File-system transaction

A group of updates treated as one unit for file-system crash consistency. The three-record append above is one explicitly chosen group. Real file-system journaling can batch several system calls or split large work across several groups; it does not automatically make an application's several writes into one file-system transaction. An atomic operation for competing software threads likewise does not by itself provide file-system transaction durability.

Creation shows why the group must include every related change. In a toy file-system directory with space for one more name, creating notes with contents EFGH needs a directory entry mapping notes to newly allocated inode 42, an inode allocation bitmap marking 42 in use, that inode with size 4 and a reference to block 9, a block allocation bitmap allocating 9, and file data block 9 with the contents. An exposed directory entry must reach an allocated inode, and its content references must reach allocated file data blocks. Protecting just the inode leaves these relationships outside the file-system transaction. This simplified five-record creation excludes parent-directory growth, timestamps and allocation counters; a real file system must include its own additional dependencies.

An acknowledged file-system transaction remains recoverable after a crash under the stated storage assumptions. In the full-data timeline, durable journal payload and a durable journal commit record suffice even before the journal checkpoint. A crash before durable commit can lose the append while still preserving file-system crash consistency. A successful ordinary write may only have reached a buffer cache; an application requiring persistence must use its documented interface and check errors. On Linux kernel-based systems, fsync on a file descriptor persists the associated file contents and file metadata, but persisting a newly created name also requires fsync on the containing file-system directory.^[ref]

A system call for requesting persistence of a file's changed contents and associated file metadata. Its scope, successful completion and separate file-system directory synchronization matter to file-system transaction durability.