Root/Documentation/filesystems/logfs.txt

1
2The LogFS Flash Filesystem
3==========================
4
5Specification
6=============
7
8Superblocks
9-----------
10
11Two superblocks exist at the beginning and end of the filesystem.
12Each superblock is 256 Bytes large, with another 3840 Bytes reserved
13for future purposes, making a total of 4096 Bytes.
14
15Superblock locations may differ for MTD and block devices. On MTD the
16first non-bad block contains a superblock in the first 4096 Bytes and
17the last non-bad block contains a superblock in the last 4096 Bytes.
18On block devices, the first 4096 Bytes of the device contain the first
19superblock and the last aligned 4096 Byte-block contains the second
20superblock.
21
22For the most part, the superblocks can be considered read-only. They
23are written only to correct errors detected within the superblocks,
24move the journal and change the filesystem parameters through tunefs.
25As a result, the superblock does not contain any fields that require
26constant updates, like the amount of free space, etc.
27
28Segments
29--------
30
31The space in the device is split up into equal-sized segments.
32Segments are the primary write unit of LogFS. Within each segments,
33writes happen from front (low addresses) to back (high addresses. If
34only a partial segment has been written, the segment number, the
35current position within and optionally a write buffer are stored in
36the journal.
37
38Segments are erased as a whole. Therefore Garbage Collection may be
39required to completely free a segment before doing so.
40
41Journal
42--------
43
44The journal contains all global information about the filesystem that
45is subject to frequent change. At mount time, it has to be scanned
46for the most recent commit entry, which contains a list of pointers to
47all currently valid entries.
48
49Object Store
50------------
51
52All space except for the superblocks and journal is part of the object
53store. Each segment contains a segment header and a number of
54objects, each consisting of the object header and the payload.
55Objects are either inodes, directory entries (dentries), file data
56blocks or indirect blocks.
57
58Levels
59------
60
61Garbage collection (GC) may fail if all data is written
62indiscriminately. One requirement of GC is that data is separated
63roughly according to the distance between the tree root and the data.
64Effectively that means all file data is on level 0, indirect blocks
65are on levels 1, 2, 3 4 or 5 for 1x, 2x, 3x, 4x or 5x indirect blocks,
66respectively. Inode file data is on level 6 for the inodes and 7-11
67for indirect blocks.
68
69Each segment contains objects of a single level only. As a result,
70each level requires its own separate segment to be open for writing.
71
72Inode File
73----------
74
75All inodes are stored in a special file, the inode file. Single
76exception is the inode file's inode (master inode) which for obvious
77reasons is stored in the journal instead. Instead of data blocks, the
78leaf nodes of the inode files are inodes.
79
80Aliases
81-------
82
83Writes in LogFS are done by means of a wandering tree. A naïve
84implementation would require that for each write or a block, all
85parent blocks are written as well, since the block pointers have
86changed. Such an implementation would not be very efficient.
87
88In LogFS, the block pointer changes are cached in the journal by means
89of alias entries. Each alias consists of its logical address - inode
90number, block index, level and child number (index into block) - and
91the changed data. Any 8-byte word can be changes in this manner.
92
93Currently aliases are used for block pointers, file size, file used
94bytes and the height of an inodes indirect tree.
95
96Segment Aliases
97---------------
98
99Related to regular aliases, these are used to handle bad blocks.
100Initially, bad blocks are handled by moving the affected segment
101content to a spare segment and noting this move in the journal with a
102segment alias, a simple (to, from) tupel. GC will later empty this
103segment and the alias can be removed again. This is used on MTD only.
104
105Vim
106---
107
108By cleverly predicting the life time of data, it is possible to
109separate long-living data from short-living data and thereby reduce
110the GC overhead later. Each type of distinc life expectency (vim) can
111have a separate segment open for writing. Each (level, vim) tupel can
112be open just once. If an open segment with unknown vim is encountered
113at mount time, it is closed and ignored henceforth.
114
115Indirect Tree
116-------------
117
118Inodes in LogFS are similar to FFS-style filesystems with direct and
119indirect block pointers. One difference is that LogFS uses a single
120indirect pointer that can be either a 1x, 2x, etc. indirect pointer.
121A height field in the inode defines the height of the indirect tree
122and thereby the indirection of the pointer.
