Ext3/Ext4 File System

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1 Operating System IIIT Kalyani 1 Ext3/Ext4 File System

2 Operating System IIIT Kalyani 2 A Bit of History Linus Torvalds used the file system of Minix a while writing the first version of Linux. The Minix was written by Andrew S. Tanenbaum b for using it in academic environment for teaching purpose. Minix 1.0 was available in 1987 as a free and open-source software. a b S. Tanenbaum

3 Operating System IIIT Kalyani 3 A Bit of History Minix file system had the following components in its structure. Boot sector, superblock, inode bitmap block, inodes, data block bitmap and data blocks. The first modification of Minix file system on Linux was Ext file system due to Rémy Card a. It came out in But it seems its performance was not satisfactory. a Card.

4 Operating System IIIT Kalyani 4 A Bit of History The next modification was again due to Rémy Card, the Ext2, the second extended filesystem a. It was there for many years. Ext2 file system was replaced around 2001 by the journaling file system b Ext3 c which is compatible to Ext2. a b file system c

5 Operating System IIIT Kalyani 5 A Bit of History The development of Ext3 was started by Stephen Tweedie a One important feature of Ext3 file system was journaling, that keeps track of changes not yet been committed to the file system. a Tweedie

6 Operating System IIIT Kalyani 6 A Bit of History This is to protect the file system against corruption due to improper unmounting a. Journaling reduces the file system check time during mounting. a Due to power failure, improper shutdown, system crash etc.

7 Operating System IIIT Kalyani 7 A Bit of History The Ext4 a file system was developed as a compatible improvement of Ext3 file system. A few of its features are large volume and file sizes, change in data block allocation and addressing scheme, checksum on journaling and meta data, improved timestamp to address year 2038 problem b etc. a Adopted in Linux around b problem

8 Operating System IIIT Kalyani 8 Disk Organization of Ext2/Ext3

9 Operating System IIIT Kalyani 9 Boot Block Block Group 0 Group (n 1) Boot Block Super Group Block Desc DB Bitmap Inode Bitmap Inode Table Data Blocks Copy Block Group i Super Block DB Bitmap Bitmap Inode Inode Table Data Blocks Block Group j DB Bitmap Inode Bitmap Inode Table Data Blocks

10 Operating System IIIT Kalyani 10 Data Structures of Ext2/Ext3 Partition We assume that logical block size is 4-KB. The whole file system partition or volume is divided in a sequence of logical blocks starting from block-0. Consecutive logical blocks are collected to form block groups a. a Blocks in a block group may have lesser access time as they may be on the same hard disk cylinder.

11 Operating System IIIT Kalyani 11 Data Structures of Ext2/Ext3 Partition The file system tries to allocate data of a file in consecutive logical blocks within a group. This reduces the access time of data as blocks most likely are on same track and cylinder.

12 Operating System IIIT Kalyani 12 Data Structures of Ext2/Ext3 Partition The first 1-KB of the block group-0 (block-0) is not part of the file system. It is reserved as the boot-sector. In a boot partition it contains MBR and part of boot loader a. The first global data structure of the file system, the superblock, starts from offset 1024 in the block group-0. a It is /dev/sda on my machine - readmbr.c++

13 Operating System IIIT Kalyani 13 Data Structures of Ext2/Ext3 Partition There are backup copies of the superblock at the beginning of a few other groups. The second global data structure, block group descriptors are present on block-1 onward of group-0.

14 Operating System IIIT Kalyani 14 Data Structures of Ext2/Ext3 Partition Following the Group descriptors in those groups where it is present, otherwise from the beginning of the group, is the block containing the bit map of the data blocks. It is followed by the bit map block of inodes. Then there are several blocks of inode table. And finally there are actual data blocks.

15 Operating System IIIT Kalyani 15 Creating Ext3 File System on USB Flash Device We create an Ext3 file system on a USB flash device for our experiment. Give the following command a $ sudo mkfs.ext3 -L Kalyani /dev/sdb1 b The file system is created with following parameter. a See man page of mkfs.ext3. b Be very careful about the device file name, otherwise you may write your hard-disk partition and destroy the system.

16 Operating System IIIT Kalyani 16 Creating Ext3 File System on USB Flash Device mke2fs (28-Dec-2013) Filesystem label=kalyani OS type: Linux Block size=4096 (log=2) Fragment size=4096 (log=2) Stride=0 blocks, Stripe width=0 blocks inodes, blocks blocks (5.00%) reserved for the super us First data block=0

17 Operating System IIIT Kalyani 17 Maximum filesystem blocks= block groups blocks per group, fragments per gr 8144 inodes per group Superblock backups stored on blocks: 32768, 98304, , , , , Allocating group tables: done Writing inode tables: done Creating journal (32768 blocks): done Writing superblocks and filesystem accounting

18 Operating System IIIT Kalyani 18 Mounting Ext3 File System on USB Flash Device We mount the formatted device at some directory, change owner, copy a few files etc. $ sudo mount /dev/sdb1./mpt $ sudo chown goutam:goutam./mpt $ cp../../lect.tar./mpt $ cd mpt $ ls -lai total drwxr-xr-x Oct 23 15:43.

