1. GSoC & grub4ext4
Peng Tao
BUPT—Broadband Network Research Center
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 1 / 37

2. Agenda
1 Google Summer of Code
2 Google Summer of Code 2008
3 Grub4ext4: enable ext4 file system as boot partition
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 2 / 37

3. About Me
Peng Tao
Broadband Network Research Center, BUPT
Graduate at Apr,2010(expected)
bergwolf@gmail.com
Copyright c 2008.
No rights reserved except that of others.
Bergwolf
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 3 / 37

4. 1 Google Summer of Code
2 Google Summer of Code 2008
3 Grub4ext4: enable ext4 file system as boot partition
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 4 / 37

5. What is Google Summer of Code?
Google Summer of Code (GSoC) is a program that offers student
developers stipends to write code for various open source projects.
Google will be working with several open source, free software, and
technology-related groups to identify and fund several projects over a
three month period.
Kicked off in 2005. In 2005, 2006 and 2007, the program has brought
together over 1,500 students with over 130 open source projects to create
millions of lines of code.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 4 / 37

6. What is Google Summer of Code?
Google Summer of Code (GSoC) is a program that offers student
developers stipends to write code for various open source projects.
Google will be working with several open source, free software, and
technology-related groups to identify and fund several projects over a
three month period.
Kicked off in 2005. In 2005, 2006 and 2007, the program has brought
together over 1,500 students with over 130 open source projects to create
millions of lines of code.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 4 / 37

7. What is Google Summer of Code?
Google Summer of Code (GSoC) is a program that offers student
developers stipends to write code for various open source projects.
Google will be working with several open source, free software, and
technology-related groups to identify and fund several projects over a
three month period.
Kicked off in 2005. In 2005, 2006 and 2007, the program has brought
together over 1,500 students with over 130 open source projects to create
millions of lines of code.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 4 / 37

8. Goals of Google Summer of Code
Google Summer of Code has several goals:
Get more open source code created and released for the benefit of all;
Inspire young developers to begin participating in open source
development;
Help open source projects identify and bring in new developers and
committers;
Provide students in Computer Science and related fields the opportunity
to do work related to their academic pursuits (think ”flip bits, not
burgers”);
Give students more exposure to real-world software development
scenarios (e.g., distributed development, software licensing questions,
mailing-list etiquette).
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 5 / 37

9. Goals of Google Summer of Code
Google Summer of Code has several goals:
Get more open source code created and released for the benefit of all;
Inspire young developers to begin participating in open source
development;
Help open source projects identify and bring in new developers and
committers;
Provide students in Computer Science and related fields the opportunity
to do work related to their academic pursuits (think ”flip bits, not
burgers”);
Give students more exposure to real-world software development
scenarios (e.g., distributed development, software licensing questions,
mailing-list etiquette).
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 5 / 37

10. Goals of Google Summer of Code
Google Summer of Code has several goals:
Get more open source code created and released for the benefit of all;
Inspire young developers to begin participating in open source
development;
Help open source projects identify and bring in new developers and
committers;
Provide students in Computer Science and related fields the opportunity
to do work related to their academic pursuits (think ”flip bits, not
burgers”);
Give students more exposure to real-world software development
scenarios (e.g., distributed development, software licensing questions,
mailing-list etiquette).
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 5 / 37

11. Goals of Google Summer of Code
Google Summer of Code has several goals:
Get more open source code created and released for the benefit of all;
Inspire young developers to begin participating in open source
development;
Help open source projects identify and bring in new developers and
committers;
Provide students in Computer Science and related fields the opportunity
to do work related to their academic pursuits (think ”flip bits, not
burgers”);
Give students more exposure to real-world software development
scenarios (e.g., distributed development, software licensing questions,
mailing-list etiquette).
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 5 / 37

12. Goals of Google Summer of Code
Google Summer of Code has several goals:
Get more open source code created and released for the benefit of all;
Inspire young developers to begin participating in open source
development;
Help open source projects identify and bring in new developers and
committers;
Provide students in Computer Science and related fields the opportunity
to do work related to their academic pursuits (think ”flip bits, not
burgers”);
Give students more exposure to real-world software development
scenarios (e.g., distributed development, software licensing questions,
mailing-list etiquette).
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 5 / 37

