[LogFS] add new flash file system

	- info and mount options for the JFS filesystem.

	- info and mount options for the JFS filesystem.

locks.txt

locks.txt

	- info on file locking implementations, flock() vs. fcntl(), etc.

	- info on file locking implementations, flock() vs. fcntl(), etc.

logfs.txt

	- info on the LogFS flash filesystem.

mandatory-locking.txt

mandatory-locking.txt

	- info on the Linux implementation of Sys V mandatory file locking.

	- info on the Linux implementation of Sys V mandatory file locking.

ncpfs.txt

ncpfs.txt

The LogFS Flash Filesystem

==========================

Specification

=============

Superblocks

-----------

Two superblocks exist at the beginning and end of the filesystem.

Each superblock is 256 Bytes large, with another 3840 Bytes reserved

for future purposes, making a total of 4096 Bytes.

Superblock locations may differ for MTD and block devices.  On MTD the

first non-bad block contains a superblock in the first 4096 Bytes and

the last non-bad block contains a superblock in the last 4096 Bytes.

On block devices, the first 4096 Bytes of the device contain the first

superblock and the last aligned 4096 Byte-block contains the second

superblock.

For the most part, the superblocks can be considered read-only.  They

are written only to correct errors detected within the superblocks,

move the journal and change the filesystem parameters through tunefs.

As a result, the superblock does not contain any fields that require

constant updates, like the amount of free space, etc.

Segments

--------

The space in the device is split up into equal-sized segments.

Segments are the primary write unit of LogFS.  Within each segments,

writes happen from front (low addresses) to back (high addresses.  If

only a partial segment has been written, the segment number, the

current position within and optionally a write buffer are stored in

the journal.

Segments are erased as a whole.  Therefore Garbage Collection may be

required to completely free a segment before doing so.

Journal

--------

The journal contains all global information about the filesystem that

is subject to frequent change.  At mount time, it has to be scanned

for the most recent commit entry, which contains a list of pointers to

all currently valid entries.

Object Store

------------

All space except for the superblocks and journal is part of the object

store.  Each segment contains a segment header and a number of

objects, each consisting of the object header and the payload.

Objects are either inodes, directory entries (dentries), file data

blocks or indirect blocks.

Levels

------

Garbage collection (GC) may fail if all data is written

indiscriminately.  One requirement of GC is that data is seperated

roughly according to the distance between the tree root and the data.

Effectively that means all file data is on level 0, indirect blocks

are on levels 1, 2, 3 4 or 5 for 1x, 2x, 3x, 4x or 5x indirect blocks,

respectively.  Inode file data is on level 6 for the inodes and 7-11

for indirect blocks.

Each segment contains objects of a single level only.  As a result,

each level requires its own seperate segment to be open for writing.

Inode File

----------

All inodes are stored in a special file, the inode file.  Single

exception is the inode file's inode (master inode) which for obvious

reasons is stored in the journal instead.  Instead of data blocks, the

leaf nodes of the inode files are inodes.

Aliases

-------

Writes in LogFS are done by means of a wandering tree.  A naïve

implementation would require that for each write or a block, all

parent blocks are written as well, since the block pointers have

changed.  Such an implementation would not be very efficient.

In LogFS, the block pointer changes are cached in the journal by means

of alias entries.  Each alias consists of its logical address - inode

number, block index, level and child number (index into block) - and

the changed data.  Any 8-byte word can be changes in this manner.

Currently aliases are used for block pointers, file size, file used

bytes and the height of an inodes indirect tree.

Segment Aliases

---------------

Related to regular aliases, these are used to handle bad blocks.

Initially, bad blocks are handled by moving the affected segment

content to a spare segment and noting this move in the journal with a

segment alias, a simple (to, from) tupel.  GC will later empty this

segment and the alias can be removed again.  This is used on MTD only.

Vim

---

By cleverly predicting the life time of data, it is possible to

seperate long-living data from short-living data and thereby reduce

the GC overhead later.  Each type of distinc life expectency (vim) can

have a seperate segment open for writing.  Each (level, vim) tupel can

be open just once.  If an open segment with unknown vim is encountered

at mount time, it is closed and ignored henceforth.

Indirect Tree

-------------

Inodes in LogFS are similar to FFS-style filesystems with direct and

indirect block pointers.  One difference is that LogFS uses a single

indirect pointer that can be either a 1x, 2x, etc. indirect pointer.

A height field in the inode defines the height of the indirect tree

and thereby the indirection of the pointer.

Another difference is the addressing of indirect blocks.  In LogFS,

the first 16 pointers in the first indirect block are left empty,

corresponding to the 16 direct pointers in the inode.  In ext2 (maybe

others as well) the first pointer in the first indirect block

corresponds to logical block 12, skipping the 12 direct pointers.

So where ext2 is using arithmetic to better utilize space, LogFS keeps

arithmetic simple and uses compression to save space.

Compression

-----------

Both file data and metadata can be compressed.  Compression for file

data can be enabled with chattr +c and disabled with chattr -c.  Doing

so has no effect on existing data, but new data will be stored

accordingly.  New inodes will inherit the compression flag of the

parent directory.

Metadata is always compressed.  However, the space accounting ignores

this and charges for the uncompressed size.  Failing to do so could

result in GC failures when, after moving some data, indirect blocks

compress worse than previously.  Even on a 100% full medium, GC may

not consume any extra space, so the compression gains are lost space

to the user.

