RHEL 6 Storage Admin - Chapter 7. The Ext4 File System

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Chapter 7. The Ext4 File System

7.1. Creating an Ext4 File System
7.2. Mounting an Ext4 File System
7.3. Resizing an Ext4 File System
7.4. Other Ext4 File System Utilities

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).

Main Features

Ext4 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, a program's writes to the file system are 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 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), which allows the system to associate several additional name/value pairs per file.
Quota journaling, which 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

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:

mkfs.ext4 /dev/device

Below is a sample output of this command, which displays the resulting file system geometry and features:

mke2fs 1.41.9 (22-Aug-2009)
Filesystem label=
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, 
        4096000

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:

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 (i.e. 16 x 4096) on a 4k-block file system, use the following commmand:

mkfs.ext4 -E stride=16,stripe-width=64 /dev/device

For more information about creating file systems, refer to man mkfs.ext4.

Note

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.