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Chapter 5. The ext4 File System

The ext4 file system is a scalable extension of the ext3 file system. With Red Hat Enterprise Linux 7, it can support a maximum individual file size of 16 terabytes, and file systems to a maximum of 50 terabytes, unlike Red Hat Enterprise Linux 6 which only supported file systems up to 16 terabytes. It also supports an unlimited number of sub-directories (the ext3 file system only supports up to 32,000), though once the link count exceeds 65,000 it resets to 1 and is no longer increased. The bigalloc feature is not currently supported.

Note

As with ext3, an ext4 volume must be umounted in order to perform an fsck. For more information, see Chapter 4, The ext3 File System.
Main Features
The ext4 file system uses extents (as opposed to the traditional block mapping scheme used by ext2 and ext3), which improves performance when using large files and reduces metadata overhead for large files. In addition, ext4 also labels unallocated block groups and inode table sections accordingly, which allows them to be skipped during a file system check. This makes for quicker file system checks, which becomes more beneficial as the file system grows in size.
Allocation Features
The ext4 file system features the following allocation schemes:
  • Persistent pre-allocation
  • Delayed allocation
  • Multi-block allocation
  • Stripe-aware allocation
Because of delayed allocation and other performance optimizations, ext4's behavior of writing files to disk is different from ext3. In ext4, when a program writes to the file system, it is not guaranteed to be on-disk unless the program issues an fsync() call afterwards.
By default, ext3 automatically forces newly created files to disk almost immediately even without fsync(). This behavior hid bugs in programs that did not use fsync() to ensure that written data was on-disk. The ext4 file system, on the other hand, often waits several seconds to write out changes to disk, allowing it to combine and reorder writes for better disk performance than ext3.

Warning

Unlike ext3, the ext4 file system does not force data to disk on transaction commit. As such, it takes longer for buffered writes to be flushed to disk. As with any file system, use data integrity calls such as fsync() to ensure that data is written to permanent storage.
Other ext4 Features
The ext4 file system also supports the following:
  • Extended attributes (xattr) — This allows the system to associate several additional name and value pairs per file.
  • Quota journaling — This avoids the need for lengthy quota consistency checks after a crash.

    Note

    The only supported journaling mode in ext4 is data=ordered (default).
  • Subsecond timestamps — This gives timestamps to the subsecond.

5.1. Creating an ext4 File System

  • To create an ext4 file system, use the following command: 
    # mkfs.ext4 block_device
    • Replace block_device with the path to a block device. For example, /dev/sdb1, /dev/disk/by-uuid/05e99ec8-def1-4a5e-8a9d-5945339ceb2a, or /dev/my-volgroup/my-lv.
    • In general, the default options are optimal for most usage scenarios.

Example 5.1. mkfs.ext4 Command Output

Below is a sample output of this command, which displays the resulting file system geometry and features: 
~]# mkfs.ext4 /dev/sdb1
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
245280 inodes, 979456 blocks
48972 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=1006632960
30 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

Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

Important

It is possible to use tune2fs to enable certain ext4 features on ext3 file systems. However, using tune2fs in this way has not been fully tested and is therefore not supported in Red Hat Enterprise Linux 7. As a result, Red Hat cannot guarantee consistent performance and predictable behavior for ext3 file systems converted or mounted by using tune2fs.

Striped Block Devices

For striped block devices (for example, RAID5 arrays), the stripe geometry can be specified at the time of file system creation. Using proper stripe geometry greatly enhances the 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 (that is, extended file system options) with the following sub-options:
stride=value
Specifies the RAID chunk size.
stripe-width=value
Specifies the number of data disks in a RAID device, or the number of stripe units in the stripe.
For both sub-options, value must be specified in file system block units. For example, to create a file system with a 64k stride (that is, 16 x 4096) on a 4k-block file system, use the following command: 
# mkfs.ext4 -E stride=16,stripe-width=64 /dev/block_device

Configuring UUID

It is also possible to set a specific UUID for a file system. To specify a UUID when creating a file system, use the -U option: 
# mkfs.ext4 -U UUID device
  • Replace UUID with the UUID you want to set: for example, 7cd65de3-e0be-41d9-b66d-96d749c02da7.
  • Replace device with the path to an ext4 file system to have the UUID added to it: for example, /dev/sda8.
To change the UUID of an existing file system, see Section 25.8.3.2, “Modifying Persistent Naming Attributes”

Additional Resources

For more information about creating ext4 file systems, see:
  • The mkfs.ext4(8) man page