However, they are not lost space to the filesystem internals.  By

cheating the user for those bytes, the filesystem gained some slack

space and GC will run less often and faster.

Garbage Collection and Wear Leveling

------------------------------------

Garbage collection is invoked whenever the number of free segments

falls below a threshold.  The best (known) candidate is picked based

on the least amount of valid data contained in the segment.  All

remaining valid data is copied elsewhere, thereby invalidating it.

The GC code also checks for aliases and writes then back if their

number gets too large.

Wear leveling is done by occasionally picking a suboptimal segment for

garbage collection.  If a stale segments erase count is significantly

lower than the active segments' erase counts, it will be picked.  Wear

leveling is rate limited, so it will never monopolize the device for

more than one segment worth at a time.

Values for "occasionally", "significantly lower" are compile time

constants.

Hashed directories

------------------

To satisfy efficient lookup(), directory entries are hashed and

located based on the hash.  In order to both support large directories

and not be overly inefficient for small directories, several hash

tables of increasing size are used.  For each table, the hash value

modulo the table size gives the table index.

Tables sizes are chosen to limit the number of indirect blocks with a

fully populated table to 0, 1, 2 or 3 respectively.  So the first

table contains 16 entries, the second 512-16, etc.

The last table is special in several ways.  First its size depends on

the effective 32bit limit on telldir/seekdir cookies.  Since logfs

uses the upper half of the address space for indirect blocks, the size

is limited to 2^31.  Secondly the table contains hash buckets with 16

entries each.

Using single-entry buckets would result in birthday "attacks".  At

just 2^16 used entries, hash collisions would be likely (P >= 0.5).

My math skills are insufficient to do the combinatorics for the 17x

collisions necessary to overflow a bucket, but testing showed that in

10,000 runs the lowest directory fill before a bucket overflow was

188,057,130 entries with an average of 315,149,915 entries.  So for

directory sizes of up to a million, bucket overflows should be

virtually impossible under normal circumstances.

With carefully chosen filenames, it is obviously possible to cause an

overflow with just 21 entries (4 higher tables + 16 entries + 1).  So

there may be a security concern if a malicious user has write access

to a directory.

Open For Discussion

===================

Device Address Space

--------------------

A device address space is used for caching.  Both block devices and

MTD provide functions to either read a single page or write a segment.

Partial segments may be written for data integrity, but where possible

complete segments are written for performance on simple block device

flash media.

Meta Inodes

-----------

Inodes are stored in the inode file, which is just a regular file for

most purposes.  At umount time, however, the inode file needs to

remain open until all dirty inodes are written.  So

generic_shutdown_super() may not close this inode, but shouldn't

complain about remaining inodes due to the inode file either.  Same

goes for mapping inode of the device address space.

Currently logfs uses a hack that essentially copies part of fs/inode.c

code over.  A general solution would be preferred.

Indirect block mapping

----------------------

With compression, the block device (or mapping inode) cannot be used

to cache indirect blocks.  Some other place is required.  Currently

logfs uses the top half of each inode's address space.  The low 8TB

(on 32bit) are filled with file data, the high 8TB are used for

indirect blocks.

One problem is that 16TB files created on 64bit systems actually have

data in the top 8TB.  But files >16TB would cause problems anyway, so

only the limit has changed.

source "fs/jffs2/Kconfig"

source "fs/jffs2/Kconfig"

# UBIFS File system configuration

# UBIFS File system configuration

source "fs/ubifs/Kconfig"

source "fs/ubifs/Kconfig"

source "fs/logfs/Kconfig"

source "fs/cramfs/Kconfig"

source "fs/cramfs/Kconfig"

source "fs/squashfs/Kconfig"

source "fs/squashfs/Kconfig"

source "fs/freevxfs/Kconfig"

source "fs/freevxfs/Kconfig"

obj-$(CONFIG_UFS_FS)		+= ufs/

obj-$(CONFIG_UFS_FS)		+= ufs/

obj-$(CONFIG_EFS_FS)		+= efs/

obj-$(CONFIG_EFS_FS)		+= efs/

obj-$(CONFIG_JFFS2_FS)		+= jffs2/

obj-$(CONFIG_JFFS2_FS)		+= jffs2/

obj-$(CONFIG_LOGFS)		+= logfs/

obj-$(CONFIG_UBIFS_FS)		+= ubifs/

obj-$(CONFIG_UBIFS_FS)		+= ubifs/

obj-$(CONFIG_AFFS_FS)		+= affs/

obj-$(CONFIG_AFFS_FS)		+= affs/

obj-$(CONFIG_ROMFS_FS)		+= romfs/

obj-$(CONFIG_ROMFS_FS)		+= romfs/

config LOGFS

	tristate "LogFS file system (EXPERIMENTAL)"

	depends on (MTD || BLOCK) && EXPERIMENTAL

	select ZLIB_INFLATE

	select ZLIB_DEFLATE

	select CRC32

	select BTREE

	help

	  Flash filesystem aimed to scale efficiently to large devices.

	  In comparison to JFFS2 it offers significantly faster mount

	  times and potentially less RAM usage, although the latter has

	  not been measured yet.

	  In its current state it is still very experimental and should

	  not be used for other than testing purposes.

	  If unsure, say N.