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## Getting Ext3/4 journal size

Ext3 is a successor of Ext2 with support for journaling which means it can have a log of all it’s recent changes it made or is going to make to the file system. This allows fsck to get the file system back in a good state when a power failure happens for example. But what is the size of the journal? Reading the manpage for tune2fs it says it needs to be between 1024 and 102400 blocks which means it can start with 1MB on a file system with a 1KB block size and 4M on a file system with 4KB block size.

So let start to see which inode contains the journal and normally this should be inode 8 unless you have a file system that was upgraded from Ext2 to Ext3/4.

$sudo LANG=C tune2fs -l /dev/sda1 | awk '/Journal inode/ {print$3}'
8


Now that we have the inode responsible for the in file system journal we can retrieve it details by doing a stat() with debugfs for that inode. Debugfs retrieve the details from the inode and on of them is de allocated size on disk.

$sudo LANG=C debugfs -R "stat <8>" /dev/sda1 | awk '/Size: /{print$6}'|head -1
debugfs 1.42.4 (12-Jun-2012)
4194304


Now let use the same procedure on an other file system:

$sudo LANG=C tune2fs -l /dev/mapper/data00-srv | awk '/Journal inode/ {print$3}'
8
$sudo LANG=C debugfs -R "stat <8>" /dev/mapper/data00-srv | awk '/Size: /{print$6}'|head -1
debugfs 1.42.4 (12-Jun-2012)
134217728


There is also an easy way as dumpe2fs provides an interface to a lot of these values directly.

\$ sudo LANG=C dumpe2fs /dev/mapper/data00-srv | grep ^Journal
dumpe2fs 1.42.4 (12-Jun-2012)
Journal inode:            8
Journal backup:           inode blocks
Journal features:         journal_incompat_revoke
Journal size:             128M
Journal length:           32768
Journal sequence:         0x0111fd4c
Journal start:            3380


Keep in mind that changing something on a live journal can destroy your file system, so never move the journal location or change it’s size unless it’s a clean unmounted file system.