13. Goals of Google Summer of Code
Google Summer of Code has several goals:
Get more open source code created and released for the benefit of all;
Inspire young developers to begin participating in open source
development;
Help open source projects identify and bring in new developers and
committers;
Provide students in Computer Science and related fields the opportunity
to do work related to their academic pursuits (think ”flip bits, not
burgers”);
Give students more exposure to real-world software development
scenarios (e.g., distributed development, software licensing questions,
mailing-list etiquette).
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 5 / 37

14. A Simple workflow
1. Open source projects who’d like to participate in Google Summer of
Code in 2008 should choose an organization administrator(s) to represent
them;
2. Organization administrators will submit the project’s application for
participation online;
3. Google will notify the organization administrators of acceptance, and
an account for the organization will be created in the Google Summer of
Code web app;
4. Students submit project proposals online to work with particular
mentoring organizations;
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 6 / 37

15. A Simple workflow
1. Open source projects who’d like to participate in Google Summer of
Code in 2008 should choose an organization administrator(s) to represent
them;
2. Organization administrators will submit the project’s application for
participation online;
3. Google will notify the organization administrators of acceptance, and
an account for the organization will be created in the Google Summer of
Code web app;
4. Students submit project proposals online to work with particular
mentoring organizations;
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 6 / 37

16. A Simple workflow
1. Open source projects who’d like to participate in Google Summer of
Code in 2008 should choose an organization administrator(s) to represent
them;
2. Organization administrators will submit the project’s application for
participation online;
3. Google will notify the organization administrators of acceptance, and
an account for the organization will be created in the Google Summer of
Code web app;
4. Students submit project proposals online to work with particular
mentoring organizations;
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 6 / 37

17. A Simple workflow
1. Open source projects who’d like to participate in Google Summer of
Code in 2008 should choose an organization administrator(s) to represent
them;
2. Organization administrators will submit the project’s application for
participation online;
3. Google will notify the organization administrators of acceptance, and
an account for the organization will be created in the Google Summer of
Code web app;
4. Students submit project proposals online to work with particular
mentoring organizations;
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 6 / 37

18. A Simple workflow
1. Open source projects who’d like to participate in Google Summer of
Code in 2008 should choose an organization administrator(s) to represent
them;
2. Organization administrators will submit the project’s application for
participation online;
3. Google will notify the organization administrators of acceptance, and
an account for the organization will be created in the Google Summer of
Code web app;
4. Students submit project proposals online to work with particular
mentoring organizations;
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 6 / 37

19. 5. Mentoring organizations rank student proposals and perform any
other due diligence on their potential mentees; student proposals are
matched with a mentor;
6. Google allocates a particular number of student slots to each
organization;
7. Students are notified of acceptance;
8. Students begin learning more about their mentoring organization and
its community before coding work starts;
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 7 / 37

20. 5. Mentoring organizations rank student proposals and perform any
other due diligence on their potential mentees; student proposals are
matched with a mentor;
6. Google allocates a particular number of student slots to each
organization;
7. Students are notified of acceptance;
8. Students begin learning more about their mentoring organization and
its community before coding work starts;
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 7 / 37

21. 5. Mentoring organizations rank student proposals and perform any
other due diligence on their potential mentees; student proposals are
matched with a mentor;
6. Google allocates a particular number of student slots to each
organization;
7. Students are notified of acceptance;
8. Students begin learning more about their mentoring organization and
its community before coding work starts;
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 7 / 37

22. 5. Mentoring organizations rank student proposals and perform any
other due diligence on their potential mentees; student proposals are
matched with a mentor;
6. Google allocates a particular number of student slots to each
organization;
7. Students are notified of acceptance;
8. Students begin learning more about their mentoring organization and
its community before coding work starts;
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 7 / 37

23. 9. Students begin coding work at the official start of the program,
provided they’ve interacted well with their community up until the
program start date;
10. Mentors and students provide mid-term progress evaluations;
11. Mentors provide a final evaluation of student progress at close of
program; students submit a final review of their mentor and the
program;
12. Student who passes final evaluation uploads completed code to a
Google Code hosted project. Program close up.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 8 / 37

