# Copyright (C) 2005-2019 Junjiro R. Okajima Basic Aufs Internal Structure Superblock/Inode/Dentry/File Objects ---------------------------------------------------------------------- As like an ordinary filesystem, aufs has its own superblock/inode/dentry/file objects. All these objects have a dynamically allocated array and store the same kind of pointers to the lower filesystem, branch. For example, when you build a union with one readwrite branch and one readonly, mounted /au, /rw and /ro respectively. - /au = /rw + /ro - /ro/fileA exists but /rw/fileA Aufs lookup operation finds /ro/fileA and gets dentry for that. These pointers are stored in a aufs dentry. The array in aufs dentry will be, - [0] = NULL (because /rw/fileA doesn't exist) - [1] = /ro/fileA This style of an array is essentially same to the aufs superblock/inode/dentry/file objects. Because aufs supports manipulating branches, ie. add/delete/change branches dynamically, these objects has its own generation. When branches are changed, the generation in aufs superblock is incremented. And a generation in other object are compared when it is accessed. When a generation in other objects are obsoleted, aufs refreshes the internal array. Superblock ---------------------------------------------------------------------- Additionally aufs superblock has some data for policies to select one among multiple writable branches, XIB files, pseudo-links and kobject. See below in detail. About the policies which supports copy-down a directory, see wbr_policy.txt too. Branch and XINO(External Inode Number Translation Table) ---------------------------------------------------------------------- Every branch has its own xino (external inode number translation table) file. The xino file is created and unlinked by aufs internally. When two members of a union exist on the same filesystem, they share the single xino file. The struct of a xino file is simple, just a sequence of aufs inode numbers which is indexed by the lower inode number. In the above sample, assume the inode number of /ro/fileA is i111 and aufs assigns the inode number i999 for fileA. Then aufs writes 999 as 4(8) bytes at 111 * 4(8) bytes offset in the xino file. When the inode numbers are not contiguous, the xino file will be sparse which has a hole in it and doesn't consume as much disk space as it might appear. If your branch filesystem consumes disk space for such holes, then you should specify 'xino=' option at mounting aufs. Aufs has a mount option to free the disk blocks for such holes in XINO files on tmpfs or ramdisk. But it is not so effective actually. If you meet a problem of disk shortage due to XINO files, then you should try "tmpfs-ino.patch" (and "vfs-ino.patch" too) in aufs4-standalone.git. The patch localizes the assignment inumbers per tmpfs-mount and avoid the holes in XINO files. Also a writable branch has three kinds of "whiteout bases". All these are existed when the branch is joined to aufs, and their names are whiteout-ed doubly, so that users will never see their names in aufs hierarchy. 1. a regular file which will be hardlinked to all whiteouts. 2. a directory to store a pseudo-link. 3. a directory to store an "orphan"-ed file temporary. 1. Whiteout Base When you remove a file on a readonly branch, aufs handles it as a logical deletion and creates a whiteout on the upper writable branch as a hardlink of this file in order not to consume inode on the writable branch. 2. Pseudo-link Dir See below, Pseudo-link. 3. Step-Parent Dir When "fileC" exists on the lower readonly branch only and it is opened and removed with its parent dir, and then user writes something into it, then aufs copies-up fileC to this directory. Because there is no other dir to store fileC. After creating a file under this dir, the file is unlinked. Because aufs supports manipulating branches, ie. add/delete/change dynamically, a branch has its own id. When the branch order changes, aufs finds the new index by searching the branch id. XIB(external inode number bitmap) ---------------------------------------------------------------------- Addition to the xino file per a branch, aufs has an external inode number bitmap in a superblock object. It is also an internal file such like a xino file. It is a simple bitmap to mark whether the aufs inode number is in-use or not. To reduce the file I/O, aufs prepares a single memory page to cache xib. As well as XINO files, aufs has a feature to truncate/refresh XIB to reduce the number of consumed disk blocks for these files. Workqueue ---------------------------------------------------------------------- Aufs sometimes requires privilege access to a branch. For instance, in copy-up/down operation. When a user process is going to make changes to a file which exists in the lower readonly branch only, and the mode of one of ancestor directories may not be writable by a user process. Here aufs copy-up the file with its ancestors and they may require privilege to set its owner/group/mode/etc. This is a typical case of a application character of aufs (see Introduction). Aufs uses workqueue synchronously for this case. It creates its own workqueue. The workqueue is a kernel thread and has privilege. Aufs passes the request to call mkdir or write (for example), and wait for its completion. This approach solves a problem of a signal handler simply. If aufs didn't adopt the workqueue and changed the privilege of the process, then the process may receive the unexpected SIGXFSZ or other signals. Also aufs uses the system global workqueue ("events" kernel thread) too for asynchronous tasks, such like handling inotify/fsnotify, re-creating a whiteout base and etc. This is unrelated to a privilege. Most of aufs operation tries acquiring a rw_semaphore for aufs superblock at the beginning, at the same time waits for the completion of all queued asynchronous tasks. Whiteout ---------------------------------------------------------------------- The whiteout in aufs is very similar to Unionfs's. That is represented by its filename. UnionMount takes an approach of a file mode, but I am afraid several utilities (find(1) or something) will have to support it. Basically the whiteout represents "logical deletion" which stops aufs to lookup further, but also it represents "dir is opaque" which also stop further lookup. In aufs, rmdir(2) and rename(2) for dir uses whiteout alternatively. In order to make several functions in a single systemcall to be revertible, aufs adopts an approach to rename a directory to a temporary unique whiteouted name. For example, in rename(2) dir where the target dir already existed, aufs renames the target dir to a temporary unique whiteouted name before the actual rename on a branch, and then handles other actions (make it opaque, update the attributes, etc). If an error happens in these actions, aufs simply renames the whiteouted name back and returns an error. If all are succeeded, aufs registers a function to remove the whiteouted unique temporary name completely and asynchronously to the system global workqueue.