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Module 5.0: I/O and Disks

• I/O hardware

• Polling and Interrupt-driven I/O

• DMA

• Bock and character devices

• Blocking and nonblocking I/O

• Secondary storage

– Disk Hardware

– Disk Scheduling

– RAID

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I/O Hardware

Incredible variety of I/O devices

Common concepts Port: serial, parallel, usb Bus (daisy chain or shared direct access)

PCI bus, IDE Bus SCSI (up to 16 devices)

Controller (host adapter)

I/O instructions control devices

Devices have addresses, used by Direct I/O instructions Memory-mapped I/O

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A Typical PC Bus Structure

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Polling vs. Interrupt-driven I/O

Polling or (Programmed I/O): Determines state of device

command-ready busy error

Busy-wait cycle to wait for I/O from device

Interrupt-driven (covered earlier)

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Interrupt-driven I/O Cycle

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Direct Memory Access Used to avoid programmed I/O (or polling) for large data movement

Requires DMA controller

Bypasses CPU to transfer data directly between I/O device and memory

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Application I/O Interface

I/O system calls encapsulate device behaviors in generic classes

Device-driver layer hides differences among I/O controllers from kernel

Devices vary in many dimensions Character-stream or block Sequential or random-access Sharable or dedicated Speed of operation read-write, read only, or write only

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A Kernel I/O Structure

ATAPI: Advanced Technology Attachment Packet Interface (for mass storage devices)

SCSI: Small Computer Standard Interface

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Block and Character Devices

Block devices include disk drives Information is stored in fixed-size blocks, each block with its own

address Commands include read, write, seek file-system access Memory-mapped file access possible

Character devices include keyboards, mice, serial ports, printers A stream of characters is sent or received without regard to any

block structure Commands include get, put

Other Devices Varying enough from block and character to have own interface E.g., Network devices, clocks, timers, etc.

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Blocking and Nonblocking I/O

• Blocking - process suspended until I/O completed– Easy to use and understand– Insufficient for some needs

• Nonblocking - I/O call returns as much as available– User interface, data copy (buffered I/O)– Extremely useful and popular with ULTs

Does not block a ULT, and thus does not prevent other ULTS from running

– Returns quickly with count of bytes read or written– Example search in Mozilla FireFox

• Asynchronous - process runs while I/O executes– Difficult to use– I/O subsystem signals process when I/O completed– Used in C#

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Secondary Storage

Secondary Storage is often arranged in a hierarchy transparent to the user with fast/expensive storage at the top and slow/cheap storage at the bottom.

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Disk Definitions

Volume – a single storage unit, e.g. CD, diskette, tape, partition

Tracks – multiple concentric circles containing data

Sectors – subsection of a track

Blocks – subsection of a sector created during formatting (512-4096B)

Cylinder – all tracks of same diameter in the disk pack

Fixed heads – one r/w head per track

Moveable heads – one r/w head per surface

Access time = seek time + latency time (15-60ms) + block transfer time (1-2ms)

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Disk Hardware

Seek time -- time to get r/w head to desired track

Rotation/latency delay – wait time for desired data to spin under r/w head

Block transfer time – time to read or write block of data

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Disk Structure

Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer.

The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially. Sector 0 is the first sector of the first track on the outermost

cylinder. Mapping proceeds in order through that track, then the rest

of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost. (assuming no interleaving).

– Block interleaving is used to allow some time for the disk controller to transfer data to memory. The controller can not keep up with the continuous bit streams from the disk.

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Disk Scheduling

The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth.

Minimize seek time. The other two factors are fixed. Reason for differences in performance

Seek time seek distance

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.

The idea is to improve both access time and bandwidth by scheduling the servicing of the disk I/O in a good order.

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Disk Scheduling (Cont.)

• Several algorithms exist to schedule the servicing of disk I/O requests.

• We illustrate them with a request queue (0-199).

98, 183, 37, 122, 14, 124, 65, 67

Head pointer 53

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FCFS

Illustration shows total head movement of 640 cylinders.

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SSTF – Shortest Seek Time First

Selects the request with the minimum seek time from the current head position.

SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests.

Illustration shows total head movement of 236 cylinders.

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SCAN

The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues.

Sometimes called the elevator algorithm.

Illustration shows total head movement of 208 cylinders.

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C-SCAN

Provides a more uniform wait time than SCAN.

The head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip.

Treats the cylinders as a circular list that wraps around from the last cylinder to the first one.

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C-LOOK

Version of C-SCAN

Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk.

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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.

The disk-scheduling algorithm should be written as a separate module of the operating system, allowing it to be replaced with a different algorithm if necessary.

Either SSTF or LOOK is a reasonable choice for the default algorithm.

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Disk Management

• Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write.

– Each sector/block (256,512,1024) has header, trailer, & data

– Header contains sector number

– Trailer contains ECC (Error-Correcting Code)

• To use a disk to hold files, the operating system still needs to record its own data structures on the disk.

– Partition the disk into one or more groups of cylinders.

– Logical formatting or “making a file system” Holds data structure for NTFS or FAT

• Boot block initializes system.

– The bootstrap is stored in ROM.

– Bootstrap loader program.

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Booting from a Disk in Windows 2000

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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.

• Swap-space management

– 4.3BSD allocates swap space when process starts; holds text segment (the program) and data segment.

– Kernel uses swap maps to track swap-space use.

– Solaris 2 allocates swap space only when a page is forced out of physical memory, not when the virtual memory page is first created.

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Disk Performance and Reliability

Technology to improving processor performance is higher than improving secondary storage.

Several improvements in disk-use techniques involve the use of multiple disks working cooperatively. RAID technology, Redundant Array of Independent Disks.

RAID schemes improve performance and improve the reliability of the storage system by storing redundant data.

Set of physical disk drives viewed by the OS as a single logical drive

Data is distributed across the physical drives of an array Redundant disk capacity is used to store parity

information Seven RAID configuration levels (RAID level 0 to RAID

level 6).

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Data Mapping for RAID level 0

RAID 0 is non-redundant, the other 6 levels are.

Data is striped across available disks

High data transfer rate Strips 0, 1, 2, and

3 can be accessed in parallel

High I/O request rate Servicing multiple

I/O requests in parallel, e.g., requests for strip 0 and strip 3 can be serviced simultaneously.

strip 0

strip 4

strip 8

strip 12

strip 1

strip 5

strip 9

strip 13

strip 2

strip 6

strip 10

strip 14

strip 3

strip 7

strip 11

strip 15

strip 0

strip 1

strip 2

strip 3

strip 4

strip 15

strip 14

strip 13

strip 12

strip11

strip 10

strip 9

strip 8

strip 7

strip 6

strip 5

ArrayManagement

Software

Logical Disk

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RAID level 1

Redundancy is achieved using mirroring (duplication of data on other disks.)

Expensive

A read can be serviced by either disks (the least access time)

A write requires updating two strips

Recovery is simple

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RAID level 2

All member disks participate in the execution of every I/O request. (Gives high transfer rate but not I/O request rate).

Spindles and heads are all synchronized to the same position

Strips are very small, a single byte or a word

Redundancy is done through Hamming Code (able to correct single-bit error and detect double bit errors). Requires smaller number of disks compared to RAID level 1 RAID level 3 requires only one extra disk. It uses parity checks.

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RAID Levels