CCE 317: Operating Systems - Mass Storage Structure

Mass Storage Structure

• The bulk of secondary storage for modern computers is hard disk drives (HDDs) and nonvolatile memory (NVM) devices.
• HDDs spin platters of magnetically-coated material under moving read-write heads.
• Drives rotate at 60 to 250 times per second.
• The transfer rate is the rate at which data flow between the drive and computer.
• Positioning time (random-access time) is the time to move the disk arm to the desired cylinder (seek time) and time for the desired sector to rotate under the disk head (rotational latency).
• A head crash results from the disk head making contact with the disk surface, which is bad.

Hard Disk Performance

• Theoretical transfer rate is 6 Gb/sec, while effective transfer rate is 1 Gb/sec.
• Seek time ranges from 3ms to 12ms, with 9ms being common for desktop drives.
• Latency is based on spindle speed, calculated as 60 / RPM.
• Average latency is half of the latency.
• Access latency is the average access time, which is the sum of average seek time and average latency.
• Average I/O time is the sum of access time, transfer time, and controller overhead.

Solid-state Disks

• Solid-state disks use nonvolatile memory like a hard disk.
• They can be more reliable than HDDs but are more expensive per MB.
• They have a shorter lifespan and require careful management.
• They are much faster and have no moving parts, so no seek time or rotational latency.

Disk Attachment

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

Address Mapping

• Disk drives are addressed as large 1-dimensional arrays of logical blocks.
• 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.

HDD Scheduling

• The operating system is responsible for using hardware efficiently, including minimizing seek time and maximizing disk bandwidth.
• There are many sources of disk I/O requests, including the OS, system processes, and user processes.
• I/O request includes input or output mode, disk address, memory address, and number of sectors to transfer.
• The OS maintains a queue of requests, per disk or device.
• Idle disk can immediately work on I/O requests, while a busy disk means work must queue.

Scheduling Algorithms

• FCFS (First-Come-First-Served) algorithm has a total head movement of 640 cylinders.
• SSTF (Shortest-Seek-Time-First) algorithm selects the request with the least seek time from the current head position.
• SCAN (Elevator Algorithm) starts at one end of the disk, moves toward the other end, servicing requests until it gets to the other end, and then reverses direction.
• C-SCAN provides a more uniform wait time than SCAN, moving from one end of the disk to the other, servicing requests as it goes, and then immediately returning to the beginning of the disk.

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, or physical formatting, divides a disk into sectors that the disk controller can read and write.
• To use a disk to hold files, the operating system still needs to record its own data structures on the disk.

Swap-Space Management

• Swap-space is virtual memory that uses disk space as an extension of main memory.
• Swap-space can be carved out of the normal file system or in a separate disk partition.

Mass Storage Structure

• The bulk of secondary storage for modern computers is hard disk drives (HDDs) and nonvolatile memory (NVM) devices.
• HDDs spin platters of magnetically-coated material under moving read-write heads.
• Drives rotate at 60 to 250 times per second.
• The transfer rate is the rate at which data flow between the drive and computer.
• Positioning time (random-access time) is the time to move the disk arm to the desired cylinder (seek time) and time for the desired sector to rotate under the disk head (rotational latency).
• A head crash results from the disk head making contact with the disk surface, which is bad.

Hard Disk Performance

• Theoretical transfer rate is 6 Gb/sec, while effective transfer rate is 1 Gb/sec.
• Seek time ranges from 3ms to 12ms, with 9ms being common for desktop drives.
• Latency is based on spindle speed, calculated as 60 / RPM.
• Average latency is half of the latency.
• Access latency is the average access time, which is the sum of average seek time and average latency.
• Average I/O time is the sum of access time, transfer time, and controller overhead.

Solid-state Disks

• Solid-state disks use nonvolatile memory like a hard disk.
• They can be more reliable than HDDs but are more expensive per MB.
• They have a shorter lifespan and require careful management.
• They are much faster and have no moving parts, so no seek time or rotational latency.

Disk Attachment

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

Address Mapping

• Disk drives are addressed as large 1-dimensional arrays of logical blocks.
• 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.

HDD Scheduling

• The operating system is responsible for using hardware efficiently, including minimizing seek time and maximizing disk bandwidth.
• There are many sources of disk I/O requests, including the OS, system processes, and user processes.
• I/O request includes input or output mode, disk address, memory address, and number of sectors to transfer.
• The OS maintains a queue of requests, per disk or device.
• Idle disk can immediately work on I/O requests, while a busy disk means work must queue.

Scheduling Algorithms

• FCFS (First-Come-First-Served) algorithm has a total head movement of 640 cylinders.
• SSTF (Shortest-Seek-Time-First) algorithm selects the request with the least seek time from the current head position.
• SCAN (Elevator Algorithm) starts at one end of the disk, moves toward the other end, servicing requests until it gets to the other end, and then reverses direction.
• C-SCAN provides a more uniform wait time than SCAN, moving from one end of the disk to the other, servicing requests as it goes, and then immediately returning to the beginning of the disk.

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, or physical formatting, divides a disk into sectors that the disk controller can read and write.
• To use a disk to hold files, the operating system still needs to record its own data structures on the disk.

Swap-Space Management

• Swap-space is virtual memory that uses disk space as an extension of main memory.
• Swap-space can be carved out of the normal file system or in a separate disk partition.

This quiz covers the structure of mass storage devices, including hard disk drives and nonvolatile memory devices, and explains key concepts such as transfer rate and positioning time.