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logfs - log-structured file system for flash devices


bind -b '#ʟ' /dev


echo fsys name config flash-device > /dev/logfsctl
echo fsys name format boot-area-size > /dev/logfsctl
echo fsys name open > /dev/logfsctl
mount -Ac /dev/logfsname dir


Logfs is a driver level implementation of the Inferno log structured filesystem, a file system designed with modern flash devices (such as NAND flash) in mind. Logfs is itself hardware independent, requiring other devices to provide the physical medium. Currently only flash(3) devices are supported.

The file system maintains two storage regions on the same medium: a log-structured hierarchical file system that implements all the functionality communicable by the Styx protocol (see intro(5)), and a boot partition that offers a fixed amount of flat storage suitable for holding such things as a kernel boot image, boot parameters etc. The boot partition can be accessed and updated without understanding all but the simplest facts about the hierarchical file system, making it easy to implement boot loaders with small footprints.

The physical layout of the file system varies from medium to medium, so that the specialised features of the medium can be accounted for.

The user table is maintained in memory; there is one table, shared between all file systems supported by this driver. There is no distinction between users and groups: a user is a group with one member. The user table records a mapping between uids and unames, as well as recording the leader and members of each group. A uid is a string naming a user or group that is stored in the file system. A uname is the string naming a user or group that is used in file system protocol messages (see intro(5)). There is a distinction so that unames can be safely reused, even though uids cannot.

Configuration and control of the file systems is by writing commands to /dev/logfsctl:

fsys name
Sets the current file system to name, which must be configured. Subsequent commands, not prefixed with fsysname, will by applied to the name file system.
[fsys name] config flash-device
Configures file system name to be written to flash(3) device flash-device. This does not initialise or format the filesystem, but simply bind name to flash-device. For each configured name, two files appear in the device name space, /dev/logfsname and /dev/logfsnameboot. The former serves the Styx protocol giving access to the hierarchical file system; the latter is a fixed size file that represents the boot partition.
[fsys name] format bootsize
Formats the underlying medium as a logfs format file system. Sufficent storage is set aside to create a boot partition of at least bootsize bytes. Some medium implementations (eg, for NAND flash) store file system parameters (eg, location, size and boot partition size) to enable automatic location of file systems and improve bad block detection. These must be part of the boot partition so that they are easily accessible to boot loaders, so such medium implementations may also enforce a minimum size for the boot partition.
[fsys name] open [-P] [-W]
Initialises the specified file system, and makes it available for use. The file system structure is verified, bad blocks are repaired and, in the case of the log structured file system, and the log replayed to regenerate the directory structure. The -W option reduces the permission control on wstat(5) requests. Specifically it allows the uid, gid, mtime, and perm to be changed by anyone. The -P option removes all file access controls, allowing anyone to open any file in any mode.
[fsys name] sweep
Forces the log to be swept, if it has been written to since last being swept. The log is normally swept automatically when space is low.
[fsys name] sync
Flush any buffered log or data to the underlying device. Use before shutting down the system if the device is not unmounted. (See notes below.)
[fsys name] trace [level]
If level is non-zero, internal diagnostics for the log file system are enabled at the given level. If level is zero, or missing, tracing is disabled.
[fsys name] unconfig
removes the configuration. The file system must not be mounted, or the boot partition open, before doing this.
uname uname uid
uname uname :uid
adds the user with uname uname and uid uid to the in-memory table
uname uname %newname
renames uname to newname, throughout the user table
uname uname =leader
sets the group leader to the uname leader
uname uname =
removes the group leader; then all members are leaders
uname uname +member
add the uname member to the group
uname uname -member
removes the uname member from the group

The file system log may be subject to a small amount of buffering for efficiency purposes; therefore, it is necessary to unmount the file system before disconnecting the power to avoid losing recent updates. Failure to do this does not result in inconsistencies in the file system, but some recent changes will be lost. Equivalently, a wstat(5) of any file or directory, with all fields set to no change (also known as a wstat flush) will cause the log to be written to disk. Note that during a dismount, and also a wstat flush, a wstat flush is also applied to the underlying flash(3) device. Furthermore, since some buffering is used on the log, needless use of wstat flush will consume log space more rapidly than normal, although it will be recovered during the next sweep.

The log is automatically swept when space is low, so there is not normally any need to use the sweep command.




flash(3), ftl(3), kfs(4)


The only medium currently supported is NAND flash. This is detected by recognising the manufacturer and device ids supplied by the status file of the flash(3) device.

LOGFS(3 ) Rev:  Thu Feb 15 14:43:36 GMT 2007