I/O Management and Disk Scheduling Chapter 11. I/O Driver OS module which controls an I/O device hides the device specifics from the above layers in the.

I/O Management and Disk Scheduling

Chapter 11

I/O Driver• OS module which controls an I/O device• hides the device specifics from the above layers in the OS/kernel • translates logical I/O into device I/O (logical disk blocks into {track, head, sector})

performs data buffering and scheduling of I/O operations structure: several synchronous entry points (device

initialization, queue I/O requests, state control, read/write) and an asynchronous entry point (to handle interrupts)

Typical driver structuredriver_strategy(request)

{

if (empty(request-queue))

driver_start(request)

else

add(request, request-queue)

block_current_process; reschedule()

}

driver_start(request) {

current_request= request;

start_dma(request);

}

driver_ioctl(request) {

}

driver_init() {

}

driver_interrupt(state) /* asynchronous part */

{

if (state==ERROR) && (retries++<MAX) {

driver_start(current_request);

return;

}

add_current_process_to_active_queue

if (! (empty(request_queue))

driver_start(get_next(request_queue))

}

User to Driver Control Flow

user

kernel

read, write, ioctl

special file ordinary file

File System

Buffer Cache

blockdevice

characterdevice

Character queue

driver_read/write driver-strategy

I/O Buffering

• before an I/O request is placed the source/destination of the I/O transfer must be locked in memory• I/O buffering: data is copied from user space to kernel buffers which are pinned to memory• works for character devices (terminals), network and disks• buffer cache: a buffer in main memory for disk sectors• character queue: follows the producer/consumer model (characters in the queue are read once)• unbuffered I/O to/from disk (block device): VM paging for instance

Buffer Cache

• when an I/O request is made for a sector, the buffer cache is checked first

• if it is missing from the cache, it is read into the buffer cache from the disk

• exploits locality of reference as any other cache

• usually replacements done in chunks (a whole track can be written back at once to minimize seek time)

• replacement policies are global and controlled by the kernel

Replacement policies

• buffer cache organized like a stack: replace from the bottom

• LRU: replace the block that has been in the cache longest with no reference to it (on reference a block is moved to the top of the stack)

• LFU: replace the block with the fewest references (counters which are incremented on reference and blocks move accordingly)

• frequency-based replacement: define a new section on the top of the stack, counter is unchanged while the block is in the new section

Least Recently Used

• The block that has been in the cache the longest with no reference to it is replaced

• The cache consists of a stack of blocks• Most recently referenced block is on the

top of the stack• When a block is referenced or brought

into the cache, it is placed on the top of the stack

Least Frequently Used

• The block that has experienced the fewest references is replaced

• A counter is associated with each block• Counter is incremented each time block

accessed• Block with smallest count is selected for

replacement• Some blocks may be referenced many times in

a short period of time and then not needed any more

Application-controlled File Caching• two-level block replacement: responsibility is split between kernel and user level• a global allocation policy performed by the kernel which decides which process will give up a block• a block replacement policy decided by the user:

kernel provides the candidate block as a hint to the process

the process can overrule the kernel’s choice by suggesting an alternative block

the suggested block is replaced by the kernel examples of alternative replacement policy: most-recently

used (MRU)

Sound kernel-user cooperation• oblivious processes should do no worse than under LRU• foolish processes should not hurt other processes• smart processes should perform better than LRU whenever possible and they should never perform worse

if kernel selects block A and user chooses B instead, the kernel swaps the position of A and B in the LRU list and places B in a “placeholder” which points to A (kernel’s choice)

if the user process misses on B (i.e. he made a bad choice), and B is found in the placeholder, then the block pointed to by the placeholder is chosen (prevents hurting other processes)

Disk Performance Parameters

• To read or write, the disk head must be positioned at the desired track and at the beginning of the desired sector

