Frequently Asked Questions
How do I get started using ext4?
Please see the Ext4 Howto page for information on getting started using ext4.
Where do I get the latest version of e2fsprogs?
How do I build e2fsprogs?
The INSTALL file in the top of the source tree gives more detailed information, but e2fsprogs uses a standard configure script, so the standard "./configure; make" will build the e2fsprogs binaries. Note that if you wish to build the ELF shared libraries, you need to add the "--enable-elf-shlibs" option to the configure invocation.
How do I create and mount a new ext4 filesystem?
First, make sure that you have e2fsprogs 1.41.0 or later installed on your system. This is required for ext4 support. If the new partition where you would like to create the ext4 filesystem is /dev/sdb1, then all you have to type is:
/sbin/mke2fs -t ext4dev /dev/sdb1
Then to mount this new filesystem, all you need to do is:
mount -t ext4dev /dev/sdb1 /mnt/test
For more information, please see the Ext4 Howto document.
Why do I get "EXT4-fs: sdb1: not marked OK to use with test code." in my dmesg?
Ext4 is currently in development, and as a safety measure, it requires that filesystems that it mounts have a flag indicating that it's OK for in-development code be able to mount the filesystem. This requirement will be dropped relatively soon, once the ext4 developers are confident that ext4 is stable.
The test_fs flag can be set by using the command "tune2fs -E test_fs /dev/sdb1". Filesystems which are created using the command "mke2fs -t ext4dev /dev/sdb1" will automatically have the test_fs flag set. When the ext4 filesystem has become stable, the command "mke2fs -t ext4 /dev/sdb1" will create a filesystem with all of the appropriate filesystem options suitable for the ext4 filesystem, but without the test_fs flag.
Understanding how it works
What are the new features in Ext4 (vs Ext2/3)?
How do I test the features in Ext4?
How do I benchmark the performance of Ext4 as against other FS? What are the tools available?
There exists a wide variety of tools and comparison, for more information on the different performance testing tools available: 
Anothere reference here: http://en.wikipedia.org/wiki/Comparison_of_file_systems.
Can I undelete files in Ext4?
No, in the same way that the ext3 journal requirements to be consistent after a crash prevent undelete of ext3 files, it isn't possible to undelete ext4 files.
Can I mount existing Ext3 as Ext4? And vice versa? Similarly from Ext2 to Ext4 and its reverse?
You can mount any ext3 filesystem as ext4 without any changes. If the filesystem is mounted as ext4 using the "extents" mount option (the default), this will enable the INCOMPAT_EXTENTS feature, and prevent the filesystem from being mounted as ext3 again. If you mount with the "-o noextents" option this will not happen.
For ext2 the filesystem would first need to have a journal created using tune2fs before it can be mounted as ext3 or ext4. At that point the filesystem is an ext3 filesystem and the above comments apply.
# tune2fs -j /dev/XXX
What is the information provided by /proc/fs/jbd2/<partition>/history?
Executing "cat /proc/fs/jbd2/<partition>/history" gives:
R/C tid wait run lock flush log hndls block inlog ctime write drop close R 2 0 107692 0 0 0 1 1 2
The purpose of this history is to provide a capture of the statistical properties on the performance of the Ext4 filesystem. It can be observed that the entries are added after a file has been copied to the file system - not immediately, but delayed by a short while, due to the asynchronous mechanism of jbd2 logging(*FIXED ME*).
What is the information provided by /proc/fs/jbd2/<partition>/info?
Executing "cat /proc/fs/jbd2/<partition>/info" gives:
56 transaction, each upto 2048 blocks average:
0ms waiting for transaction 57671ms running transaction 0ms transaction was being locked 28ms flushing data (in ordered mode) 14ms logging transaction 2383 handles per transaction 6 blocks per transaction 7 logged blocks per transaction
How to online resize the Ext4 filesystem?
Online resizing of ext4 works in a similar manner as ext3, using either resize2fs or ext2resize, but there is currently a limit (around 4TB or so) to the maximum filesystem size. Implementing online resize with the META_BG feature would allow this limit to be exceeded.
What is the difference between extents mapping and traditional indirect block mapping?
To quote from the paper: http://ext2.sourceforge.net/2005-ols/2005-ext3-paper.pdf:
Currently, the ext2/ext3 ﬁlesystem, like other traditional UNIX ﬁlesystems, uses a direct, indi- rect, double indirect, and triple indirect blocks to map ﬁle offsets to on-disk blocks. This scheme, sometimes simply called an indirect block mapping scheme, is not efﬁcient for large ﬁles, especially large ﬁle deletion. In order to address this problem, many modern ﬁlesystems (including XFS and JFS on Linux) use some form of extent maps instead of the traditional indirect block mapping scheme. Since most ﬁlesystems try to allocate blocks in a contiguous fashion, extent maps are a more efﬁcient way to represent the mapping between logical and physical blocks for large ﬁles. An extent is a single descriptor for a range of contiguous blocks, instead of using, say hundreds of entries to describe each block individually.
What is delayed allocation? What are its advantages in Ext4?
Delayed allocation worked by deferring the allocation of new blocks in the ﬁlesystem to disk blocks until writeback time. This helps in three ways:
1. Reduced fragmentation.
2. Reduced CPU cycles spent in get_block() calls.
3. It may avoid the need for disk updates for metadata creation, which in turn reduces impact on fragmentation.
What is multiblock allocation (mballoc)?
mballoc is a mechanism to allow many blocks to be allocated to a file in a single operation, in order to dramatically reduce the amount of CPU usage searching for many free blocks in the filesystem. Also, because many file blocks are allocated at the same time, a much better decision can be made to find a chunk of free space where all of the blocks will fit.
The mballoc code is active when using the O_DIRECT flag for writes, or if the delayed allocation (delalloc) feature is being used. This allows the file to have many dirty blocks submitted for writes at the same time, unlike the existing kernel mechanism of submitting each block to the filesystem separately for allocation.
What is this bitmap allocator?
Can you say something about the history of Ext4?
Check here: http://en.wikipedia.org/wiki/Ext4.
When was Ext4 first annouced to the LKML?
Check here: http://kerneltrap.org/node/6776
What are the key differences between ext3 and ext4?
The main new features in ext4 are below, and are described more fully in New_ext4_features:
- extent-mapped files for more efficient storage of file metadata (EXTENTS)
- multi-block and delayed allocation for faster/better file allocations
- support for larger filesystems (up to 2^48 blocks, currently 2^60 bytes) (64_BIT)
- optimized storage of filesystem metadata like bitmaps and inode table (FLEX_BG)
- less overhead for e2fsck, on-disk checksum of group descriptors (GDT_CSUM)
- removed 32000 subdirectory limit (DIR_NLINKS)
- nanosecond inode timestamps (EXTRA_ISIZE)
What are the key differences between jbd and jbd2?
The code between jbd and jbd2 is nearly identical, but jbd2 adds a few new features in a compatible way:
- support for 64-bit filesystems (64_BIT)
- checksumming of journal transactions (CHECKSUM)
- asynchronous transaction commit block write (ASYNC_COMMIT)