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authorAndré Fabian Silva Delgado <emulatorman@parabola.nu>2016-03-25 03:53:42 -0300
committerAndré Fabian Silva Delgado <emulatorman@parabola.nu>2016-03-25 03:53:42 -0300
commit03dd4cb26d967f9588437b0fc9cc0e8353322bb7 (patch)
treefa581f6dc1c0596391690d1f67eceef3af8246dc /Documentation/filesystems/aufs/design/06fhsm.txt
parentd4e493caf788ef44982e131ff9c786546904d934 (diff)
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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.