A journaling (or journalling) file system is a file system that logs changes to a journal (usually a circular log in a specially-allocated area) before actually writing them to the main file system. In computing, a file system (often also written as filesystem) is a method for storing and organizing computer files and the data they contain to make it easy to find and access them. ...

Metadata-only journaling

Journaling can have a severe impact on performance because it requires that all data be written twice. Metadata-only journaling is a compromise between reliability and performance that stores only changes to file metadata (which is usually relatively small and hence less of a drain on performance) in the journal. This still ensures that the file system can recover quickly when next mounted, but leaves an opportunity for data corruption because unjournaled file data and journaled metadata can fall out of sync with each other. Metadata (Greek meta after and Latin data information) are data that describe other data. ... Metadata (Greek meta after and Latin data information) are data that describe other data. ...

For example, appending to a file on a Unix file system typically involves three steps:

1. Increasing the size of the file in its inode.
2. Allocating space for the extension in the free space map.
3. Actually writing the appended data to the newly-allocated space.

In a metadata-only journal, it would not be clear after a crash whether step 3 was done or not, because it would not be logged. If step 3 was not done but steps 1 and 2 are replayed anyway after a crash, the file will gain a tail of garbage. In computing, an inode is a data structure on a traditional Unix-style file system such as ext2. ...

The write cache in most operating systems will traditionally order its writes with an elevator sort (or some similar scheme) to maximize throughput. To avoid an out-of-order write hazard, writes for file data must be ordered in the sort so that they are committed to storage before their associated metadata. This can be tricky to implement because it requires coordination within the operating system kernel between the file system driver and write cache. I think, that elevator is an subsystem, that turn sequence of sectors to write into sequence of sectors with growing offset. ...

FFS implementations typically do without a journal by ordering all meta-data writes to disk to ensure that the FS is recoverable. This is reliable as long as the disk does not "lie" about its internal write cache status and that it can write blocks atomically. Soft updates take a variation of this approach by allowing only asynchronous meta-data writes that do not render the on-disk file system inconsistent, or that the only inconsistency that ever happens is a storage leak. It avoids having to do synchronous meta-data writes by temporarily "rolling back" any part of a meta-data block that depends on another potentially non-flushed one when writing it. This also permits to selectively flush certain files without having to flush all meta-data blocks or a whole journal. Recovery then simply becomes a matter of running a background walk of the file system when it is next mounted to garbage collect any data orphaned in leaks. FFS can stand for: Front des Forces socialistes or the Front of Socialist Forces, an Algerian political party. ... In computer file systems, soft updates are an approach to maintaining disk integrity after a crash or power outage. ... In computer science, garbage collection (also known as GC) is a form of automatic memory management. ...

See also

• Comparison of file systems