What is the journal?

what is journal in ubuntu

Many modern file systems employ a journal including NTFS, Ext3/4, XFS, HFS+, and others. The journal helps with two things, avoiding file system corruption and speeding up recovery after a failure such as a power loss or system crash. There are two major components to a file system, the data and the metadata. The data is the contents of a file, image, video, documents, and ultimately it is what gives the file system it’s value. The metadata is what describes the structure of the filesystem including how files are named, stored in directories, access permissions, file modification times, and recording areas of the disk that are in use or are free to be allocated to other files as they grow or are created. If the metadata becomes corrupted because of a system crash, it could lead to further data loss/corruption. For example, part of the disk might be selected for allocation to a file, but if it’s not recorded correctly before a crash, it might be added to the file’s list of data blocks, but still in the list of free data blocks and allocated to a second file later on. Now, there are two files that are sharing the same data blocks/content.

The journal is a place on the disk reserved for recording changes to the file system. The exact details of what is being changed it written first to the journal located in a single location on disk, then, after the journal is updated, the changes are applied to the appropriate locations on the disk which might require several writes. One the updates are done, the journal entry is marked as complete. After a crash, the system only need to examine the journal for incomplete entries and complete them to fix the file system. This speeds recovery and ensures a change is made entirely or not at all. If a block is allocated to a growing file, it will be added to the file’s block list and also marked as in use.

Also, on file systems that support journalling, most often it’s only metadata journalling to preserve the structure, but not the data itself. Full data journalling is normally quite expensive and slow, but less crucial than metadata. It’s possible to enable for data as well if needed.

Also, to complete this, there are some file systems that don’t currently offer journalling including FAT32, exFAT, Ext2, and UDF. If there’s a crash during an update, a full scan of the filesystem needs to be done to track down any errors or corruption.

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