CCE 317: Operating Systems - Mass Storage Structure

Mass Storage Structure

• Bulk of secondary storage for modern computers is Hard Disk Drives (HDDs) and Nonvolatile Memory (NVM) devices
• HDDs consist of spinning platters of magnetically-coated material under moving read-write heads
• Drives rotate at 60 to 250 times per second
• Transfer rate is the rate at which data flows between the drive and the computer
• Positioning time (random-access time) consists of seek time (time to move disk arm to desired cylinder) and rotational latency (time for desired sector to rotate under the disk head)

Hard Disk Performance

• Transfer rate: theoretical - 6 Gb/sec, effective - 1 Gb/sec
• Seek time: 3ms to 12ms, with 9ms being common for desktop drives
• Latency: based on spindle speed, calculated as 1 / (RPM / 60) = 60 / RPM
• Average latency: half of the latency
• Access latency: average access time = average seek time + average latency
• Average I/O time: average access time + (amount to transfer / transfer rate) + controller overhead

Solid-state Disks

• Nonvolatile memory used like a hard disk
• More reliable than HDDs
• More expensive per MB
• May have a shorter life span, requiring careful management
• Much faster than HDDs
• No moving parts, so no seek time or rotational latency

Disk Attachment

• Host-attached storage accessed through I/O ports talking to I/O buses
• Several buses available, including ATA, SATA, eSATA, SAS, USB, and FC
• Data transfers on a bus carried out by special electronic processors called controllers (or host-bus adapters, HBAs)
• Host controller on the computer end of the bus, device controller on device end

Address Mapping

• Disk drives are addressed as large 1-dimensional arrays of logical blocks
• Logical block is the smallest unit of transfer
• Low-level formatting creates logical blocks on physical media
• The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially
• Logical to physical address should be easy, except for bad sectors

HDD Scheduling

• The operating system is responsible for using hardware efficiently
• Minimize seek time
• Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer
• I/O request includes input or output mode, disk address, memory address, number of sectors to transfer
• OS maintains a queue of requests, per disk or device
• Idle disk can immediately work on I/O request, busy disk means work must queue

Scheduling Algorithms

• FCFS (First-Come-First-Served): total head movement of 640 cylinders
• SSTF (Shortest-Seek-Time-First): selects the request with the least seek time from the current head position
• SCAN (Elevator Algorithm): disk arm starts at one end of the disk, moves toward the other end, servicing requests until it gets to the other end, where the head movement is reversed
• C-SCAN: provides a more uniform wait time than SCAN, treats the cylinders as a circular list that wraps around from the last cylinder to the first one

Selecting a Disk-Scheduling Algorithm

• SSTF is common and has a natural appeal
• SCAN and C-SCAN perform better for systems that place a heavy load on the disk
• Performance depends on the number and types of requests
• Requests for disk service can be influenced by the file-allocation method and metadata layout

Storage Device Management

• Low-level formatting: dividing a disk into sectors that the disk controller can read and write
• Partitioning the disk into one or more groups of cylinders, each treated as a logical disk
• Logical formatting or “making a file system”
• Most file systems group blocks into clusters to increase efficiency
• Disk I/O is done in blocks, file I/O is done in clusters

Swap-Space Management

• Swap-space: virtual memory uses disk space as an extension of main memory
• Swap-space can be carved out of the normal file system, or, more commonly, it can be in a separate disk partition
• File data is written to swap space until a write to the file system is requested
• Other dirty pages go to swap space due to no other home
• Text segment pages are thrown out and reread from the file system as needed

This quiz covers the basics of mass storage structure, including hard disk drives and nonvolatile memory devices, their components, and performance metrics.