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