© Original by Donald Acton; Changes by George Tsiknis

Unix File System

The Structure of the Unix file system is not covered in the text

Unit 13 2

Learning Objectives We have seen the physical characteristics of a disk

and how data is transferred between disk and memory

Now we’ll see how OS organizes the data on the disk. You should be able to:draw a picture of the structure of a file in terms of inodes

and data blocks given its sizecompare the structure of two files given their sizes list the actions performed when a file is created, modified

or deletedestimate the number of read/write operations OS has to

perform in order to read/write data from/to a file identify caching as a way to improve I/O performance

Unit 13 3

The File System

Works directly with the view of the disk provided by the controller

Provides a device independent view of a disk

Further levels of abstraction on file system are what the application sees

Application

Unix I/O

File System

Disk Drive

File System

Layering

Unit 13 4

Purpose of a file system

Provides an abstraction so that applications don’t access disk directly

Hides complexity of different drive types Manages the device Provides a structure for organizing the data Provides protection

Unit 13 5

File System Properties

Persistent – survive reboots Correct – reflect the operations performed Robust – must be able to recover after a crash Efficient – must make good use of disk space

and must be fast

Unit 13 6

Unix File System Semantics

A file is simply a sequence of m bytesB0, B1, .... , Bk , .... , Bm-1

Everything is a fileAll I/O devices are represented as filesExamples:

– /dev/ad0s4e (/var disk partition)– /dev/ttyd0 (terminal)– /dev/rmt0 (tape drive)

Even the running kernel is represented as a file:– /dev/kmem

Unit 13 7

Unix File Types

Regular files: Binary/text files they are all the same

Directory file It’s still a file but it contains names and locations of other files

Special files Block (disks), character (terminal) devices

Interprocess communication Named pipes Sockets

Unit 13 8

Blocks – the basic unit

A file is a sequence of bytes stored in a set of fixed sized blocks

Block sizes are multiples of the sector size (e.g. 512, 1024, 4096, 8192)

The sectors comprising a particular block are contiguous All blocks in a particular file system are the same size With respect to a file, the smallest amount of data the

file system transfers to or from a disk is a block Blocks are the virtualization of a disk’s logical blocks

Unit 13 9

File System Primary Services

FS Must be able to:Keep track of which blocks are free

Determine which blocks belong to a particular file

Maintain administrative data like file permissions,

creation and modified times, etc.

Find the list of free blocks, root directory, and some

“other stuff” when the system is started

Unit 13 10

The Primary Pieces

Super block – helps locate everything

Inode – maintains the file’s administrative data,

tracks the file’s data blocks

Data blocks – multipurpose blocks used for File’s data blocks

Indirect blocks

Unit 13 11

File System Data Structures

Inode list – list of free inodes (inodes are just

numbers)

Inode map – given an inode can locate the disk

block the inode is insometimes this is just a formula

Free block list

Superblock

Unit 13 12

Single Indirect Block

UNIX InodeType/mode Link count

File Size

UID GID

Various modification

access and creation

times

Direct Data Block 0

Direct Data Block 1

Direct Blocks 2 - 9

Single Indirect Block

Double Indirect Block

Additional bookkeeping

information

Data Block

Data Block

0

……127

Data Block

Data Block

0

……127

0

……127

Data Block

Double Indirect Block

0

……127

0

……127

0

……127

0

……127

0

……127

Data Block

Direct Data Block 1

Direct Data Block 0Data Block

0

……127

Triple Indirect BlockTriple Indirect Block

The indirect block is exactly

the size of a block

Data Block

Unit 13 13

(Some) Inode Parts

Type/Mode Type (regular file, directory, device, etc) Permissions

– Group, user, other - 3 bits each (e.g. rwxr-x—x, rw-r-----)

Link count How many directory entries point to this file

UID User ID number – identifies owner of file

GID Group ID number – identifies group of file

Unit 13 14

How big is the largest file that could be created in the original Unix file system?

