A presentation on the Ext4 file system and the evolution of Ext filesystem in Linux operating system. Linux uses virtual filesystem. The comparison of the ext filesystem generations is provided.

1. The ext filesystem
A four generation retrospective
Presented by –
Neha Kulkarni (5202)
ME Computer
Pune Institute of Computer Technology

2. Outline
• Linux file system
• Virtual file system
• Ext file system
• Ext2 file system
• Ext3 file system
• Ext4 file system
• Comparison
• Conclusion
• References

3. Linux filesystem
• A file system is an abstraction that supports the
creation, deletion, and modification of files, and
organization of files into directories.
• Linux uses the File System Hierarchy (FSH)
standard.
• Modern UNIX systems allow disks to be
partitioned into two or more separate physical
entities, each containing a distinct file system.
• Modern UNIX file systems are virtual file systems,
designed to handle many different types of
underlying physical file systems.

4. In the FHS, all files and directories appear under the
root directory /, even if they are stored on different
physical or virtual devices

6. • The superblock contains all of the information
about how the file system is configured, such
as block size, block address range, and mount
status.
• The i-nodes contain the file attributes and a
map indicating where the blocks of the file are
located on the disk. They are of 128 bytes.
• The data blocks are where file contents are
stored.
• i-node in position 2 of the table usually points
to the entry for the root directory file in the
file system.

7. inode pointer structure

8. Linux virtual file system

9. ext filesystem
• Implemented in April 1992 as the first file system
specifically for the Linux kernel
• It was the first implementation that used the
virtual file system, for which support was added
in the Linux kernel in version 0.96c, and it could
handle file systems up to 2GB in size.
• Filenames were allowed up to 255 characters.
• Only one timestamp, no data modification
timestamps
• Use of linked-list for free space- caused
fragmentation

10. Inode description for ext

11. ext2 filesystem
• Designed by Remy Card in January 1993
• Designed and implemented to fix some problems
present in the first extended file system.
• Maximal file size 2GB, can be extended up to 4TB
• Provides long file names up to 255 characters
• Reserves around 5% of the blocks for super-user
• User can set attributes on files or directories
• Allows the administrator to choose the logical
block size when creating the file system (1024B,
2048B, 4096B)

12. • Implements fast symbolic links (upto 60
characters)
• Keeps track of the file system state
• Immutable files can only be read. Nobody can
write or delete them.
• Directories are managed as linked lists of
variable length entries
• Data modification timestamps were available
• ext2 partitions can be converted to ext3 and vice-
versa without any need for backing up the data
and repartitioning

14. Bitmaps

15. • The superblock is defined in struct ext2_super_block, line 339
of include/linux/ext2_fs.h
• A group descriptor is defined by ext2_group_descr structure,
line 148 of ext2_fs.h
• Block number of the first block of inode table is stored in the
bg_inode_table field of the group descriptor.
• i_mode determines the type and access rights of a file. They
are present in sys/stat.h

16. Directory inode
Directory entries in the inode table require special attention. To test if an inode refers
to a directory file we can use the S_ISDIR(mode) macro:
if (S_ISDIR(inode.i_mode))
In the case of directory entries, the data blocks pointed by i_block[] contain a list of
the files in the directory and their respective inode numbers.

17. Locating a file
int fd = open("/home/ealtieri/hello.txt", O_RDONLY);
The desired file is hello.txt, while its path is /home/ealtieri/hello.txt.
Once the inode of the file is known, the data blocks belonging to the hello.txt are specified
by the inode.block[] array.

18. ext3 filesystem
• ext3, or third extended filesystem, is a
journalled filesystem.
• The filesystem was merged with the mainline
Linux kernel in November 2001 from 2.4.15
onward.
• Its main advantage over ext2 is journaling,
which improves reliability and eliminates the
need to check the file system after an unclean
shutdown.

19. • ext3 adds the following features to ext2:
- A journal
- Online file system growth
- HTree indexing for larger directories – 32
bit
• Maximum file size is 16GB-2TB
• Maximum file system size is 2TB-32TB
• The max number of blocks for ext3 is 232

20. • Journalling levels:
– Journal (Lowest risk) -
Both metadata and file contents are written to the
journal before being committed to the main file
system.
– Ordered (medium risk) -
Only metadata is journalled, file contents are written
to the disk before associated metadata is marked as
committed in the journal.
– Writeback (highest risk) -
Only metadata is journalled, file contents are not.
Contents might be written before or after the journal
I is updated.

