Devices

Solid-State Drive (SSD)

• based on DRAM or NAND/NOR flash memory. no mechanical parts.
• NAND flash memory
  • organisation: cells, pages, blocks
  • controller can read/program by page
  • controller can erase by block
  • random/sequential access latency is comparable, but more efficient at reading than writing.

Hard Disk (HDD)

• platters and cylinders in a disk, as many tracks as heads. each track has N sectors
• addressing modes:
  • CHS (cylinder-head-sector), virtual/physical
  • LBA (logical block addressing), only virtual
• in order to transfer data, you have to seek, rotate, and actually transfer. this adds up.
• so how do you optimise it?
  • aaand we’re back to scheduling. this time we’re scheduling the disk arm.
    • first come first serve: do requests in order of arrival. but that’s not actually an optimisation.
    • shortest seek first: do requests with a shortest seek time. but that has to be calculated.
    • elevator algorithm: go up the disk, then down the disk, then back up the disk…do requests along the way.
• error handling
  • programming errors (if you’re requesting a nonexistent sector)
  • transient errors (e.g. shit getting on the head of the arm)
  • permanent errors (if a disk block is physically damaged)
  • seek errors (the arm went to the wrong place)
  • controller errors (controller is a lil bitch and stops accepting commands)
  • disk track with bad sector
    • in this case you can remap
    • switch spare for bad sector, or shift all sectors to bypass the bad one

Clock

• Hardware function
  • simple clocks send hardware interrupts every voltage cycle of power supply
  • advanced clocks have own crystal oscillator which decrements a counter in a register. send hardware interrupt every time the counter is 0
• Software function
  • Maintains time of day (real time) — system boot time (backup cock) + uptime (ticks)
  • Stops processes from running too long — on every tick, decrement current process’ quantum
  • Accounts for CPU usage — on every tick, increment current process’ CPU time
  • Handle alarm syscall — on every tick, decrement alarm counter
  • Give the system watchdog/software timers — generate system notifications for synchronous alarms
  • Profile, monitor, gather statistics — on every tick, increment some set of counters