19 Operating System IIIT Kalyani drwxrwxr-x Oct 23 06: rw-rw-r Oct 23 15:42 11 drwx Oct 23 15:35 lo $ tar xvf lect.tar./lect/./lect/lect8.dvi./lect/lect5.tex... $ rm lect.tar $ cd lect $ ls -lai

20 Operating System IIIT Kalyani 20 total drwxrwxr-x Oct 23 15:35. 2 drwxr-xr-x Oct 23 15: rwxr-xr-x Jul 5 12:30 bat rw-r--r Jul 27 13:55 Le...

21 Operating System IIIT Kalyani 21 Reading Information from the Superblock The structure of the superblock is available in the header file ext3 fs.h. We keep it here in ext3sb.h a typedef unsigned long long int le64; typedef unsigned int le32, u32; typedef unsigned short int le16; typedef unsigned char u8; a A more civilized way is to include ext3 fs.h. But the kernel source is not available on my machine.

22 Operating System IIIT Kalyani 22 struct ext3_super_block { /*00*/ le32 s_inodes_count; /* Inodes count le32 s_blocks_count; /* Blocks count */... le32 s_free_inodes_count; /* Free ino le32 s_first_data_block; /* First Data B le32 s_log_block_size; /* Block size *...

23 Operating System IIIT Kalyani 23 Reading Information from the Superblock We open the device (/dev/sdb1) a as a file. Skip the first 1 KB of space of boot block. Read the superblock in a variable of type struct ext3 super block. Finally print different fields from the superblock. a This is to be done in superuser mode.

24 Operating System IIIT Kalyani 24 Reading Information from the Superblock $ g++ -Wall ext3readsb.c++ $ sudo./a.out /dev/sdb1 The output is: Total inode count: Total block count: Free-block count: Free-inode count: Block size: 4096 Blocks per group: 32768

25 Operating System IIIT Kalyani 25 Inodes per group: 8144 First data block: 0 Mount time: Mon Oct 23 15:41: Magic signature: ef53 1st non-reserve Inode: 11 Size of inode: 256 Block Gr. No.: bit UUID: 30a895c0d99340d981d9c4ada7e6c1f Last mount point: /home/goutam/iiitkalyani/ope Volume name: Kalyani

26 Operating System IIIT Kalyani 26 Writing in the Superblock Writing in the superblock is dangerous. Write a less sensitive data, the volume name. Open the device in read-write mode. Read the superblock in a buffer of type struct ext3 super block Update the field s volume name. Write back the superblock in its proper place on the device.

27 Operating System IIIT Kalyani 27 Writing in the Superblock $ g++ -Wall ext3writevolname.c++ -o wr $ sudo./wr /dev/sdf1

28 Operating System IIIT Kalyani 28 Command dumpe2fs The command dumpe2fs prints information about ext2/ext3/ext4 filesystems. $ sudo dumpe2fs /dev/sdf1 Filesystem volume name: IIT-Kalyani Last mounted on: /home/goutam/iiitkal operatingsystem/lect/ Filesystem UUID: 30a895c0-d993-40d9-8 Filesystem magic number: 0xEF53 Filesystem revision #: 1 (dynamic)

29 Operating System IIIT Kalyani Filesystem OS type: Linux Inode count: Block count: Block size: Blocks per group: Fragments per group: Inodes per group: 8144 Inode blocks per group: 509

30 Operating System IIIT Kalyani 30 Filesystem created: Mon Oct 23 15:35:25... First inode: 11 Inode size: Group 0: (Blocks ) Primary superblock at 0, Group descriptors a Reserved GDT blocks at Block bitmap at 479 (+479), Inode bitmap at Inode table at (+481) free blocks, 8133 free inodes, 2 direc

31 Operating System IIIT Kalyani 31 Free blocks: , , ,... Free inodes: Group 1: (Blocks ) Backup superblock at 32768, Group descriptor Reserved GDT blocks at Block bitmap at (+479), Inode bitmap a Inode table at (+481) free blocks, 8144 free inodes, 0 direc Free blocks: Free inodes:

32 Operating System IIIT Kalyani 32 Group 2: (Blocks ) Block bitmap at (+0), Inode bitmap at Inode table at (+2) free blocks, 7741 free inodes, 14 dire Free blocks: , Free inodes: Group 59: (Blocks ) Block bitmap at (+0), Inode bitmap a Inode table at (+2) free blocks, 8144 free inodes, 0 direc

33 Operating System IIIT Kalyani 33 Free blocks: Free inodes:

34 Operating System IIIT Kalyani 34 Block Group of a File The output of dumpe2fs /dev/sdb1 shows that most of the files are in block group 2. How to find the block group of a given file (inode)?

