Novell LINUX ENTERPRISE SERVER 11 - STORAGE ADMINISTRATION GUIDE 2-23-2010, Руководство администратора, Страница: 15 / 140

Overview of File Systems in Linux 15
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1.2.2 Ext3
Ext3 was designed by Stephen Tweedie. Unlike all other next-generation file systems, Ext3 does not
follow a completely new design principle. It is based on Ext2. These two file systems are very
closely related to each other. An Ext3 file system can be easily built on top of an Ext2 file system.
The most important difference between Ext2 and Ext3 is that Ext3 supports journaling. In summary,
Ext3 has three major advantages to offer:
Š
“Easy and Highly Reliable Upgrades from Ext2” on page 15
Š “Reliability and Performance” on page 15
Š
“Converting an Ext2 File System into Ext3” on page 15
Easy and Highly Reliable Upgrades from Ext2
The code for Ext2 is the strong foundation on which Ext3 could become a highly-acclaimed next-
generation file system. Its reliability and solidity are elegantly combined in Ext3 with the advantages
of a journaling file system. Unlike transitions to other journaling file systems, such as ReiserFS or
XFS, which can be quite tedious (making backups of the entire file system and recreating it from
scratch), a transition to Ext3 is a matter of minutes. It is also very safe, because re-creating an entire
file system from scratch might not work flawlessly. Considering the number of existing Ext2
systems that await an upgrade to a journaling file system, you can easily see why Ext3 might be of
some importance to many system administrators. Downgrading from Ext3 to Ext2 is as easy as the
upgrade. Just perform a clean unmount of the Ext3 file system and remount it as an Ext2 file system.
Reliability and Performance
Some other journaling file systems follow the “metadata-only” journaling approach. This means
your metadata is always kept in a consistent state, but this cannot be automatically guaranteed for
the file system data itself. Ext3 is designed to take care of both metadata and data. The degree of
“care” can be customized. Enabling Ext3 in the
data=journal
mode offers maximum security
(data integrity), but can slow down the system because both metadata and data are journaled. A
relatively new approach is to use the
data=ordered
mode, which ensures both data and metadata
integrity, but uses journaling only for metadata. The file system driver collects all data blocks that
correspond to one metadata update. These data blocks are written to disk before the metadata is
updated. As a result, consistency is achieved for metadata and data without sacrificing performance.
A third option to use is
data=writeback
, which allows data to be written into the main file system
after its metadata has been committed to the journal. This option is often considered the best in
performance. It can, however, allow old data to reappear in files after crash and recovery while
internal file system integrity is maintained. Ext3 uses the
data=ordered
option as the default.
Converting an Ext2 File System into Ext3
To convert an Ext2 file system to Ext3:
1
Create an Ext3 journal by running
tune2fs -j
as the
root
user.
This creates an Ext3 journal with the default parameters.
To specify how large the journal should be and on which device it should reside, run
tune2fs
-J
instead together with the desired journal options
size=
and
device=
. More information
about the
tune2fs
program is available in the
tune2fs
man page.
2
Edit the file
/etc/fstab
as the
root
user to change the file system type specified for the
corresponding partition from
ext2
to
ext3
, then save the changes.