Chapter 7. The Ext4 File System
The ext4 file system is a scalable extension of the ext3 file system, which was the default file system of Red Hat Enterprise Linux 5. Ext4 is now the default file system of Red Hat Enterprise Linux 6, and can support files and file systems of up to 16 terabytes in size. It also supports an unlimited number of sub-directories (the ext3 file system only supports up to 32,000).
7.1. Creating an Ext4 File System
To create an ext4 file system, use the
mkfs.ext4 command. In general, the default options are optimal for most usage scenarios, as in:
Below is a sample output of this command, which displays the resulting file system geometry and features:
mke2fs 1.41.9 (22-Aug-2009)
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
1954064 inodes, 7813614 blocks
390680 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=4294967296
239 block groups
32768 blocks per group, 32768 fragments per group
8176 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done
For striped block devices (e.g. RAID5 arrays), the stripe geometry can be specified at the time of file system creation. Using proper stripe geometry greatly enhances performance of an ext4 file system.
When creating file systems on lvm or md volumes,
mkfs.ext4 chooses an optimal geometry. This may also be true on some hardware RAIDs which export geometry information to the operating system.
To specify stripe geometry, use the
-E option of
mkfs.ext4 (i.e. extended file system options) with the following sub-options:
Specifies the RAID chunk size.
Specifies the number of data disks in a RAID device, or the number of stripe units in the stripe.
For both sub-options,
must be specified in file system block units. For example, to create a file system with a 64k stride (i.e. 16 x 4096) on a 4k-block file system, use the following commmand:
mkfs.ext4 -E stride=16,stripe-width=64 /dev/
For more information about creating file systems, refer to
It is possible to use
tune2fs to enable some ext4 features on ext3 file systems, and to use the ext4 driver to mount an ext3 file system. These actions, however, are not supported in Red Hat Enterprise Linux 6, as they have not been fully tested. Because of this, Red Hat cannot guarantee consistent performance and predictable behavior for ext3 file systems converted or mounted thusly.