Buffer Cache

7/24/2019 Buffer Cache

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UNIX Kernel Architecture

libraries

User level

Kernel level

User programs

hardware

Kernel level

Hardware level

trap

Sytem call interface

File subsytem

Buffer cache

Chracter block

Device drivers

Hardware control

Inter-processcommunication

scheduler

Memory

management

Process

control

sybsystem


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System Calls System call are similar to ordinary functions

Libraries map these function call to primitivesneeded to enter OS

Assembly programs can directly invoke system

call System calls can be categorized into :

File subsystem related calls

Process control system related calls

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File subsystem related calls

Allocating file space

Free space management

File access control

open, close read, write, stat, chown, chmode etc

Process control system related calls

Process synchronization

Interprocess communication

Memory management

Process scheduling

Fork, exec, wait, exit etc. 3


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File system layout

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Boot block Super block Inode list Data blocks

Boot block can contain the boot strap code

A file system may have an empty bootblock

describe state of file system

Size, number file it can store, location of freespace etc


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Buffering

File subsystem accesses file data usingbuffering mechanism

The mechanism regulates the flow between

buffer and secondary storage The mechanism interacts with block i/o

device driver to initiate the data transfer

block I/O devices are random accessstorage devices

Device drivers are kernel module that

controls the device operation 5


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Buffer Cache

When a process wants to access data from a file,

the kernel brings the data into main memory,alters it and then request to save in the file

system

Buffering increases the response time ,throughput,

Buffering minimizes the frequency of disk

access by keeping a pool of internal data buffercalled buffer cache.


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Buffer Cache

Buffer cache contains the data in recently useddisk blocks

When reading data from disk, the kernel

attempts to read from buffer cache. If data is already in the buffer cache, the kernel

does not need to read from disk

If data is not in the buffer cache, the kernelreads the data from disk and cache it


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Buffer Headers

A buffer consists of two partsa memory array

buffer header

disk block : buffer = 1 : 1

Figure 3.1 Buffer Header

device num

block num

status

ptr to next buf on hash queue

ptr to previous buf on hash queue

ptr to next buf on free list

ptr to previous buf on free list

ptr to data area


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Buffer Headers

device num logical file system number

block num

block number of the data on disk

status

The buffer is currently locked.

The buffer contains valid data.

delayed-write

The kernel is currently reading or writing the contents of buffer to disk.

A process is currently waiting for the buffer to become free.

kernel identifies the buffer content by examing device num andblock num.


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Structures of the buffer pool

Buffer pool according to LRU

The kernel maintains a free list of buffer

doubly linked list

take a buffer from the head of the free list.

When returning a buffer, attaches the buffer to the

tail.

free list

head

buf 1 buf 2 buf n

Forward ptrs

Back ptrs

Figure 3.2 Free list of Buffers


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Structures of the buffer pool

When the kernel accesses a disk block

separate queue (doublely linked circular list)

hashed as a function of the device and block num

Every disk block exists on one and only on hash

queue and only once on the queue

4

517

105098

99353

28 64

97

blkno0 mod 4

blkno1 mod 4

blkno2 mod 4

blkno3 mod 4

Hash queue headers

Figure 3.3 Buffers on the Hash Queues


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Scenarios for retrieval of a buffer

Determine the logical device num and block num The algorithms for reading and writing disk blocks use

the algorithmgetblk

The kernel finds the block on its hash queue

The buffer is free.

The buffer is currently busy.

The kernel cannot find the block on the hash queue

The kernel allocates a buffer from the free list.

In attempting to allocate a buffer from the free list, finds a buffer onthe free list that has been marked delayed write.

The free list of buffers is empty.


