System information

Chapter 5. The Ext3 File System
The ext3 file system is essentially an enhanced version of the ext2 file system. These improvements
provide the following advantages:
Availab ilit y
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
e2fsck program. 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 run fsck -n on 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
run fsck on 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 run fsck -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 is d ata= o rd ered (default).
Dat a In t eg rit y
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.
Sp eed
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.
Easy Tran sit io n
It is easy to migrate from ext2 to ext3 and gain the benefits of a robust journaling file system
without reformatting. Refer to Section 5.2, “ Converting to an Ext3 File System” for more
information on how to perform this task.
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