System Administration adding disks

1
System Administration adding disks

• Overview
• Disk standards and comparison
• Current standards
• IDE (ATA) ATAPI
• IDE arose from the older standards disk
  controller was on separate interface, now
  integrated
• LBA idea arose from limitations of CHS field
  sizes
• IDE bus is almost ISA
• Interleaving isnt well supported
• ATAPI is SCSI-like upper layer, IDE lower
• Completely standardized, unlike SCSI (nonstandard
  adapters)
• Lower level (register is summarized in many
  places) lives in fixed place in PC I/O space

2
System Administration SCSI

• SCSI is actually several standards and
  generations
• Cabling
• Most older cabling is obsolete
• Differential vs single-ended idea
• Single-ended introduces common-ground crosstalk
• Result is cable length limitation
• Speed
• 5 to 20 MHz
• Full speed only for synchronous transfers
• Setup is done asynchronously at minimum speed
• Bus width
• 8 to 32 bit bus width
• Layered protocol
• Bus acquisition
• Target acquisition
• Data phase contains higher level (transaction
  description or data itself)

3
System Administration adding disks

• The problems SCSI and IDE dont handle well
• Plug-and-play
• Hot-pluggable devices
• New physical standards
• Transaction interleaving
• File systems spread over several devices
• RAID is somewhat separate (RAID controllers)
• Emerging standards
• Firewire
• High-speed network-like protocol
• Can be fibre or wire
• Fibre channel
• Fiber more speed, isolation

4
Information sources

• SCSI/IDE book from readinglist
• See readinglist on course page
• Describes history and details of both IDE (ATA)
  and SCSI standards
• Linux Kernel book
• Describes current and 2.4 kernel approaches to
  virtual device file system
• PC hardware
• Compares IDE and SCSI for server and workstation
• SCSI needed for servers better to economize on
  CPU and get SCSI for interleaving
• Describes realities of current disks which ones
  last which for occasional use

5
System Administration adding disks

• The current disk market
• Warehouse.com
• You should research this yourself
• Significant items are prices, capacity, rpm
  determines speed, most standard brands OK
• SCSI is much higher priced than IDE, but often
  more durable 36G seems to be typical size
• Look at warehouse catalog in class
• Speed, etc
• RPM vs access time
• Lower bound is ½ revolution
• Interleaved requests and caching give much more
  speed, especially in server use
• Failure times are given in transactions usually
  300K 800K
• Much statistical fiction
• Failures usually due to tropical environment if
  it acts bad, replace in time

6
Disk partitioning PC architecture

• Disk partitioning PC architecture
• Hard disks
• Device files have names like /dev/hda2, /dev/sdc1
• Partition table
• fdisk or cfdisk allows examine-modify partition
  table no effect inside partitions
• Commands are such as m-menu, p-print, w-write,
  q-quit, n-new, t-type
• Individual partitions
• Format ext type using format (or initialization
  menu)
• Set type for DOS/Windows dont format
• Fix /etc/fstab
• Other devices
• Zip disks
• CDs and Roxio

7
Adding disks in typical Linux/Unix

• Adding disks in typical Linux/Unix
• Conventional standard device files
• Partition and format if needed (fdisk and format)
• Add device file if needed (major number is
  needed)
• Add partitions to /etc/fstab
• If new driver needed
• Set it up as module or driver
• Include it in /etc/master
• Rebuild kernel
• New variety Linux 2.4 kernels
• New devices get autoconfigured new device file
  appears in /dev/devfs

8
A few war stories

• Recovering a deleted partition
• Deleted DOS Fat32 partition
• The deletion removed only the partition table
  entry and the first sector
• Attempting to make a first sector (referring to
  the DOS programmmers guide) revealed that there
  is hidden info here
• Fdisk showed size of file system
• Method that worked
• Make file system on another disk with spare space
• dd copy first sector in Linux
• Add a partition (linux ext2) in existing disk
  dont format
• dd it into first sector in existing disk
• Change type back to fat32 with fdisk - Voila