https://ext4.wiki.kernel.org/index....ifference_between_ext2.2C_ext3.2C_and_ext4.3F
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What is the difference between ext2, ext3, and ext4?
The ext2, ext3, and ext4 file systems are a family of file systems that have a strong amount of backwards and forward compatibility. In fact, they can be considered a single filesytem format with a number of feature extensions, and ext2, ext3, and ext4 are merely the names of the implementations found in the Linux kernel. This way of looking at things is supported by the fact that they share the same userspace utilities (e2fsprogs), and that many filesystems can be mounted on different filesystems. For example, a filesystem which is created for use with ext3 can be mounted using either ext2 or ext4. However, a filesystem with ext4-specific extensions can not be mounted using ext2 or ext3, and the ext3 file systems code in the kernel requires the presence of a journal, which is generally not present in partitions formatted for use by the ext2 file system. The ext4 code has the ability to mount and use a filesystem without a journal.
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Can I mount existing Ext3 as Ext4? And vice versa? Similarly from Ext2 to Ext4 and its reverse?
With recent versions of ext4 (2.6.29 and later), you can mount any ext2 or ext3 filesystem as ext4 without any changes. You must use tune2fs to enable the new ext4 features:
# tune2fs -O extents,uninit_bg,huge_file /dev/DEV
# e2fsck -f /dev/DEV
If you want to create a journal on an ext2 filesystem that you have mounted as ext4, you must also issue the command:
# tune2fs -j /dev/DEV
Once you have enabled extents a former ext2 or ext3 filesystem, it is an ext4 filesystem and cannot be reverted to the old format.
If you have created a journal on a former ext2, it can be removed if it needs to be reverted to ext2:
# tune2fs -O ^has_journal /dev/DEV
Some ext4 features cannot be enabled on an existing ext3 filesystem.
See the Ext4 Howto for more details.
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