Hard Disk Drives

Everything You (N)Ever Wanted to Know About Hard Drives

Craig M. Buchek

St. Louis Linux Users Group

July 17, 2003

"To promote the progress of science and useful arts, by securing for limited times to

authors and inventors the exclusive right to their respective writings and discoveries"

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History of the Hard Disk Drive

  • Invented by IBM in 1956
    • RAMAC - Random Access Method of Accounting and Control
      • 50 platters, each 24 inches across
      • Stored 5 MB total
  • IBM model 3340, introduced in 1973
    • Stored 30 MB on each of 2 spindles
    • Nick-named "Winchester", after the 30/30 riffle
    • Ancestor of the modern hard drive
  • Seagate ST-412
    • Available with IBM PC/XT
    • 10 MB version of ST-506

Pictures of RAMAC


RAMAC hard drive RAMAC spindle

Stallman as St. Ignutius
with a Hard Drive Platter on His Head

Physical Layout

  • Platters
    • Disc-shaped, metal or glass substrate
    • Coated with a ferrite oxide
      • Holds magnetic charge
      • Modern drives use a thin-film layer
    • Modern drives typically have 1 to 4 platters
  • Spindle
    • Collection of platters that spin together
  • Spindle motor
    • Rotates the spindle at high speeds
  • Actuator motor
    • Moves the read/write heads from track to track
    • Read/write arm pivots on an axis, using strong magnets and voice coil
    • Positioning done with servo system feedback loop
      • Servo information on drive media between data helps position the heads

Physical Layout (Continued)

  • Read/write heads
    • Detect/modify magnetic domains in the media
    • One read head and one write head per platter surface
      • Both on same armature
    • Very tiny, very sensitive
    • Most expensive part of the drive
      • Highest-tech part of the drive
    • Read head technology:
      • Ferrite
      • Metal in Gap
      • TF - Thin Film
      • MR - Magneto-Resistive
      • GMR - Gigantic Magneto-Resistive

Hardware Interfaces

  • ST-506
    • Two ribbon cables: one for data (20 pins), one for control (34 pins)
    • Driver on PC had to do all the work on the raw data from the read/write heads and actuator
    • MFM or RLL encoding
  • ESDI
    • Enhanced Small Device Interface
    • Moved some of the intelligence to the drive
    • Somewhere between ST-506 and IDE in technology
  • IDE
    • Integrated Drive Electronics
      • Controller logic is on the drive itself
      • Generic term - actually applies to ATA and SCSI
        • Common usage implies ATA
  • ATA
  • SCSI


  • Data is stored in magnetic domains
    • Magnetized area pointing "N" or "S"
    • Read heads can't tell which is "N" and which is "S"
      • If a bunch of "S" were in a row it would be hard to tell how many there were
      • They read flux reversals -- changes from "N" to "S" or "S" to "N"
    • Encodings prevent too many non-reversals in a row
  • FM - Frequency Modulation
    • 1.5 reversals per bit
    • 0 = RN, 1 = RR
  • MFM - Modified Frequency Modulation
    • 0.75 reversals per bit
  • RLL - Run Length Limited
    • 0.46 reversals per bit
    • 11 = RNNN, 10 = NRNN, ... 0011 = NNNNRNNN
  • PRML - Partial Response Maximum Likelihood
    • Statistical methods must be used, because flux reversals partially overlap
    • Samples analog signals instead of expecting discrete reversals


  • AT Attachment
    • Controller on the original IBM AT
      • Connected directly to the ISA bus
        • 16-bit data transfers
  • Most common interface - found on most PCs
    • 40-pin ribbon cable
      • Pin 1 marked with red stripe
    • 80-pin cable (but 40-pin connectors) for ATA/66 and faster
    • 44-pin connector for 2.5" notebook drives (4 added for power)
  • Lots of marketing names
    • IDE, EIDE, FastATA, UltraATA, Ultra/66, Ultra/100, Ultra/133
  • Master/Slave
    • Both drives on same data / control channel
    • Must configure drives with jumpers
    • Master drive controls the slave’s controller
    • Hurts performance

ATA (Continued)

  • ATA through ATA-6
    • 3.3 MB/s to 133 MB/s
    • Transfer modes
      • PIO - Programmed IO
        • CPU reads IO ports to get the data
        • PIO-1 to PIO-4: 3.3 MB/s to 16.7 MB/s
      • DMA - Direct Memory Access
        • DMA controller transfers memory
        • Bus Master - Drive controller transfers memory
        • DMA Mode 0 to Multiword DMA Mode 2: 2.1 MB/s to 16.7 MB/s
      • UltraDMA
        • Transfers on the clock edges (twice as often)
        • UDMA-0 to UDMA-6: 16.7 MB/s to 133 MB/s
  • ATAPI - ATA Packet Interface
    • Extension to allow CD-ROMs
  • SerialATA
    • Currently reaching the limits of parallel interface
    • Smaller cables (less signal lines)
    • No more master/slave


  • Small Computer Systems Interface
  • Various flavors
    • SCSI-1, SCSI-2, SCS-3
    • Fast, Ultra, Ultra2, Ultra3, Ultra160, Ultra320
    • 5 MB/s to 320 MB/s
    • Narrow, wide
      • 50-pin, 68-pin ribbon cable
    • SCA (84-pin connector)
      • Hot pluggable
    • FC-AL (Fibre-Channel Arbitrated Loop)

SCSI (Continued)

