Datasheet

“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 384 — #410
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18.2.2 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:
Easy and Highly Reliable Upgrades from Ext2
As Ext3 is based on the Ext2 code and shares its on-disk format as
well as its metadata format, upgrades from Ext2 to Ext3 are incredibly
easy. Unlike transitions to other journaling file systems, such as
ReiserFS, JFS, 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, as the recreation of an
entire file system from scratch might not work flawlessly. Consider-
ing the number of existing Ext2 systems that await an upgrade to a
journaling file system, you can easily figure out 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 Other journaling file systems follow the
“metadata-only” journaling approach. This means your metadata
is always kept in a consistent state but the same cannot be automat-
ically guaranteed for the file system data itself. Ext3 is designed to
take care of both metadata and data. The degree of “care” can be cus-
tomized. Enabling Ext3 in the data=journal mode offers maximum
security (i.e., data integrity), but can slow down the system as both
metadata and data are journaled. A relatively new approach is to use
the data=ordered mode, which ensures both data and metadata in-
tegrity, but uses journaling only for metadata. The file system driver
collects all data blocks that correspond to one metadata update. These
blocks are grouped as a “transaction” and written to disk before the
metadata is updated. As a result, consistency is achieved for meta-
data 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 in-
ternal file system integrity is maintained. Unless you specify some-
thing else, Ext3 is run with the data=ordered default.
384 18.2. Major File Systems in Linux