Compute the number of blocks in the file

Direct Blocks 10 blocksSingle Indirect 128 “Double Indirect 128

2 “

Triple Indirect 1283

“

Total 2,113,674 blocksx 512

1,082,201,088 bytes

Unit 13 15

Block Address Ranges

Direct 0 to 5,119 (10 x 512 – 1)

Single indirect 5120 to 70,655 (5,120 + 128 x 512 – 1)

Double indirect 70,656 to 8,459,263

(70,656 + 1282 x 512 - 1) Triple indirect

8,459,264 to 1,082,201,087(8,459,264 + 1283 x 512 – 1)

Unit 13 16

Locate byte 1,000,000,033

Convert to blocks1,000,000,033 / 512 = 1,953,125The remainder of 33 is offset into the blockBased on result determine if direct access, single,

double, or triple indirectThis case is triple indirect Determine which of the 1283 blocks it is

– 1,953,125 – (10 + 128 + 1282) = 1,936,603

Unit 13 17

Continuing the hunt

1,936,603 0x1D8CDB Observe that 128 is 27 so if we look at the above

in binary and then group 7 bits at a time we can quickly get the indexes to the needed indirect blocks

1 D 8 C D B

0001 1101 1000 1100 1101 1011

Unit 13 18

FreeBSD 6.0 dinode.hstruct ufs1_dinode {

u_int16_t di_mode; /* 0: IFMT, permissions; see below. */

int16_t di_nlink; /* 2: File link count. */

union {

u_int16_t oldids[2]; /* 4: Ffs: old user and group ids. */

} di_u;

u_int64_t di_size; /* 8: File byte count. */

int32_t di_atime; /* 16: Last access time. */

int32_t di_atimensec; /* 20: Last access time. */

int32_t di_mtime; /* 24: Last modified time. */

int32_t di_mtimensec; /* 28: Last modified time. */

int32_t di_ctime; /* 32: Last inode change time. */

int32_t di_ctimensec; /* 36: Last inode change time. */

ufs1_daddr_t di_db[NDADDR]; /* 40: Direct disk blocks. */

ufs1_daddr_t di_ib[NIADDR]; /* 88: Indirect disk blocks. */

u_int32_t di_flags; /* 100: Status flags (chflags). */

int32_t di_blocks; /* 104: Blocks actually held. */

int32_t di_gen; /* 108: Generation number. */

u_int32_t di_uid; /* 112: File owner. */

u_int32_t di_gid; /* 116: File group. */

int32_t di_spare[2]; /* 120: Reserved; currently unused */

};

Unit 13 19

Superblock

Located in first available sector after boot sector

Maintains global file system information such as: Location of inode map and free inode list

Location of the inode that corresponds to the root directory

Location of list of free blocks

Block size being used

Sector size (anything except 512 is unusual)

Dirty flag

Loss of the super block is a catastrophe

For security, it may be copied to other well known

locations

Unit 13 20

Installing a Disk

As shipped the disk does not contain a file system (old disk drives didn’t even contain tracks and sectors)

When disk is installed must: Lay down tracks and sectors (nowadays done by manufacturer) Determine if all the sectors are good (nowadays done by

manufacturer) Partition the “disk” into logical functional regions (optional)

Create a file system in each partition (win: format , Unix: newfs ) Put down a superblock Create an inode map and free inode list Build the list of free blocks

Unit 13 21

Metadata

Data about data File metadata:

Basically information in inodeDescribes owner of data, file size, etc

File system metadata:Superblock informationBlock size, inode map, block lists, etc

Unit 13 22

Directory File

A directory is just a file that contains information to locate other files and directories

A directory entry has a structure like the following

typedef struct dirent32 {

ino32_t d_ino; /* "inode number" of entry */

off32_t d_off; /* offset of disk directory

entry */

uint16_t d_reclen; /* length of this record */

char d_name[1];/* name of file */

} dirent32_t;From: FreeBSD 5.2

Unit 13 23

Superblock + Inodes + data blocks and directories

Inode Map

Free Blocks

Inode of ‘/’

SuperblockInode Inode Inode Inode

Inode Map

Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Inode

Block

usr

Directory

Inode

Data

Block

Data

Block

Indirect

Data

Block

Data

BlockData

BlockData

Block

Data

Block

Unit 13 24

How it might look on disk

Spindle

Unit 13 25

Creating a file

When a file is created it is emptyLocate a free inode Fill in inode fieldsRemove inode from free list

Locate directory to add file to Add a new directory entry which includes

– Filename– Inode of file

Unit 13 26

Creating a File

Free Inodes

Inode Map

Free Blocks

…

Superblock

Inode Inode Inode Inode

Inode Map

Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Some Directory

•

••

FileA

FileC

NewFile

Second Directory

•

••

Link Count = 1Link Count = 2

NewFile2

Data

Block

Unit 13 27

Steps in Writing a File Block

If data goes into an existing blockMake changes to in memory copy, write changed

block

OtherwiseLocate a free block, write dataAdd free block to inode or indirect blockMay have to get a block to use as indirect block first

