1. Do Journaling filesystems guarantee
against corruption after a power failure
Rudhramoorthi A

2. Introduction
Journaling File system
Journaling file system is a file system that keeps track of changes not
yet commited to file system’s main part by recording the intentions of
such changes in a data structure known as "journal"
when system crash or power failure, such file systems can be brought
back online more quickly with lower likelihood of becoming
corrupted.

3. Why Journaling file system?
Updating a file systemS to reflect changes to files and directories
requires many seperate write options
ex: Deleting an file need 3 steps:
step1:Removing its directory entry
step2:Release the i-node number(free inodes)
step3:Returning all used block to the pool of free disk blocks
To prevent this, a journaled file system allocates a special area called
journel and it records the changes it will make ahead of time.
It may be succedded or not replayed at all.

4. Techniques
Different file system treat a journal in different format
Normal text file
Hidden file
Kept in external device
Solid state drive
Battery backed non-volatile RAM
Some changes to journal may themselves be journeled into some other
portions that is distributed across multiple physical volumes to
protect again device failure

5. Linux Journaling file sytems
If Non-jounaling file system is impropely shutdown,Then the
Operating system detect this and check performance by the utility
called "Fsck"
This file system fsck scans the whole pc(this take more times) and
fixed any issues that be safely corrected In this also some corruption
of data will hapens
To prevent this Journaling Files systems was created
If a crash accurs, jounrnal can be used as checkpoint to record the
unsaved inforamation and avoid corrupting file system metadata

6. A Typical Journaling file system

7. Linux journaling file system history
The first journaled file system was the IBM® Journaled File
System(JFS).JFS was first released in 1990, but the current version
supported in Linux is the later-developed JFS2.
In 1994, Silicon Graphics introduced the high-performance XFS for
the IRIX operating system. XFS was ported into Linux in 2001.
The most commonly used journaling file system, ext3fs (or third
extended file system) is an extension of ext2 with journaling
capabilities. The ext3fs has been supported in Linux since 2001
Finally, the ReiserFS journaling file system blazed many new trails
when it was introduced and found wide adoption. Its evolution is now
diminished because of the legal issues of its original author.

8. Variations on journaling
Journaling file systems use a journal to buffer changes to the file
system (which is also used in crash recovery) but can use different
strategies for when and what is journaled.
Three of the most common strategies are
Writeback mode
Ordered mode
Data mode
Writeback mode: **Here, only the metadata is journaled and data is
written to the file on the disk. In a crash, the file system is recoverable,
but the physical data can be corrupted. Corruption can occur if a crash
happens after the journal is made of the metadata, but before the
writing of the physical data. File system recovery is the worst, but the
performance is the best.
Ordered mode(default):

9. This mode is the reverse of writeback. The physical data is
writt en first before the metadata is journaled. The ordered
mode allows the data and file system to be uncorrupted if a
system crashes before the journal is written. File system
recovery is medial.
Data mode:
In the data mode, the metadata and file contents are journaled.
System performance can be poorer than the other two modes,
but the fault tolerance is much better.

10. Journaling file systems today
JFS2:
JFS2(also called the enhanced journaled file system)
64-bit file system that,although based on original JFS
Also provides extent-based file allocation for performance
There is no internal commit policy
XFS:
XFS uses the writeback journaling policy
It is one of early journaling file systems so it was already
mature and reliable
Along with extent-based allocation, XFS uses Delayed
allocation
A Guaranteed rate input/output and direct I/O (Through
bandwidth reservation for files system users),so datawhere

11. data is copied directly between the disk and user space
buffer(rather than being staged through multiple buffers)
Third extended file system (ext3fs)
Ext3fs permits three types of journaling (writeback, ordered,
and data) but uses ordered as the default mode
The journal commit policy is configurable but by default is
based on filling 1/4 of the journal or through timeout of one of
the commit timers.
ReiserFS:
The default method for journaling is ordered and supports
online resizing to grow the file system
Developed from ground up with journaling in mind
ReiserFS also included tail packing to dynamically reduce
fragmentation. For smaller files, ReiserFS tends to be much
faster than ext3fs (when tail packing is enabled).
Reiser4:

12. Resier4 was designed for better journaling through the use of
wandering logs and delayed allocation of blocks until the
journal is committed (as was done in XFS).
Reiser4 was also designed with a flexible plugin architecture
(to support capabilities such as compression and
encryption)but was rejected by the Linux community so
Reiser4 has stopped
Fourth extended file system
The ext4 file system is designed as a backward- and forward-
compliant replacement for ext3fs but with many new advanced
features (some of which break the compatibility)
Ithas also been designed to use extents, but if this is used, then
compatibility with ext3fs is lost
The contents of the journal are also checksummed to make the
journal more reliable
An online defragmentation tool (e4defrag) has been developed
to address this. You can use the tool to defragment individual

13. files or an entire file system
In ext3, the minimum resolution for timestamp was one
second. Ext4fs is looking toward the future: Where processor
and interface speeds continue to increase, better resolution is
needed. For this reason, the minimum timestamp resolution
in ext4 is 1 nanosecond.

14. Conclusion:
Journaling file systems provide reliability and protect against
corruption in the face of system crash or power loss
Additionally, the crash recovery time for journaling file systems is
dramatically reduced compared to more traditional file system
methods (such as those that rely on fsck).
Overall, you need to decide if a journal is needed to help with recovery
of data. If so, the Journal mode needs to be determined, but usually
the default of the ordered type gives a median of performance and
recovery
The journal only assists in data recovery for a file system error, not a
hardware failure.
Nothing ever compares to data backups for full recovery of data.