35 Operating System IIIT Kalyani 35 Block Group of a File Read the superblock and extract the inode number per group (innpg). The group number is (inn - 1)/ innpg, where inn is the inode number (there is no 0 inode).

36 Operating System IIIT Kalyani 36 Block Group of a File $ g++ -Wall ext3inodenotogrno.c++ $ sudo./a.out /dev/sdb1 Enter an Inode number Group No: 2 GD offset: 4160 Free inode count: 7741 Free block count: 31602

37 Operating System IIIT Kalyani 37 From Inode Number to File We read the inode number of a file and read file data directly from the device partition of the file system. $ g++ -Wall ext3inodetofileblock.c++ $ sudo./a.out /dev/sdb1 less Enter an Inode number Inode Table block: Inode no in group: 158

38 Operating System IIIT Kalyani 38 Inode Table offset: Inode offset: File Size: Links count: 1 File mode: 81a4 Blocks count: 48 Data Block no: 1521 Data Block no: 1522 Data Block no: 1523 Data Block no: 1524 Data Block no: 1525

39 Operating System IIIT Kalyani 39 Data Block no: 1526 Data Block no: 0 Data Block no: 0 Data Block no: 0 Data Block no: 0 Data Block no: 0 Data Block no: 0 Data Block no: 0 Data Block no: 0 Data Block no: 0 < File Data >

40 Operating System IIIT Kalyani 40 \documentclass{seminar} \usepackage{fancybox,color} \def\printlandscape{\special{landscape}} \usepackage{amsmath}... \end{document} %% End lect1.tex

41 Operating System IIIT Kalyani 41 Linux Index Node

42 Operating System IIIT Kalyani 42 Other meta data Data Direct Address Data Data Data Indirect Indirection Block Data 2 Indirect 3 Indirect Double Ind. Data Data I Node Triple ind. Indirection Block Data Data Double Ind. Indirection Block

43 Operating System IIIT Kalyani 43 From Inode Number to File Open the device and read the superblock. Read the inode number from the keyboard (may be passed as a command line argument). Calculate the group number using inode number and the superblock data inode per group. Read the corresponding group descriptor.

44 Operating System IIIT Kalyani 44 From Inode Number to File The group descriptor will supply the block number of the inode table of the group. The position of the inode in the table can be determined using the offset of the table, the index of the inode in the inode table and inode size. All these information are available in the superblock and the group descriptor.

45 Operating System IIIT Kalyani 45 From Inode Number to File The inode is read in a buffer. It contains the data block numbers of the file which can be read.

46 Operating System IIIT Kalyani 46 Creating Ext4 File System on USB Flash Device Exactly in the same way we create an Ext4 file system on a USB Flash drive and copy a few files. $ sudo mkfs.ext4 /dev/sdb1 mke2fs (28-Dec-2013) Filesystem label= OS type: Linux Block size=4096 (log=2) Fragment size=4096 (log=2)

47 Operating System IIIT Kalyani 47 Stride=0 blocks, Stripe width=0 blocks inodes, blocks blocks (5.00%) reserved for the super us First data block=0 Maximum filesystem blocks= block groups blocks per group, fragments per gr 8080 inodes per group Superblock backups stored on blocks: 32768, 98304, , , , ,

48 Operating System IIIT Kalyani 48 Allocating group tables: done Writing inode tables: done Creating journal (32768 blocks): done Writing superblocks and filesystem accounting

49 Operating System IIIT Kalyani 49 Note a Few Differences Total number of inodes and blocks are slightly different. The number of inodes per block is different compared to Ext3 file system. Number of blocks per group are same.

50 Operating System IIIT Kalyani 50 Mount Device and Copy Files $ sudo mount /dev/sdb1./mpt $ df -T... /dev/sdb1 ext % /home/goutam/iiitkalyani/operatingsystem/l $ sudo chown goutam:goutam./mpt $ cp../../lect.tar./mpt $ ls -ai./mpt lect.tar 11 lost+found

51 Operating System IIIT Kalyani 51 $ cd./mpt $ tar xvf lect.tar./lect/./lect/lect7.pdf... $ ls -lai./lect/lect1.tex rw-r--r--. 1 goutam goutam Jul 27 13:55./lect/Lect1.tex

52 Operating System IIIT Kalyani 52 Inode to File We use the inode number of Lect1.tex on /dev/sdb1, go through the superblock, group descriptor, inode table and inodes to locate the blocks of the file. The initial part of the procedure is identical to the Ext3 file system. But data block list (i block[15]) is stored in a different way.