• Seek time– time it takes to position the head at the

desired track

• Rotational delay or rotational latency– time its takes for the beginning of the

sector to reach the head

Disk Performance Parameters

• Access time– Sum of seek time and rotational delay– The time it takes to get in position to read

or write

• Data transfer occurs as the sector moves under the head

Disk I/O Performance• disks are at least four orders of magnitude slower than the main memory• the performance of disk I/O is vital for the performance of the computer system as a whole• disk performance parameters

seek time (to position the head at the track): 20 ms rotational delay (to reach the sector): 8.3 ms transfer time: 1-2 MB/sec

access time (seek time+ rotational delay) >> transfer time for a sector

therefore the order in which sectors are read matters a lot

Disk Scheduling Policies

• Seek time is the reason for differences in performance

• For a single disk there will be a number of I/O requests

• If requests are selected randomly, we will get the worst possible performance

Disk Scheduling Policies

• First-in, first-out (FIFO)– Process request sequentially– Fair to all processes– Approaches random scheduling in

performance if there are many processes

Disk Scheduling Policies

• Priority– Goal is not to optimize disk use but to meet

other objectives– Short batch jobs may have higher priority– Provide good interactive response time

Disk Scheduling Policies

• Last-in, first-out– Good for transaction processing systems

• The device is given to the most recent user so there should be little arm movement

– Possibility of starvation since a job may never regain the head of the line

Disk Scheduling Policies

• Shortest Service Time First– Select the disk I/O request that requires the

least movement of the disk arm from its current position

– Always choose the minimum Seek time

Disk Scheduling Policies

• SCAN– Arm moves in one direction only, satisfying

all outstanding requests until it reaches the last track in that direction

– Direction is reversed

Disk Scheduling Policies

• C-SCAN– Restricts scanning to one direction only– When the last track has been visited in one

direction, the arm is returned to the opposite end of the disk and the scan begins again

Disk Scheduling Policies

• N-step-SCAN– Segments the disk request queue into

subqueues of length N– Subqueues are process one at a time, using

SCAN– New requests added to other queue when

queue is processed

• FSCAN– Two queues– One queue is empty for new request

Disk Scheduling Policies• usually based on the position of the requested sector rather than according to the process priority• shortest-service-time-first (SSTF): pick the request that requires the least movement of the head• SCAN (back and forth over disk): good distribution• C-SCAN(one way with fast return):lower service variability but head may not be moved for a considerable period of time• N-step SCAN: scan of N records at a time by breaking the request queue in segments of size at most N• FSCAN: uses two subqueues, during a scan one queue is consumed while the other one is produced

RAID• Redundant Array of Independent Disks (RAID)• idea: replace large-capacity disks with multiple smaller-capacity drives to improve the I/O performance• RAID is a set of physical disk drives viewed by the OS as a single logical drive• data are distributed across physical drives in a way that enables simultaneous access to data from multiple drives• redundant disk capacity is used to compensate the increase in the probability of failure due to multiple drives• size RAID levels (design architectures)

RAID Level 0• does not include redundancy• data is stripped across the available disks

disk is divided into strips strips are mapped round-robin to consecutive disks a set of consecutive strips that map exactly one

strip to each array member is called stripe

strip 0 strip 3strip 2strip 1

strip 7strip 6strip 5strip 4

...

RAID Level 1• redundancy achieved by duplicating all the data• each logical disk is mapped to two separate physical disks so that every disk has a mirror disk that contains the same data

a read can be serviced by either of the two disks which contains the requested data (improved performance over RAID 0 if reads dominate)

a write request must be done on both disks but can be done in parallel

recovery is simple but cost is highstrip 0 strip 1strip 0strip 1

strip 3strip 2strip 3strip 2

...

RAID Levels 2 and 3

...

• parallel access: all disks participate in every I/O request• small strips (byte or word size)• RAID 2: error correcting code (Hamming) is calculated across corresponding bits on each data disk and stored on log(data) parity disks; necessary only if error rate is high• RAID 3: a single redundant disk which keeps the parity bit

P(i) = X2(i) + X1(i) + X0(i)• in the event of failure, data can be reconstructed but only one request at the time can be satisfied

b0 b1 b2 P(b) X2(i) = P(i) + X1(i) + X0(i)

RAID Levels 4 and 5

strip 0 P(0-2)strip 2strip 1

P(3-5)strip 5strip 4strip 3

• independent access: each disk operates independently, so multiple I/O request can be satisfied in parallel• large strips• RAID 4: for small writes: 2 reads + 2 writes

example: if write performed only on strip 0:

P’(i) = X2(i) + X1(i) + X0’1(i) =

X2(i) + X1(i) + X0’(i) + X0(i) + X0(i) =

P(i) + X0’(i) + X0(i)

• RAID 5: parity strips are distributed across all disks

UNIX SVR4 I/O