As of this writing (mid-July, 2008, just after the release of 2.6.26), the ext4 code is functionally complete and functional enough that a few people are using it in production. However, it is still being tested and although the developers haven't lost any data yet, please be cautious and keep plenty of backups!
Getting Ext4 code
For people who build their own kernel
1. Start with a 2.6.26 kernel. Optionally apply the 2.6.26-ext4-1 patchset to get improved performance through delayed allocation. In your kernel's .config file, enable EXT4DEV_FS (along with EXT4DEV_FS_XATTR and EXT4DEV_FS_POSIX_ACL if you like).
For people who are running Fedora
Fedora Core 9 has a kernel based on 2.6.25 that has basic ext4 support. (It is missing some of the latest fixes and performance optimizations, such as delayed allocation.)
There is a Yum repository with an updated e2fsprogs (and updated kernel?) available. XXX need location and more detail
Creating ext4 filesystems
Creating a new ext4 filesystem is very easy once you have upgraded to e2fsprogs 1.41 or later. Simply type:
$ mke2fs -t ext4dev /dev/DEV
Once the filesystem is created, it can be mounted as follows:
$ mount -t ext4dev /dev/DEV /wherever
Converting an ext3 filesystem to ext4
To convert an existing ext3 filesystem to use ext4, use the command
$ tune2fs -O extents -E test_fs /dev/DEV
If the filesystem was created with 128 byte inodes, it can be converted to use 256 byte for greater efficiency via:
$ tune2fs -I 256 /dev/DEV
WARNING: Once the extent feature has been turned on, the filesystem will no longer be mountable using the ext3 filesystem!