24. 9. Students begin coding work at the official start of the program,
provided they’ve interacted well with their community up until the
program start date;
10. Mentors and students provide mid-term progress evaluations;
11. Mentors provide a final evaluation of student progress at close of
program; students submit a final review of their mentor and the
program;
12. Student who passes final evaluation uploads completed code to a
Google Code hosted project. Program close up.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 8 / 37

25. 9. Students begin coding work at the official start of the program,
provided they’ve interacted well with their community up until the
program start date;
10. Mentors and students provide mid-term progress evaluations;
11. Mentors provide a final evaluation of student progress at close of
program; students submit a final review of their mentor and the
program;
12. Student who passes final evaluation uploads completed code to a
Google Code hosted project. Program close up.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 8 / 37

26. 9. Students begin coding work at the official start of the program,
provided they’ve interacted well with their community up until the
program start date;
10. Mentors and students provide mid-term progress evaluations;
11. Mentors provide a final evaluation of student progress at close of
program; students submit a final review of their mentor and the
program;
12. Student who passes final evaluation uploads completed code to a
Google Code hosted project. Program close up.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 8 / 37

27. 1 Google Summer of Code
2 Google Summer of Code 2008
3 Grub4ext4: enable ext4 file system as boot partition
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 9 / 37

28. Google Summer of Code 2008
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 9 / 37

29. Google Summer of Code 2008
GSoC 2008 was officially announced on February 25 and concluded on
September 2, 2008.
177 open source organizations with more than 3000 ideas.
over 1000 students from all over the world participated. Eighty-three
percent of them received passing final evaluations from their mentors and
communities, the highest rate of success in all four years running the
program.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 10 / 37

30. Google Summer of Code 2008
GSoC 2008 was officially announced on February 25 and concluded on
September 2, 2008.
177 open source organizations with more than 3000 ideas.
over 1000 students from all over the world participated. Eighty-three
percent of them received passing final evaluations from their mentors and
communities, the highest rate of success in all four years running the
program.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 10 / 37

31. Google Summer of Code 2008
GSoC 2008 was officially announced on February 25 and concluded on
September 2, 2008.
177 open source organizations with more than 3000 ideas.
over 1000 students from all over the world participated. Eighty-three
percent of them received passing final evaluations from their mentors and
communities, the highest rate of success in all four years running the
program.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 10 / 37

32. GSoC 2008 organizations
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 11 / 37

33. GSoC 2008 organizations
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 12 / 37

34. openSUSE in GSoC 2008
openSUSE provided 30 project ideas this year. Ten of them became a
GSoC project and all ten projects passed final evaluation!
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 13 / 37

35. openSUSE mentors and students
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 14 / 37

36. GSoC 2008 and Me
Preparation: Started from last winter, I began to do the RTFC things. I
collected documents and manuals about GRUB legacy, got a slight idea
of how to patch GRUB from GRUB’s side.
Application: There was about two weeks to write my proposal(Officially
only one week, but there was one week’s delay). Communication is very
important while writing proposal, especially with the mentor.
Working on it: Reading kernel and e2fsprogs code, asking questions on
IRC, writing patch, bla bla. A lot of things happened.
Submitting: I submitted my code and documents to Coly Li on August
14.
The end: With Coly Li’s help, my patch is now in openSUSE 11.1 beta1.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 15 / 37

37. GSoC 2008 and Me
Preparation: Started from last winter, I began to do the RTFC things. I
collected documents and manuals about GRUB legacy, got a slight idea
of how to patch GRUB from GRUB’s side.
Application: There was about two weeks to write my proposal(Officially
only one week, but there was one week’s delay). Communication is very
important while writing proposal, especially with the mentor.
Working on it: Reading kernel and e2fsprogs code, asking questions on
IRC, writing patch, bla bla. A lot of things happened.
Submitting: I submitted my code and documents to Coly Li on August
14.
The end: With Coly Li’s help, my patch is now in openSUSE 11.1 beta1.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 15 / 37