  • Higher performance than ATA
    • Optimized for server-type loads
      • Queues requests better than ATA
    • Allows access to more devices simultaneously
      • 8 / 16 (including controller)
  • Much higher price than ATA
    • You do get more features/performance for the higher price
    • Manufacturers rarely make the exact same drive with both interfaces
    • SCSI drives now tend to come in smaller capacities than ATA
  • Termination
    • Last device on each end of the channel must be properly terminated
    • Prevents signal from "bouncing back"
    • General must set jumpers or resistors
  • SCSI IDs
    • Each SCSI device must be set with a unique ID on the channel
    • Use jumpers, or auto-configure on new drives

Block Addressing

  • Data is accessed one block at a time
    • Blocks are also known as sectors
  • CHS - Cylinders / Heads / Sectors
  • LBA - Logical Block Addressing
    • Each block gets a single sequential number
  • BIOS Limitations
    • 528 MB (1024x16x63)
    • 8.4 GB (1024x256x63, 16383x16x63)
    • 137 GB (ATA 28-bit LBA)
    • 144 PB (ATA 48-bit LBA)
    • OS limits
      • DOS had 2 GB partition limit
      • Boot loaders require boot code within BIOS-accessible areas
      • File systems have limits, but are usually very large
    • Other limits are generally BIOS bugs

Partition Table

  • MBR (Master Boot Record)
    • First sector on the disk
    • Contains boot code and the partition table
    • MBR code actually defines the structure of the partition table
  • Partition table has only room for only 4 partitions
  • Primary partitions
    • Partition defined directly in the partition table
    • DOS/Windows/NT require boot partition to be a primary
  • Extended partitions
    • Takes up a primary partition slot
    • Contains a "sub" partition table
      • Still only 4 partitions, but extended partitions can be chained
    • Only one extended partition is allowed
  • Logical partitions
    • Partitions contained within extended partitions

Boot Process

  • BIOS loads MBR into memory and executes it
  • MBR code looks at partition table
  • MBR code determines "active" (bootable) partition
  • MBR code loads boot sector of active partition and executes it
  • Boot sector chain loads rest of boot loader
  • Boot loader loads the OS and transfers execution to it
  • OS loads drivers, which detect and initialize hardware
  • File system drivers allow access to files on the drive


  • Partitions divide the drive into logical "chunks"
  • Partitions contain file systems
  • First sector of a partition may hold the boot sector
  • BSD uses slices
    • Slice is what DOS calls the extended partition
    • Slice contains BSD partitions
  • Creating and modifying partitions
    • fdisk
    • cfdisk
    • GUI versions are distro-specific

Block Devices

  • Linux naming conventions
    • hda - entire ATA hard drive
    • sda - entire SCSI hard drive
    • hda1 - hda4 - primary partitions
    • hda5 - hda63 - logical partitions
  • FreeBSD naming conventions
    • sd0 - entire hard drive
    • sd0s1d - 4th partition (d) on 1st slice (s1) of first disk (sd0)
  • Boot loaders
    • Some use Linux conventions
    • Some use numbers instead of letters
    • Some start numbering at 0
  • /dev/rdsk
    • Raw disk interface (no buffering)
  • /dev/dsk
  • In OpenBSD slice 3 is the whole disk

File Systems

  • Logical structure of files and directories
  • Many to choose from:
    • Ext2, Ext3
    • ReiserFS, Reiser4
    • XFS, JFS
    • NTFS
    • FAT
      • FAT12, FAT16, FAT32
      • FAT, VFAT (long file names)
  • Creating a file system:
    • mkfs, mke2fs
  • Checking a file system:
    • fsck (file system check)


  • Spindle speed
    • Measured in RPMs: 4200, 5400, 7200, 10K
      • Faster drives have smaller platters, due to wobble and momentum problems
    • Faster means more data passes by the read/write heads in a given amount of time
  • Access / seek times
    • Seek - time to move from one track to another
    • Access - seek time plus latency for desired sector to come around
    • Full-stroke - from inner-most to outer-most track
    • Average - average time form one random track to another
    • Definitions not precisely agreed upon (Marketing Alert!)
  • Master / slave
    • Fastest devices should always be master
    • Avoid slaves if possible
    • Two devices that are used simultaneously should be on separate channels

I/O Performance

  • Transfer rates
    • Disk to controller
      • Depends a lot on physical characteristics
    • Controller to RAM
      • Most commonly used rating
      • You’ll never reach this rate in real life
        • Command overhead
        • Unlikely you'd ask for contiguous sectors that are cached on controller
  • PIO, DMA, UltraDMA
    • Make sure controller, device, and OS drivers all use fastest available
  • Buffers
    • Larger buffers can cache more data on the drive (2-8 MB typical)
    • Allows for read-ahead of sectors you may ask for next

OS-Level Performance

  • Use the appropriate file system
  • Partitioning schemes
    • Use read-only partitions when possible
    • Keep partitions that may fill up separate
    • Place commonly-used partitions first (inner part of drive)
  • Tune the file system
  • Tune the controller / driver
    • hdparm
    • Make sure block transfers are as large as possible
    • Make sure DMA is enabled
    • Make sure fastest DMA/UltraDMA mode is used


  • Redundant Array of Inexpensive Disks
  • Level 0
    • Striping (no redundancy)
    • Related: spanning, JBOD
  • Level 1
    • Mirroring
  • Level 5
    • Distributed parity disk
  • Level 10
    • 1+0, not a "real" level


  • Storage access is split into several layers to abstract various functions
  • Hard drives have gained more built-in intelligence
    • Removes complexity from the computer and OS
    • Allows added features, such as bad block remapping
    • Increases performance (buffering, etc.)
  • BIOS controls the initialization of the hard drive
    • Transfers control to the OS in several steps
  • SCSI vs. ATA
    • Depends what your goals are
  • Partitions divide up a drive into logical chunks
    • Partitions contain file systems
  • File systems organize files and directories





Presentation Info