21. Block allocator
• When allocating a block for a file, the Ext3 block
allocator always starts from the block group
where the i-node structure is stored to keep the
meta-data and data blocks close to each other
• In case of multiple files allocating blocks
concurrently, the Ext3 block allocator uses block
reservation to make sure that subsequent
requests for blocks for a particular file get served
before it is interleaved with other files.
• Ext3 still uses the bitmap to search for the free
blocks to reserve

22. Disadvantages of ext3
• Ext3 lacks recent features such as extents, dynamic
allocation of inodes, block sub-allocation.
• A directory can have at the most 31998 subdirectories.
• No online defragmentation tool
• Does not support the recovery of deleted files
• Does not have native support for snapshots
• No check-summing while writing the journal
• Ext3 stores dates as Unix time using four bytes in the
file header. 32 bits does not give enough scope to
continue processing files beyond January 18, 2038.

23. ext4 filesystem
• It supports 48-bit block addressing, so it will
have 1 EB of maximum filesystem size and 16
TB of maximum file size.
• 1 EB = 1,048,576 TB (1 EB = 1024 PB, 1 PB =
1024 TB, 1 TB = 1024 GB)
• Ext4 allows an unlimited number of sub
directories

24. Extents
• Extent mapping is used in Ext4 to represent new files, rather
than the double, triple indirect block mapping used in Ext2/3.

25. Ext4 block allocator
• Main goal is to provide better allocation for
small and large files
• Ext4 uses a "multiblock allocator" (mballoc).
• Persistent pre-allocation :
Applications tell the filesystem to preallocate
the space.
• The feature is available via the
libc posix_fallocate() interface

26. Delayed Allocation
• Delayed allocation defers block allocations from
write() operation time to page flush time.
• It increases the opportunity to combine many
block allocation requests into a single request
reducing fragmentation
• Saves CPU cycles, and avoids unnecessary block
allocation for short-lived files
• The current Ext4 delayed allocation only supports
data=writeback journalling mode.

27. Inode features
• Larger inodes: Ext4 will default to 256 bytes. This is needed
to accommodate some extra fields (like nanosecond
timestamps or inode versioning), and the remaining space
of the inode will be used to store extend attributes that are
small enough to fit it that space. This will make the access
to those attributes much faster, and improves the
performance of applications that use extend attributes by a
factor of 3-7 times.
• Inode reservation consists in reserving several inodes when
a directory is created, expecting that they will be used in
the future.
• Nanoseconds timestamps means that inode fields like
"modified time" will be able to use nanosecond resolution
instead of the second resolution of Ext3.

28. Journalling
• Ext4 checksums the journal data to know if
the journal blocks are failing or corrupted.
• In Ext4 the journaling feature can be disabled,
which provides a small performance
improvement.

29. Other features
• Ext4 supports online defragmentation
• There's a e4defrag tool which can defragment
individual files or the whole filesystem
• Fsck- In Ext4, at the end of each group's inode
table will be stored a list of unused inodes
(with a checksum, for safety), so fsck will not
check those inodes. The result is that total fsck
time improves from 2 to 20 times, depending
on the number of used inodes.

30. Difference between ext versions
Point ext2 ext3 ext4
Maximum
individual file size
16GB – 2TB 16GB – 2TB 16GB – 16TB
Maximum file
system size
2TB – 32TB 2TB – 32TB 1EB
Journalling Not available Available Available and can
be turned “off” too
Number of
directories
31998 31998 Unlimited
Journal checksum No No Yes
Multi-block
allocation and
delayed allocation
No No Yes

31. Conclusion
• Linux file system follows the FSH standard
• Ext, ext2, ext3, ext4 are the versions of the ext
file system
• Ext is no longer supported by Linux kernel
• Ext4 filesystem supports 48-bit block
addressing, multi-block allocation, extents,
persistent pre-allocation, delayed allocation,
unlimited number of subdirectories, journal
checksumming and improved timestamps.

32. References
[1] ”Design and Implementation of the Seconded Extended
Filesystem”, Remy Card, Proceedings of the First Dutch
International Symposium on Linux, ISBN 9036703859
[2] Ext4 Linux Kernal Newbies – Online Documentation
[3] Introduction to Linux : Chapter 3 – General Overview of the
Linux Filesystem
[4] ”Ext4 block and inode allocator improvements”, Aneesh
Kumar et al, Proceesings of the Linux Symposium, 2008
[5] Linux Filesystem Hierarchy, Binh Nguyen
[6] www.tldp.org_LDP_tlk_fs_filesystem
[7] www.wikipedia.org – ext2, ext3, ext4