1. Grub4ext4
grub4ext4: Current Status and Future Plans
Bergwolf@bupt.org
Broad-band Network Research Center

2. Grub4ext4
Agenda
1 Introduction to grub4ext4
2 Current Status
3 Future Plans

3. Grub4ext4
Introduction to grub4ext4
What is grub4ext4?
grub4ext4 is a 2008 Google Summer of Code program,
working with opensuse.org mentored by Colyli.
grub4ext4 aims at enabling GRUB legacy to boot an ext4
partition.
The program is hosted at
http://code.google.com/p/grub4ext4/,
Development notes can be found at
http://bergwolf.googlepages.com/grub4ext4.

4. Grub4ext4
Introduction to grub4ext4
Proposed features of grub4ext4
The initial motivation of ext4 file system is to support larger single
partition using larger block numbers.
There are several incompatible features in ext4 that disables GRUB
legacy from booting an ext4 partition.
Replace indirect mapping with extent mapping.
64-bit metadata(superblock, inode, group descriptor) changes.
Flexible block groups. This allows metadata blocks to be
grouped together outside of the block group that they
administer, allowing contiguous regions of data blocks to span
multiple block groups, and making metadata intensive
workloads seek less.
Multiple mount protection. Not implemented yet(as of Linux
kernel 2.6.25.1)

5. Grub4ext4
Current Status
Agenda
1 Introduction to grub4ext4
2 Current Status
3 Future Plans

6. Grub4ext4
Current Status
Current status of grub4ext4
Now GRUB legacy is able to read extents enabled
partitions(without other ext4 incompatible features).
Each file in the file system is divided into several logical
blocks, and stored on disk in forms of several physical blocks.
Ext4 file system uses extent mapping to map the logical block
number of a file to physical block number, which is different
from indirect mapping in ext2/3.

7. Grub4ext4
Current Status
Indirect mapping in ext2/3
Figure: ext2/3 indirect mapping, image taken from ’Taking advantage of
Ext3 journaling file system in a forensic investigation’ by
gnarvae@yahoo.com

8. Grub4ext4
Current Status
Extent mapping
The biggest advantage of extent mapping over indirect mapping is
that it avoids disk seeking when the file is allocated continuously.
Figure: ext4 extent mapping, image taken from ’Ext4: The Next
Generation of Ext2/3’ by Theodore Ts’o

9. Grub4ext4
Current Status
i block(60 bytes) in ext4 inode structure can first store one
ext4 extent header and four ext4 extent. When the file needs more
than four ext4 extent, it expands to an extent tree.
Figure: ext4 extent tree, image taken from ’The new ext4 filesystem:
current status and future plans’ by Andreas Dilger and Alex Tomas

10. Grub4ext4
Current Status
What I have done?
Based on ext4 ext find extent, I implemented static int
ext4fs block map(int logical block), which looks for the physical
block number mapped to logical block instead of building the
extent path which traversal the extents tree.
It working in this way:
(1)Check eh depth in extent header. eh depth==0, go to(3),
otherwise go to(2)
(2)the current level if index. Find the extent index closest
to(which means the smallest ei block that is larger than the
logical block) the logical block we are looking for. Read the
block pointed by the index found in (2) into buffer. Go to (1)
(3)the current level is the leaf. So look for the closest extent
and return the physical block.

11. Grub4ext4
Current Status
Test results
I have run a simple test on my code. It worked just fine on 8
partitions(four ext3 partitions and four ext4 partitions with extents
feature enabled) on my USB storage device.
You can check the test procedure at
http://bergwolf.googlepages.com/test case1, and build your own
test case if you are interested.

12. Grub4ext4
Future Plans
Future work
support incompatible 64-bit metadata changes. This is mainly
about supporting changes in group descriptor. I can check
EXT4 FEATURE INCOMPAT 64BIT in superblock and
determine which type of group descriptor to use.
support flexible block groups.

13. Grub4ext4
Future Plans
Q&A
Thank You!
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