Adding a Disk

This contains links to information used in the lectures.

SCSI

• Connectors (Centronics, DB-50, DB-25, Mini-micro)
• SCSI devices mostly female
• SCSI bus consists of a daisy-chain
• Most devices have 2 SCSI ports (in and out)
• Termination - external v internal
• SCSI device number 0 to 7 - set with thumbwheel, DIP switches or jumpers
• SCSI-2 : faster but bus limited to 2m
• "Fast" SCSI - 10 Megatransfers per sec
• Wide means each transfer is 2 or 4 bytes rather than 1
• "Ultra" - 20 megatransfers of 2 bytes
• Differential SCSI : 0 to 15 devices
• The controller takes one address

Problems with configuration

• Internal devices
• Duplicate addresses
• "auto-terminating" devices - some don't when should
• unannounced termination - later devices vanish
• faulty (backward) thumbwheels
• SCSI bus too long - cables inside add a lot
• SCSI controller has address

Disk Geometry

• Platter
• Surface
• Track/Cylinder
• Sector
• Head

Installing a Disk

• Connect disk
• Create device files using mknod or SAM (or insf under HP-UX)
• Format disk
  SCSI preformatted but good to reformat in case more bad blocks created during shipping. Use mediainit on HP and format on SunOS.
• Label and partition disk
  On SunOS format also partitions disks (menu driven). HP-UX had only one partition and swap partition. Later versions permitted software striping. HP-UX 10.XX has logical volume manager.
  Hints for partitions:
  • Make a backup root partition on second disk
  • Increase swap space with memory - 2 to 4 times memory
  • Split (stripe) swap space across disks
  • Make partitions smaller than backup device capacity
  • Create separate partitions for files that are likely to go out of control (/tmp, /usr/tmp, /var if used for logging, news)
• Create UNIX filesystems on partitions
  Use mkfs or newfs, which is a user friendly front end. Now usually can just do newfs partition.
• Verify filesystems
• Setup automatic mounting at boot time
  This is done using a file which gives the partitions and mount points. This is /etc/fstab under SunOS and HP-UX 10.X and /etc/checklist under HP-UX 9.XX, and vftab under Solaris. The third field gives the filesystem type: nfs for nfs mounted files, local files differ with vendor (ufs, 4.2 under SunOS, hfs under HP-UX). The fourth gives mount options. The sixth specifies the fsck pass. The fifth is a dump frequency which is mostly unused.

  /etc/fstab is read by mount, umount, swapon, and fsck commands. One can call mount with just the mount point name :

  mount /usr/local
  

  mount -a mounts all the filesystems listed in /etc/fstab. It is called at boot time. Note that it will read the file sequentially so filesystems must appear in the correct order of mounting.
• Setting up swapping /etc/fstab file.

fsck : Check and Repair Filesystems

fsck is used to fix inconsistencies introduced into Unix filesystems during a crash. It will automatically fix most minor damage. The five most common types of damage are:

• Unreferenced inodes
• Large link counts
• Unused data blocks not recorded in block maps (free list)
• Data blocks listed as free that are also in files
• Incorrect summary information in superblock

Errors other than those above are deemed serious and fsck prints a message and quits. You must then run fsck manually (without -p option). Fixing the disk by hand requires extensive knowledge, extreme bravery and the type of foolhardiness that led to the charge of the Light Brigade. Lacking these give the default answer to all the questions fsck asks. Note that there is an option to fsck that answers yes to all the questions, but yes is not always the default !! One should try to copy files fsck wishes to delete. Files whose parent directory cannot be found will be copied to lost+found under their inode numbers. ncheck can sometimes be used to discover pathnames corresponding to inodes, and clri can be used to clear inodes which fsck will not fix.