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ext3 filesystem

ext3 filesystem

what is journling in ext3 filestem?,i know it's used for recovery options but still can someone explain me better
Thanks in advance!
3 REPLIES
Goran Koruga
Honored Contributor

Re: ext3 filesystem

Hello.

Why not use some of your web search skills?

http://en.wikipedia.org/wiki/Ext3

http://en.wikipedia.org/wiki/Journaling_file_system

Regards,
Goran
Jupinder Bedi
Respected Contributor

Re: ext3 filesystem

Hello Sarvana ,

here is the answer

A journaling file system is a file system that keeps track of the changes it intends to make in a journal (usually a circular log in a dedicated area of the file system) before committing them to the main file system. In the event of a system crash or power failure, such file systems are quicker to bring back online and less likely to become corrupted.


Journaling levels
There are three levels of journaling available in the Linux implementation of ext3:

Journal (lowest risk)
Both metadata and file contents are written to the journal before being committed to the main file system. Because the journal is relatively continuous on disk, this can improve performance in some circumstances. In other cases, performance gets worse because the data must be written twice - once to the journal, and once to the main part of the filesystem.[8]
Ordered (medium risk)
Only metadata is journaled; file contents are not, but it's guaranteed that file contents are written to disk before associated metadata is marked as committed in the journal. This is the default on many Linux distributions. If there is a power outage or kernel panic while a file is being written or appended to, the journal will indicate the new file or appended data has not been "committed", so it will be purged by the cleanup process. (Thus appends and new files have the same level of integrity protection as the "journaled" level.) However, files being overwritten can be corrupted because the original version of the file is not stored. Thus it's possible to end up with a file in an intermediate state between new and old, without enough information to restore either one or the other (the new data never made it to disk completely, and the old data is not stored anywhere). Even worse, the intermediate state might intersperse old and new data, because the order of the write is left up to the disk's hardware.[9][10] XFS uses this form of journaling.[11]
Writeback (highest risk)
Only metadata is journaled; file contents are not. The contents might be written before or after the journal is updated. As a result, files modified right before a crash can become corrupted. For example, a file being appended to may be marked in the journal as being larger than it actually is, causing garbage at the end. Older versions of files could also appear unexpectedly after a journal recovery. The lack of synchronization between data and journal is faster in many cases. JFS uses this level of journaling, but ensures that any "garbage" due to unwritten data is zeroed out on reboot.



All things excellent are as difficult as they are rare
Viktor Balogh
Honored Contributor

Re: ext3 filesystem

in short: ext3 = ext2 + journaling.
with the help of keeping journals you can skip the long process of fsck, what fsck does is reading the journal and doing the actions it founds in it.
(ok it was a raw, simplistic explanation)
take a look here:

http://www.linuxtopia.org/HowToGuides/ext3JournalingFilesystem.html
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