UNIX%20System%20Administration

1
UNIX System Administration

• Adding a Disk
• Chapter 8

2
Disk Interfaces

• Proprietary interfaces
• Big. Yellow. Different.
• SCSI
• Small Computer Standardized Interface
• IDE
• Integrated Drive Electronics
• Fibre Channel
• NKOTB

3
(IDE) ATA helping if you gave me one.

• IDE - The idea of incorporating the drives
  controller (NOT the bus controller) onto the
  drive unit itself.
• PC IDE is attached to main system bus via only
  buffering/isolation components - there is no IDE
  interface per-se
• More properly referred to as ATA - Advanced
  Technology Attachment
• Defined protocol for DISK DRIVES
• ATAPI - ATA Packet Interface - protocol
  encapsulated in ATA for other IDE devices (CD-?
  Drives, misc removable storage devices)

4
ATA Performance

• Limited to two devices per channel, two channels
  per system
• Cheap, fast, single-user storage
• ATA protocols have less overhead than SCSI
• No error correction (until ATA-4)
• No behind-the-scenes management (e.g. integrity
  checks)
• Less extensible than SCSI
• Very simple protocol will not support more than
  two drives per channel
• Drives are not multitasking
• ATA bus is not shared with neighbor device -
  each drive is master of its own domain while
  handling an I/O request
• Host CPU must manage drives
• 60-100 CPU time of a PIII might be spent on a
  large transfer

5
SCSI

• SCSI - Small Computer Sstandardized Interface
• Began as Shugart Associates Standardized
  Interface in 1979, adopted as ANSI standard
  (SCSI-1) in 1986 with a name change
• Separate, intelligent I/O bus attached to main
  bus. All devices on bus are intelligent as well.
• Managing device - initiator - typically is the
  host interface though this is not required.
  Peripherals are targets.

6
SCSI too

• Initiator controls whether a SCSI device is
  connected to the bus (separate control lines
  facilitate initiator/target control
  communications)
• Multiple I/O requests may be issued against
  multiple targets
• Controllers typically use DMA (bus mastering) to
  offload the work of transferring data to memory
  from the host CPU

7
SCSI Buses

• Single Ended (SE)
• Oldest variety
• Most susceptible to noise due to low signal
  levels - required strict adherence to cable
  specifications
• Uses common ground for all signal lines
• Differential varieties
• Developed to overcome SE limitations - noise
  immunity
• Uses a and - line set for each signal
• High Voltage Differential (HVD) - 12 volts
• Low Voltage Differential (LVD) - 5 volts
• Less expensive version of HVD
• LVD devices will detect connection to a SE bus
  and fall back to SCSI-2 speeds
• All are incompatible
• All require termination. SE - passive H/LVD -
  active
• Absorbs signal at ends of the bus to prevent
  reflections

8
Feeling a little...irregular? Add some fibre to
your diet.

• Fibre Channel - Member of a class of high-speed,
  serial interfaces for disk storage (serial
  referring to the transmission order of data
  bytes)
• SSA (Serial Storage Architecture) is IBMs
  proprietary attempt at a serial interface for
  storage. As always, Big Blue sticks to that
  P-word. Regardless, SSA is too little, too late.
  Revel in the irony.
• Fibre Channel media
• Coaxial copper cable
• Twisted Pair copper cable
• Multimode or Singlemode fiber (required for
  speeds over 1Gb/s 100MB/s) - two fibers
  required (Tx/Rx) in a P2P link configuration.
• Fibre channel protocols
• Provide capability to support FC-specific
  protocols as well as SCSI, IP, and IEEE 802.2
  (ethernet)
• Supports class of service - datagram,
  fractional speed, connectionless, etc.

9
FC Topo- and Termi- nology

• Point-to-point
• exactly two devices. Exclusive media access.
• Arbitrated loop
• up to 127 devices, chained in a loop. Shared
  media access.
• Fabric-switched
• up to 224 devices in a cross-point switched
  configuration. Requires a switch, provides
  exclusive media access.

10
Drive Terminology

• Sector - atomic unit of storage, typically 512B
• Block - abstracted sector free from track/surface
  designations
• Track - sectors in a single groove on one surface
  of a platter
• Cylinder - from a vertical perspective, set of
  tracks on all disk surfaces at equal distance
  from the center
• Partition - Logical grouping of a set of
  consecutive cylinders or blocks
• LUN - Logical Unit Number - SCSI construct that
  permits dividing a single target into up to 7
  logical subdevices. All targets have LUN 0,
  devices like multi-tray CD changers and drive
  arrays utilize other LUN values as required.