38. GSoC 2008 and Me
Preparation: Started from last winter, I began to do the RTFC things. I
collected documents and manuals about GRUB legacy, got a slight idea
of how to patch GRUB from GRUB’s side.
Application: There was about two weeks to write my proposal(Officially
only one week, but there was one week’s delay). Communication is very
important while writing proposal, especially with the mentor.
Working on it: Reading kernel and e2fsprogs code, asking questions on
IRC, writing patch, bla bla. A lot of things happened.
Submitting: I submitted my code and documents to Coly Li on August
14.
The end: With Coly Li’s help, my patch is now in openSUSE 11.1 beta1.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 15 / 37

39. GSoC 2008 and Me
Preparation: Started from last winter, I began to do the RTFC things. I
collected documents and manuals about GRUB legacy, got a slight idea
of how to patch GRUB from GRUB’s side.
Application: There was about two weeks to write my proposal(Officially
only one week, but there was one week’s delay). Communication is very
important while writing proposal, especially with the mentor.
Working on it: Reading kernel and e2fsprogs code, asking questions on
IRC, writing patch, bla bla. A lot of things happened.
Submitting: I submitted my code and documents to Coly Li on August
14.
The end: With Coly Li’s help, my patch is now in openSUSE 11.1 beta1.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 15 / 37

40. GSoC 2008 and Me
Preparation: Started from last winter, I began to do the RTFC things. I
collected documents and manuals about GRUB legacy, got a slight idea
of how to patch GRUB from GRUB’s side.
Application: There was about two weeks to write my proposal(Officially
only one week, but there was one week’s delay). Communication is very
important while writing proposal, especially with the mentor.
Working on it: Reading kernel and e2fsprogs code, asking questions on
IRC, writing patch, bla bla. A lot of things happened.
Submitting: I submitted my code and documents to Coly Li on August
14.
The end: With Coly Li’s help, my patch is now in openSUSE 11.1 beta1.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 15 / 37

41. GSoC 2008 and Me
Preparation: Started from last winter, I began to do the RTFC things. I
collected documents and manuals about GRUB legacy, got a slight idea
of how to patch GRUB from GRUB’s side.
Application: There was about two weeks to write my proposal(Officially
only one week, but there was one week’s delay). Communication is very
important while writing proposal, especially with the mentor.
Working on it: Reading kernel and e2fsprogs code, asking questions on
IRC, writing patch, bla bla. A lot of things happened.
Submitting: I submitted my code and documents to Coly Li on August
14.
The end: With Coly Li’s help, my patch is now in openSUSE 11.1 beta1.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 15 / 37

42. 1 Google Summer of Code
2 Google Summer of Code 2008
3 Grub4ext4: enable ext4 file system as boot partition
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 16 / 37

43. Introduction
The ext4 file system is merged into Linux kernel as ext4dev.
However, an ext4-formated partition can not be used as a boot partition,
because GRUB Legacy(referred to as grub in the following text), which is
the most popular bootloader nowadays, doesn’t yet support mounting
and reading ext4 partitions.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 16 / 37

44. Introduction
The ext4 file system is merged into Linux kernel as ext4dev.
However, an ext4-formated partition can not be used as a boot partition,
because GRUB Legacy(referred to as grub in the following text), which is
the most popular bootloader nowadays, doesn’t yet support mounting
and reading ext4 partitions.
As part of Google Summer of Code 2008 program, grub4ext4 aims at
enabling gurb to boot on ext4 partitions.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 16 / 37

45. Introduction(ctd.)
Project home:
http://bergwolf.googlepages.com/grub4ext4
Packages:
http://download.opensuse.org/repositories/home:/bergwolf:/grub4ext4/
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 17 / 37

46. Linux VFS concepts
Generally, Linux Virtual File System(VFS) accomplishes following tasks:
1 Manage kernel level file abstractions in one format for all file systems
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 18 / 37

47. Linux VFS concepts
Generally, Linux Virtual File System(VFS) accomplishes following tasks:
1 Manage kernel level file abstractions in one format for all file systems
2 receive system calls from user level(e.g.write, open, stat, link)
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 18 / 37

