administration category returns nothing - presumably because the 50% threshold is reached. Boolean mode ignores this threshold so a search for 'cron' returns several hits
  • in Boolean mode preceding a word with a '+' means the result must include that word, a '-' means it must not
  • in Boolean mode '+crontab -anacron' means match articles about crontab that DO NOT mention anacron
  • to match a phrase e.g. 'manage system' check the Boolean mode radio button and enclose the phrase in quotes "some phrase ..."
  • in Query expansion mode the search context is expanded beyond the keywords you entered - relevancy of hits may well be degraded

HARDWARE

SCSI

  • Small Computer Standard Interface (SCSI)

    Small Computer Standard Interface (SCSI)

    • More expensive than IDE (ATA), offers greater flexibility, expandability and faster throughput.
    • PC SCSI adapters have their own firmware which is used for device configuration.
    Type Width Bandwidth Comments
    SCSI-1, SCSI-2 8 bit 5 MBps Interchangeable, ok for low-end tape drives, too slow for modern disks
    Wide SCSI 16 bit 10 MBps Same clock rate as SCSI-1, SCSI-2
    Fast SCSI 8 bit 10 MBps Clock rate is doubled
    Fast Wide SCSI 16 bit 20 MBps Higher clock rate and wider bus
    Ultra SCSI 8 bit 20 MBps Higher clock rate
    Ultra Wide SCSI 16 bit 40 MBps Wider bus
    Ultra 2 SCSI 8 bit 40 MBps Higher clock rate
    Wide Ultra 2 SCSI 16 bit 80 MBps Higher clock rate, wider bus
    SCSI-3 16 bit 1-2 GBps Fibre channel, kms distance, hot pluggable, Single Connector Attachment (SCA) for 16 bit

    There later scsi types such as scsi-4, scsi5 along with other scsi interfaces that include - usb attached scsi, parallel scsi, scsi express, ssa, fibre channel, ...

    SCSI termination

    • Conditions signal and reduces noise
    • Bus must be terminated at both ends
    • Termination can be built into a device or be separate
    • If mixing 8 and 16 bit devices then must use 16 bit terminators
  • SCSI addressing
    /proc/scsi/scsi
    • When mixing 8 and 16 bit devices 8 bit devices should use IDs 0 - 7 (provides higher priority for the slower devices).
    • 16 bit devices can use any IDs, 0 - 15.
    Width ID range Priority Comments
    8 bit 0 - 7 0 - 7 7 devices, 1 Controller(ID = 7), 7 = highest priority
    16 bit 0 - 15 8 - 15, 0 - 7 15 devices, 1 Controller(ID = 7), 7 = highest priority, 8 = lowest
    16 bit SCA auto Single Connector Attachment, 126 devices

    Logical Unit Numbers (LUN)

    • Devices such as RAID controllers will report a single SCSI ID to the bus although they may well incorporate several disks.
    • Tapes, single disk drives either report a LUN = 0 or do not report one.
    • SCSI adapters may support more than one cable or channel.
    • There may be multiple adapters in a system.
    Full SCSI address format

    Adapter Number, Channel Number, Device ID, LUN

    $ more /proc/scsi/scsi
    Attached devices:
      Host: scsi0 Channel: 00 Id: 00 Lun: 00
        Vendor: ATA      Model: ST1000LM024 HN-M Rev: 2AR1
        Type:   Direct-Access                    ANSI  SCSI revision: 05
      Host: scsi1 Channel: 00 Id: 00 Lun: 00
        Vendor: PLDS     Model: DVD-RW DS8A8SH   Rev: KL31
        Type:   CD-ROM                           ANSI  SCSI revision: 05
    .....
    

    Possible addressing example

    Device Address Linux Name
    Raid controller 00,0,0,0 /dev/sda
    Raid controller 10,0,0,1 /dev/sdb
    Tape 0,0,1,0 /dev/st0
    CD-rom 0,0,2,0 /dev/scd0
    Controller 0,0,7,0

    SCSI device naming

    SCSI device First Second Third and so on ..
    Disk /dev/sda /dev/sdb /dev/sdc /dev/sdd
    Tape /dev/st0 /dev /st1/dev/st2 /dev/st3
    CDrom /dev/sr0 /dev/sr1 /dev/sr2 /dev/sr3
    CDrom /dev/scd0 /dev/scd1 /dev/scd2 /dev/scd3
  • Access scsi modes pages
    /usr/bin/sdparm

    Access SCSI modes pages; read VPD pages; send simple SCSI commands - potentially changing mode pages. See man pages for options.

    sdparm [options] device
    

    Get SCSI information about /dev/sda

    $ sudo sdparm  /dev/sda
        /dev/sda: ATA       ST1000LM024 HN-M  2AR1
    Read write error recovery mode page:
      AWRE        1  [cha: n, def:  1]
      ARRE        0  [cha: n, def:  0]
      PER         0  [cha: n, def:  0]
    Caching (SBC) mode page:
      WCE         1  [cha: y, def:  1]
      RCD         0  [cha: n, def:  0]
    Control mode page:
      SWP         0  [cha: n, def:  0]
    
  • IDE CD burner support < 2.6 kernels
    • Standard linux burn tools are written to support scsi burners.
    • A scsi-ide translation module has been written to bridge the gap, not needed for 2.6 kernels, important for 2.4.

    For 2.4 or less kernels the following kernel parameters need to be enabled:

    SCSI support
    SCSI CR-ROMS and SCSI generic support 
    ATA/IDE/MFM/RLL support
    IDE, ATA and ATAPI Block devices
    SCSI emulation support 
    

    Re-compile the kernel

    Configure /etc/modules (Redhat/ Fedora) or /etc/modules.d/local (Debian)

    alias scd0 sr_mod
    pre-install sg modprobe ide-scsi
    pre-install sg_mod modprobe ide-scsi
    pre-install ide-scsi modprobe ide-cd
    

    load the appropriate modules on boot.