11
(No Transcript)
12
Preparing your disk

• Connect the disk to the computer
• Create device files so disk can be accessed
• Format the disk
• Partition the disk
• Create filesystems on partitions
• Configure filesystem mounting

13
Let there be light!

• Connect the disk
• Create device files to access the disk
• All devices have entries in the /dev structure
• Disks (and like storage media) have two
• Character (or raw) - a low-level device used
  for maintenance interaction (fsck, etc) -
  /dev/rdsk
• Block - conceptually higher-level device used
  for filesystem interaction (backups, mounting,
  etc) - /dev/dsk
• Typically established by a reconfiguration
  sequence (though can be established manually
  under certain circumstances)
• All /dev entries for LUN 0 will be created
  initially, extra LUNs may require extra steps
• Format the disk
• In terms of low-level format, disks are
  pre-formatted by the manufacturer. This procedure
  should not be necessary, and should only be
  attempted with sufficient knowledge of the
  ramifications.

14
Click to add title

• Partition the disk
• Actual method is O/S dependent
• Logically groups cylinders (or blocks) into a
  usable portion which can be referenced
• Partitions are associated with and referred to by
  a device entry
• CwTxDySz
• C controller (w) - number of SCSI controller in
  the system
• T target (x) - SCSI ID of target on bus. 0-7 or
  0-15
• D LUN (y) - 0 always exists, typically only one
  LUN. 0-7
• S slice (z) - partition ID. 0-7
• Slice 2 is the backup slice - a special
  partition that refers to the entire disk, used
  for low-level disk manipulation
• Partitions should not overlap (but the customer
  is always right)
• A disks label refers to its partition table map.
  Labeling the disk writes this map onto the disk
  to be used by the system for interfacing with
  that drive.
• Remember to partition for O/S, swap, and other
  file storage needs (e.g. home, log, application
  directories)

15
Click harder!

• Create a filesystem on new partitions
• Partitions (slices) need to be initialized to
  accept a filesystem.
• Actual procedure varies by O/S - typically the
  newfs command
• Requires the /dev/rdsk filename of the partition
  to initialize
• Writes out the required filesystem data
  structures to disk (superblocks, disk block maps,
  inode tables, etc)

16
NOT THAT HARD!

• Configure filesystem mounting
• Procedure is (AGAIN) O/S dependent
• Filesystem must be mounted before can be
  accessible
• Add entries for automatic mounting
• /etc/vfstab
• Mount points are directories on an existing
  filesystem (or / if this is the root disk), but
  once mounted, the new filesystem will overlay and
  hide anything contained in that mount point
  directory.
• You must mount in order of dependence if mounting
  a filesystem on a mounted filesystem other than /
  

17
When good disks go bad

• fsck - Filesystem check and repair
• Capable of repairing most soft errors
• Runs automatically on boot (per settings in
  /etc/vfstab)
• Required to be run against filesystems that were
  potentially made inconsistent by an improper
  shutdown - mount will check a flag that indicates
  the possible state of the filesystem and will
  usually refuse to accept inconsistent filesystems
• A fsck should only be done on an unmounted
  filesystem especially if changes will be made -
  O/S buffers will become inconsistent and the
  system may panic
• Certain repair operations may be destructive (but
  would be required before disk could be mounted
  again)
• Running fsck on a journaled filesystem takes
  little time as the journal log clears up
  inconsistencies (fsck will flush the log, which
  is very small)

18
Solaris

• Connect the disk (power off, please) and set the
  SCSI Ids and place terminators (or IDE
  master/slave jumper)
• Power on and enter PROM mode
• Issue probe-scsi (or probe-scsi-all) to verify
  disk presence
• Use probe-ide for IDE machines
• Issue boot -r for reconfig boot
• Use the format command to label (partition) the
  disk
• Ensure your slices do not overlap - no direct
  indication would be provided of this oversight
  (the overlap exception is slice 2 - the backup
  slice - dont ever touch it). Zero-out any unused
  partitions to avoid accidental use. Dont forget
  to issue the label command before exiting.
• Use newfs to initialize the filesystem
• Mount the filesystem to verify
• Add entries in /etc/vfstab as required - enable
  journaling, but be careful!!