Unit 13 28

Writing a File Block

Free Inodes

Inode Map

Free Blocks

…

Superblock

Inode Inode Inode Inode

Inode Map

Data

Block Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Some Directory

•

••

FileA

FileC

NewFile

Indirect

Block

Unit 13 29

Steps in Deleting a File

Directory entry is removed and directory updated on disk

Link count on file being removed decremented If link count 0

Return disk blocks to free listReturn inode to free inode list

Unit 13 30

Deleting a File

Free Inodes

Inode Map

Free Blocks

…

Superblock

Inode Inode Inode Inode

Inode Map

Data

Block Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Some Directory

•

••

FileA

FileC

NewFile

Indirect

Block

Link Count = 1Link Count = 0

Unit 13 31

Order of Writing Meta Data

If data not written in proper order file system could be inconsistent if system crashes

Examples: Block claimed by two inodes

– (block added to inode before old inode updated)

Inode referenced from two directories when not a link – Inode freed and then immediately reused but directory of initial

reference not update

Unit 13 32

Meta Data Update Requirements

Indirect blocks and inodes written synchronously upon deallocation

Directory updates consisting of:Deleting a fileAdding a fileChanging (renaming) a file are all done synchronously

Causes significant file system slowdown

Unit 13 33

Ordering Operations (BAD)

Free Inodes

Inode Map

Free Blocks

…

Superblock

Inode Inode Inode Inode

Inode Map

Data

Block Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Some Directory

•

••

FileA

FileC

NewFile

Indirect

Block

Unit 13 34

Ordering Operations (Good)

Free Inodes

Inode Map

Free Blocks

…

Superblock

Inode Inode Inode Inode

Inode Map

Data

Block Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Some Directory

•

••

FileA

FileC

NewFile

Indirect

Block

Unit 13 35

Actions to Read File Data

To open a file and read data the system must:Read the inode mapRead the inode for the fileRead 0 – 3 indirect blocksRead the data block

With this approach 3 – 6 reads are needed

Unit 13 36

Actions to Write File Data

To open a file and write data the system must:Read the inode mapRead the inode for the fileRead 0 – 3 indirect blocks

Allocate and insert 0 – 3 indirect blocks Read and or allocate the data block, change it,

write it back

Unit 13 37

The Case for Caches

Assume IDE disk with 16KB blocks Transfer time for 16KB ≈16x10-5ms Average access time 10ms (really about 12.5ms)

Time to read 16KB of data 10ms * 3(6) = 30(60)ms

Effective transfer rate 16KB/30(60)ms = 533 KB/s (267KB/s)

Manufacturer claimed transfer rate 150MB/s (SATA)

Unit 13 38

Speeding things up

Obvious problem for all the disk accesses Idea

Save (i.e. cache) copies of important data to avoid going to disk

Could cache:– Inode map– Inodes– Even individual disk blocks (indirect blocks would be an

especially good candidate)

Unit 13 39

Where are caches used?

Adapted from: Computer Systems: A Programmer’s Perspective

Registers

On-chip L1cache (SRAM)

Main memory(DRAM)

Local secondary storage(local disks)

Larger, slower,

and cheaper (per byte)storagedevices

Remote secondary storage(distributed file systems, Web servers)

Local disks hold files retrieved from disks on remote network servers.

Main memory holds disk blocks retrieved from local disks.

Off-chip L2cache (SRAM)

L1 cache holds cache lines retrieved from the L2 cache.

CPU registers hold words retrieved from cache memory.

L2 cache holds cache lines retrieved from memory.

L0:

L1:

L2:

L3:

L4:

L5:

Smaller,faster,and

costlier(per byte)storage devices

Unit 13 40

The Cost of Caching

It requires storage, that storage could be used for something else

If changes are made to the cached copy then the copy and entity being cached are inconsistent

Keeping things consistent can be a complex task especially if there are multiple cached copies

Unit 13 41

File System - Summary

Components of a Unix file system: superblock, inodes, directories, indirect blocks, data blocks

The inode does not contain the file’s name A file can be associated with several different names When the inode’s reference count goes to 0 the

resources used by the file can be reclaimed All the interesting information about a file (e.g. owner,

permissions etc.) is contained in the inode To read/write a file block, need to access multiple blocks Caching can speedup access, but it is expensive to

maintain consistency of cached data