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Chapter 4. The ext3 File System
The ext3 file system is essentially an enhanced version of the ext2 file system. These improvements provide the following advantages:
- Availability
- After an unexpected power failure or system crash (also called an unclean system shutdown), each mounted ext2 file system on the machine must be checked for consistency by the
e2fsckprogram. This is a time-consuming process that can delay system boot time significantly, especially with large volumes containing a large number of files. During this time, any data on the volumes is unreachable.It is possible to runfsck -non a live filesystem. However, it will not make any changes and may give misleading results if partially written metadata is encountered.If LVM is used in the stack, another option is to take an LVM snapshot of the filesystem and runfsckon it instead.Finally, there is the option to remount the filesystem as read only. All pending metadata updates (and writes) are then forced to the disk prior to the remount. This ensures the filesystem is in a consistent state, provided there is no previous corruption. It is now possible to runfsck -n.The journaling provided by the ext3 file system means that this sort of file system check is no longer necessary after an unclean system shutdown. The only time a consistency check occurs using ext3 is in certain rare hardware failure cases, such as hard drive failures. The time to recover an ext3 file system after an unclean system shutdown does not depend on the size of the file system or the number of files; rather, it depends on the size of the journal used to maintain consistency. The default journal size takes about a second to recover, depending on the speed of the hardware.Note
The only journaling mode in ext3 supported by Red Hat isdata=ordered(default). - Data Integrity
- The ext3 file system prevents loss of data integrity in the event that an unclean system shutdown occurs. The ext3 file system allows you to choose the type and level of protection that your data receives. With regard to the state of the file system, ext3 volumes are configured to keep a high level of data consistency by default.
- Speed
- Despite writing some data more than once, ext3 has a higher throughput in most cases than ext2 because ext3's journaling optimizes hard drive head motion. You can choose from three journaling modes to optimize speed, but doing so means trade-offs in regards to data integrity if the system was to fail.
Note
The only journaling mode in ext3 supported by Red Hat isdata=ordered(default). - Easy Transition
- It is easy to migrate from ext2 to ext3 and gain the benefits of a robust journaling file system without reformatting. For more information on performing this task, see Section 4.2, “Converting to an ext3 File System” .
Note
Red Hat Enterprise Linux 7 provides a unified extN driver. It does this by disabling the ext2 and ext3 configurations and instead uses
ext4.ko for these on-disk formats. This means that kernel messages will always refer to ext4 regardless of the ext file system used.
4.1. Creating an ext3 File System
After installation, it is sometimes necessary to create a new ext3 file system. For example, if a new disk drive is added to the system, you may want to partition the drive and use the ext3 file system.
- Format the partition or LVM volume with the ext3 file system using the
mkfs.ext3utility:#mkfs.ext3 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.
- Label the file system using the
e2labelutility:#e2label block_device volume_label
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.ext3 -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 ext3 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
- The mkfs.ext3(8) man page
- The e2label(8) man page