48. Linux VFS concepts
Generally, Linux Virtual File System(VFS) accomplishes following tasks:
1 Manage kernel level file abstractions in one format for all file systems
2 receive system calls from user level(e.g.write, open, stat, link)
3 Interact with a specific file system based on mount point traversal
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 18 / 37

49. Linux VFS concepts
Generally, Linux Virtual File System(VFS) accomplishes following tasks:
1 Manage kernel level file abstractions in one format for all file systems
2 receive system calls from user level(e.g.write, open, stat, link)
3 Interact with a specific file system based on mount point traversal
4 receive request from other parts of the kernel, mostly from memory
management
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 18 / 37

50. Linux VFS concepts(ctd.)
VFS data structures:
1 VFS handle for the file: inode
2 user instantiated file handle: file
3 the whole filesystem metadata: superblock
4 a name to inode translation: dentry
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 19 / 37

51. Linux VFS concepts(ctd.)
Inodes:
Inode object is the kernel representation of the low level file
Inode has operations:
const struct inode operations *i op;
const struct file operations *i fop;
Inodes contain an FS specific information
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 20 / 37

52. Linux VFS concepts(ctd.)
File:
Store the process’s information about the file
File information: use count, flags, mode, owner, uid, gid, file operations,
etc.
File operations are copied from the i fop field of the inode object to the
file object by the kernel
Two file operation tables are maintained per FS: one for directories and
the other for regular files
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 21 / 37

53. Linux VFS concepts(ctd.)
Superblock:
Handel metadata only
Responsible for retrieving and storing metadata from FS media or peers
Struct superblock hold things like:
– block device, blocksize, filesystem type, flags, list of (inodes, dirty
inodes, etc) – super operations – wait queue – pointer to the root dentry
of the FS – etc...
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 22 / 37

54. Linux VFS concepts(ctd.)
Dentry:
The kernel uses dentries to represent the file system structure
Dentry is a name to inode translation structure
Dentry has Pointers to store the parent-child relationship of the files
Cached aggressively by VFS
Eliminate lookups by FS & private caches, faster lookups for NFS
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 23 / 37

55. From ext3 to ext4
There are four incompatible ext4 features against ext3 defined in
fs/ext4/ext4.h:
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 24 / 37

56. From ext3 to ext4
There are four incompatible ext4 features against ext3 defined in
fs/ext4/ext4.h:
1 EXT4 FEATURE INCOMPAT EXTENTS
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 24 / 37

57. From ext3 to ext4
There are four incompatible ext4 features against ext3 defined in
fs/ext4/ext4.h:
1 EXT4 FEATURE INCOMPAT EXTENTS
2 EXT4 FEATURE INCOMPAT 64BIT
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 24 / 37

58. From ext3 to ext4
There are four incompatible ext4 features against ext3 defined in
fs/ext4/ext4.h:
1 EXT4 FEATURE INCOMPAT EXTENTS
2 EXT4 FEATURE INCOMPAT 64BIT
3 EXT4 FEATURE INCOMPAT MMP(not implemented yet)
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 24 / 37

59. From ext3 to ext4
There are four incompatible ext4 features against ext3 defined in
fs/ext4/ext4.h:
1 EXT4 FEATURE INCOMPAT EXTENTS
2 EXT4 FEATURE INCOMPAT 64BIT
3 EXT4 FEATURE INCOMPAT MMP(not implemented yet)
4 EXT4 FEATURE INCOMPAT FLEX BG
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 24 / 37

60. From ext3 to ext4
There are four incompatible ext4 features against ext3 defined in
fs/ext4/ext4.h:
1 EXT4 FEATURE INCOMPAT EXTENTS
2 EXT4 FEATURE INCOMPAT 64BIT
3 EXT4 FEATURE INCOMPAT MMP(not implemented yet)
4 EXT4 FEATURE INCOMPAT FLEX BG
To enable grub to boot on ext4 filesystem is to provide grub with support for
these incompatible features.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 24 / 37

61. EXT4 FEATURE INCOMPAT EXTENTS
Ext2/3 uses Indirect mapping to store data on disk.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 25 / 37

62. extents
ext2/3 indirect block maps are incredibly inefficient for large files
one extra block read per 1024 blocks
very obvious when moving or deleting big CD/DVD image files
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 26 / 37