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Retrieval of a Buffer:1stScenario (a)

The kernel finds the block on the hash queue and its buffer isfree

4

517

105098

99353

28 64

97

blkno0 mod 4

blkno1 mod 4

blkno2 mod 4

blkno3 mod 4

Hash queue headers

freelist header

Search for block 4


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Retrieval of a Buffer:1stScenario (b)

4

517

105098

99353

28 64

97

blkno0 mod 4

blkno1 mod 4

blkno2 mod 4

blkno3 mod 4

freelist header

Remove block 4 from free list


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Retrieval of a Buffer: 2ndScenario (a)

The kernel cannot find the block on the hash queue, so it

allocates a buffer from free list

4

517

105098

99353

28 64

97

blkno0 mod 4

blkno1 mod 4

blkno2 mod 4

blkno3 mod 4

Hash queue headers

freelist header

Search for block 18: Not in cache


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Retrieval of a Buffer: 2ndScenario (b)

Hash queue headers

18

4

517

105098

9935

28 64

97

blkno0 mod 4

blkno1 mod 4

blkno2 mod 4

blkno3 mod 4

freelist header

Remove 1stblock from free list: Assign to 18


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Retrieval of a Buffer: 3rdScenario (a)

The kernel cannot find the block on the hash queue, and finds delayed write

buffers on hash queue

4

517

105098

99353

28 64

97

blkno0 mod 4

blkno1 mod 4

blkno2 mod 4

blkno3 mod 4

Hash queue headers

freelist header

Search for block 18, Delayed write blocks on free list

delay

delay


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Retrieval of a Buffer: 3rdScenario (b)

517

105098

99353

28 64

97

blkno0 mod 4

blkno1 mod 4

blkno2 mod 4

blkno3 mod 4

Hash queue headers

freelist header

18

writing

writing

(b) Writing Blocks 3, 5, Reassign 4 to 18

Figure 3.8


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Retrieval of a Buffer: 4th Scenario

The kernel cannot find the buffer on the hash queue, and the free list is empty

28blkno0 mod 4

blkno1 mod 4

blkno2 mod 4

blkno3 mod 4

Hash queue headers

freelist header

4 64

5 9717

105098

99353

Search for block 18, free list empty


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Retrieval of a Buffer: 5th Scenario

Kernel finds the buffer on hash queue, but it is currently busy

4

517

105098

99353

28 64

97

blkno0 mod 4

blkno1 mod 4

blkno2 mod 4

blkno3 mod 4

Hash queue headers

freelist header

Search for block 99, block busy

busy


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File System Algorithms

Lower Level File system lgorithms

namei

alloc free ialloc ifree

iget iput

buffer allocation algorithms

getblk brelse bread bwrite


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Inode

contains the information necessary for a process to

access a file

exits in a static form on disk and the kernel reads

them into an in-core inode


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Inodes

consists of- file owner identifier

- file type

- file access permissions

- file access times

- number of links to the file

- table of contents for the disk address of data in afile

- file size


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Inodes in-core copy of the inode contains

- status of the in-core inode indicating

- logical device number of file system

- inode number

- pointers to other in-core inodes

- reference count


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inode contains the table of content to locate a file

data

Since each block on a disk is addressable bynumber , the table of content consists of set of

disk block number

If file were stored in contiguous manner, startblock number and file size would have been

enough .

This strategy would not allow expansion/ contraction

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File data may not be necessarily be stored in

contiguous manner

This would enable file to be of any size and can be

easily expanded or contracted

This would require basic unit of allocation unit to be a

blockThe table of content would set of block number

The size of table of content will vary with size of file

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Structure of a regular file

direct0

direct1

direct2

direct3

direct4

direct5

direct6

direct7

direct8

direct9

single indirect

double indirect

triple indirect

Inode Data Blocks


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Structure of a regular file

Assume that a logical block on the file system holds 1K bytes,

A block number is addressable by a 32 bit integer, then a block can hold up to 256block numbers

10 direct blocks with 1K bytes each=10K bytes

1 indirect block with 256 direct blocks= 1K*256=256K bytes

1 double indirect block with 256 indirect blocks=256K*256=64M bytes

1 triple indirect block with 256 double indirect blocks=64M*256=16G bytes

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