53 Operating System IIIT Kalyani 53 Inode to File $ g++ -Wall ext4inodetofileblock.c++ $ sudo./a.out /dev/sdb1 Enter an Inode number Inode Table block: Inode no in group: 166 Inode Table offset: Inode offset: File Size: 24166

54 Operating System IIIT Kalyani 54 Links count: 1 File mode: 81a4 Blocks count: 48 Index: Index: 1 4 Index: 2 0 Index: 3 0 Index: 4 6 Index: Index: 6 0 Index: 7 0

55 Operating System IIIT Kalyani 55 Index: 8 0 Index: 9 0 Index: 10 0 Index: 11 0 Index: 12 0 Index: 13 0 Index: 14 0

56 Operating System IIIT Kalyani 56 Content of i block[15] In Ext3 file system the inode field i block[15] holds the data block numbers (directly or indirectly) of the file. These block numbers are stored in a different way in Ext4 file system. The 60 byte space of i block[15] is used to store a data structure (part of it) known as extent tree to store data block numbers.

57 Operating System IIIT Kalyani 57 Extent Tree of Ext4 File System Each node of the extent tree has a header followed by either index data structures (for an internal node) or a sequence of extent data structure (for a leaf node). An index structure points to a disk block containing the subtree of the internal node. An extent data structure points to a set of consecutive data blocks of the file.

58 Operating System IIIT Kalyani 58 Extent Tree of Ext4 File System Size of header, index and entry data structures are 12 bytes each. The root node of the extent tree is stored in the space of i block[15] of the inode. The root node has the header (12 bytes) and maximum four (4) (12 bytes each) other types of nodes in this 60 byte space.

59 Operating System IIIT Kalyani 59 Three structures of Extent Tree struct ext4_extent_header { le16 eh_magic; /* probably will supp le16 eh_entries; /* number of valid en le16 eh_max; /* capacity of store le16 eh_depth; /* has tree real unde le32 eh_generation; /* generation of the };

60 Operating System IIIT Kalyani 60 Three structures of Extent Tree struct ext4_extent { le32 ee_block; le16 ee_len; le16 ee_start_hi; le32 ee_start; }; /* first logical bloc /* number of blocks c /* high 16 bits of ph /* low 32 bits of phy

61 Operating System IIIT Kalyani 61 Byte Dump of i block and Extent Tree Byte dump of ind_block[60] a f d Data from extent tree Extent magic: f30a Number of entries: 1

62 Operating System IIIT Kalyani 62 Extent depth: 0 Number of Data Blocks: 6 First Data Block No:

63 Operating System IIIT Kalyani 63 Byte Dump of i block and Extent Tree Note the Little Endian (lsb) order of byte storage. a f3 is 0xf30a, 4d is 0x d = = One (1) in the second field of the extent header indicates that there is one entry after the header.

64 Operating System IIIT Kalyani 64 Byte Dump of i block and Extent Tree Zero (0) of the third field of the extent header indicates that the node is a leaf and the following data structure is of type extent. The six (6) in the second field of the extent data structure indicates that there are six consecutive data blocks. The last field of extent provides the block number of the first data block.

65 Operating System IIIT Kalyani 65 File Data from Data Block < File Data > \documentclass{seminar} \usepackage{fancybox,color} \def\printlandscape{\special{landscape}} \usepackage{amsmath} \usepackage{bm}...

66 Operating System IIIT Kalyani 66 A Few Changes in Ext4 Increase in maximum file system size and file size. Maximum number of subdirectories under a directory a. Introduction of extent tree in place of direct and indirect mappings of each data block in Ext3. An extent tree stores the mappings of the beginning and the count of each set of consecutive data blocks. a Reference 6 and 7.

67 Operating System IIIT Kalyani 67 A Few Changes in Ext4 The single block allocator of Ext3 is replaced by multi block allocator. Delayed allocation of a disk block. Checksum of journal data.

68 Operating System IIIT Kalyani 68 Bibliography 1. Operating System Concepts by Abraham Silberschatz, Peter B Galvin & Gerg Gagne, 9 th ed., Wiley Pub., 2014, ISBN Understanding the Linux Kernel by Daniel P Bovet & Marco Cesati, 3 rd ed., O Reilly, ISBN A Linus Programming Interface by Michael Kerrisk, Pub. no starch press, 2010, ISBN

69 Operating System IIIT Kalyani 69 Disk Layout

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