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-
-# Copyright (C) 2011-2016 Junjiro R. Okajima
-
-File-based Hierarchical Storage Management (FHSM)
-----------------------------------------------------------------------
-Hierarchical Storage Management (or HSM) is a well-known feature in the
-storage world. Aufs provides this feature as file-based with multiple
-writable branches, based upon the principle of "Colder, the Lower".
-Here the word "colder" means that the less used files, and "lower" means
-that the position in the order of the stacked branches vertically.
-These multiple writable branches are prioritized, ie. the topmost one
-should be the fastest drive and be used heavily.
-
-o Characters in aufs FHSM story
-- aufs itself and a new branch attribute.
-- a new ioctl interface to move-down and to establish a connection with
-  the daemon ("move-down" is a converse of "copy-up").
-- userspace tool and daemon.
-
-The userspace daemon establishes a connection with aufs and waits for
-the notification. The notified information is very similar to struct
-statfs containing the number of consumed blocks and inodes.
-When the consumed blocks/inodes of a branch exceeds the user-specified
-upper watermark, the daemon activates its move-down process until the
-consumed blocks/inodes reaches the user-specified lower watermark.
-
-The actual move-down is done by aufs based upon the request from
-user-space since we need to maintain the inode number and the internal
-pointer arrays in aufs.
-
-Currently aufs FHSM handles the regular files only. Additionally they
-must not be hard-linked nor pseudo-linked.
-
-
-o Cowork of aufs and the user-space daemon
-  During the userspace daemon established the connection, aufs sends a
-  small notification to it whenever aufs writes something into the
-  writable branch. But it may cost high since aufs issues statfs(2)
-  internally. So user can specify a new option to cache the
-  info. Actually the notification is controlled by these factors.
-  + the specified cache time.
-  + classified as "force" by aufs internally.
-  Until the specified time expires, aufs doesn't send the info
-  except the forced cases. When aufs decide forcing, the info is always
-  notified to userspace.
-  For example, the number of free inodes is generally large enough and
-  the shortage of it happens rarely. So aufs doesn't force the
-  notification when creating a new file, directory and others. This is
-  the typical case which aufs doesn't force.
-  When aufs writes the actual filedata and the files consumes any of new
-  blocks, the aufs forces notifying.
-
-
-o Interfaces in aufs
-- New branch attribute.
-  + fhsm
-    Specifies that the branch is managed by FHSM feature. In other word,
-    participant in the FHSM.
-    When nofhsm is set to the branch, it will not be the source/target
-    branch of the move-down operation. This attribute is set
-    independently from coo and moo attributes, and if you want full
-    FHSM, you should specify them as well.
-- New mount option.
-  + fhsm_sec
-    Specifies a second to suppress many less important info to be
-    notified.
-- New ioctl.
-  + AUFS_CTL_FHSM_FD
-    create a new file descriptor which userspace can read the notification
-    (a subset of struct statfs) from aufs.
-- Module parameter 'brs'
-  It has to be set to 1. Otherwise the new mount option 'fhsm' will not
-  be set.
-- mount helpers /sbin/mount.aufs and /sbin/umount.aufs
-  When there are two or more branches with fhsm attributes,
-  /sbin/mount.aufs invokes the user-space daemon and /sbin/umount.aufs
-  terminates it. As a result of remounting and branch-manipulation, the
-  number of branches with fhsm attribute can be one. In this case,
-  /sbin/mount.aufs will terminate the user-space daemon.
-
-
-Finally the operation is done as these steps in kernel-space.
-- make sure that,
-  + no one else is using the file.
-  + the file is not hard-linked.
-  + the file is not pseudo-linked.
-  + the file is a regular file.
-  + the parent dir is not opaqued.
-- find the target writable branch.
-- make sure the file is not whiteout-ed by the upper (than the target)
-  branch.
-- make the parent dir on the target branch.
-- mutex lock the inode on the branch.
-- unlink the whiteout on the target branch (if exists).
-- lookup and create the whiteout-ed temporary name on the target branch.
-- copy the file as the whiteout-ed temporary name on the target branch.
-- rename the whiteout-ed temporary name to the original name.
-- unlink the file on the source branch.
-- maintain the internal pointer array and the external inode number
-  table (XINO).
-- maintain the timestamps and other attributes of the parent dir and the
-  file.
-
-And of course, in every step, an error may happen. So the operation
-should restore the original file state after an error happens.