63. extents
ext4 introduces a new on-disk data storage structure: extents.
ext2/3 indirect block maps are incredibly inefficient for large files
one extra block read per 1024 blocks
very obvious when moving or deleting big CD/DVD image files
an extent is a single descriptor for a chunk of contiguous blocks
efficient to represent large contiguous files
fewer metadata IO, better CPU utilization
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 26 / 37

64. on-disk extent format
12 bytes ext4 extent structure
48 bit physical block number to address 1EB filesystem
15 bit extent to store at most 128MB in a single extent
32 bit logical block number to address 16TB filesystem
struct ext4 extent {
le32 ee block; /* first logical block extent covers */
le16 ee len; /* number of blocks covered by extent */
le16 ee start hi; /* high 16 bits of physical block */
le32 ee start lo; /* low 32 bits of physical block */
};
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 27 / 37

65. extent mapping
Ext4 uses extent mapping, which is an advantage when storing large
continues files.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 28 / 37

66. Peng Tao (BUPT) Introduction to GSoC and grub4ext4 29 / 37

67. Peng Tao (BUPT) Introduction to GSoC and grub4ext4 30 / 37

68. EXT4 FEATURE INCOMPAT 64BIT
Currently(kernel 2.26.1), only changes in group descriptor are marked
with this feature. Group descriptor is fixed sized(32 bytes) in ext2/3 but can
be flexible in ext4(at least 64 bytes).
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 31 / 37

69. size of group descriptor
size of group descriptor is stored in s desc size of ext4 superblock which
is not present in ext2/3 superblock.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 32 / 37

70. size of group descriptor
size of group descriptor is stored in s desc size of ext4 superblock which
is not present in ext2/3 superblock.
In grub4ext4, test EXT4 FEATURE INCOMPAT 64BIT bit to decide
whether to read s desc size.
#define EXT2 DESC SIZE(s)
(EXT4 HAS INCOMPAT FEATURE(s,EXT4 FEATURE INCOMPAT 64BIT)?
s–>s desc size : EXT4 MIN DESC SIZE)
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 32 / 37

71. EXT4 FEATURE INCOMPAT FLEX BG
flex bg feature allows bitmaps and inode tables for a block group to be
placed anywhere on the storage media.
greatly decrease fsck time
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 33 / 37

72. EXT4 FEATURE INCOMPAT FLEX BG
flex bg feature allows bitmaps and inode tables for a block group to be
placed anywhere on the storage media.
greatly decrease fsck time
grub doesn’t interfere with bitmaps but uses group desc structure to
access the inode table, so this feature is naturally supported.
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 33 / 37

73. Solved problem 1
How about ext3 files that are left unchanged on ext4 file system?
Peng Tao (BUPT) Introduction to GSoC and grub4ext4 34 / 37

74. Solved problem 1
How about ext3 files that are left unchanged on ext4 file system?
There might be nonextent files on ext4 filesystem.
i flag in each file’s inode should be checked before mapping its logical block
number to physical block number in grub.
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75. Solved problem 2
What if physical block number of a file exceeds 32bit limitation on a
48bit ext4 filesystem?
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76. Solved problem 2
What if physical block number of a file exceeds 32bit limitation on a
48bit ext4 filesystem?
Physical block number is 64bit in ext4.
In grub4ext4, high 16 bits is checked to avoid reading files whose physical
block number exceeds 32bit limitation.
If a request for such files is received, grub will return ERR FILELENGTH
with error message ”Filesystem compatibility error, cannot read whole file”.
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77. Thanks
grub4ext4 is originally Coly Li’s idea. Without his insightful guiding,
grub4ext4 would not be what it is today.
And special thanks goes to Aneesh Kumar, who have helped me a lot in
understanding new ext4 features.
Also, without Google Summer of Code program, this project would not
have been grown so quickly. Google, you rock!
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78. Thank you!
Q&A
Peng Tao
Broadband Network Research Center
State Key Laboratory of Network and Switching Technology
Beijing University of Posts and Telecommunications
Beijing, 100876, China
bergwolf@gmail.com
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