123
124Another difference is the addressing of indirect blocks. In LogFS,
125the first 16 pointers in the first indirect block are left empty,
126corresponding to the 16 direct pointers in the inode. In ext2 (maybe
127others as well) the first pointer in the first indirect block
128corresponds to logical block 12, skipping the 12 direct pointers.
129So where ext2 is using arithmetic to better utilize space, LogFS keeps
130arithmetic simple and uses compression to save space.
131
132Compression
133-----------
134
135Both file data and metadata can be compressed. Compression for file
136data can be enabled with chattr +c and disabled with chattr -c. Doing
137so has no effect on existing data, but new data will be stored
138accordingly. New inodes will inherit the compression flag of the
139parent directory.
140
141Metadata is always compressed. However, the space accounting ignores
142this and charges for the uncompressed size. Failing to do so could
143result in GC failures when, after moving some data, indirect blocks
144compress worse than previously. Even on a 100% full medium, GC may
145not consume any extra space, so the compression gains are lost space
146to the user.
147
148However, they are not lost space to the filesystem internals. By
149cheating the user for those bytes, the filesystem gained some slack
150space and GC will run less often and faster.
151
152Garbage Collection and Wear Leveling
153------------------------------------
154
155Garbage collection is invoked whenever the number of free segments
156falls below a threshold. The best (known) candidate is picked based
157on the least amount of valid data contained in the segment. All
158remaining valid data is copied elsewhere, thereby invalidating it.
159
160The GC code also checks for aliases and writes then back if their
161number gets too large.
162
163Wear leveling is done by occasionally picking a suboptimal segment for
164garbage collection. If a stale segments erase count is significantly
165lower than the active segments' erase counts, it will be picked. Wear
166leveling is rate limited, so it will never monopolize the device for
167more than one segment worth at a time.
168
169Values for "occasionally", "significantly lower" are compile time
170constants.
171
172Hashed directories
173------------------
174
175To satisfy efficient lookup(), directory entries are hashed and
176located based on the hash. In order to both support large directories
177and not be overly inefficient for small directories, several hash
178tables of increasing size are used. For each table, the hash value
179modulo the table size gives the table index.
180
181Tables sizes are chosen to limit the number of indirect blocks with a
182fully populated table to 0, 1, 2 or 3 respectively. So the first
183table contains 16 entries, the second 512-16, etc.
184
185The last table is special in several ways. First its size depends on
186the effective 32bit limit on telldir/seekdir cookies. Since logfs
187uses the upper half of the address space for indirect blocks, the size
188is limited to 2^31. Secondly the table contains hash buckets with 16
189entries each.
190
191Using single-entry buckets would result in birthday "attacks". At
192just 2^16 used entries, hash collisions would be likely (P >= 0.5).
193My math skills are insufficient to do the combinatorics for the 17x
194collisions necessary to overflow a bucket, but testing showed that in
19510,000 runs the lowest directory fill before a bucket overflow was
196188,057,130 entries with an average of 315,149,915 entries. So for
197directory sizes of up to a million, bucket overflows should be
198virtually impossible under normal circumstances.
199
200With carefully chosen filenames, it is obviously possible to cause an
201overflow with just 21 entries (4 higher tables + 16 entries + 1). So
202there may be a security concern if a malicious user has write access
203to a directory.
204
205Open For Discussion
206===================
207
208Device Address Space
209--------------------
210
211A device address space is used for caching. Both block devices and
212MTD provide functions to either read a single page or write a segment.
213Partial segments may be written for data integrity, but where possible
214complete segments are written for performance on simple block device
215flash media.
216
217Meta Inodes
218-----------
219
220Inodes are stored in the inode file, which is just a regular file for
221most purposes. At umount time, however, the inode file needs to
222remain open until all dirty inodes are written. So
223generic_shutdown_super() may not close this inode, but shouldn't
224complain about remaining inodes due to the inode file either. Same
225goes for mapping inode of the device address space.
226
227Currently logfs uses a hack that essentially copies part of fs/inode.c
228code over. A general solution would be preferred.
229
230Indirect block mapping
231----------------------
232
233With compression, the block device (or mapping inode) cannot be used
234to cache indirect blocks. Some other place is required. Currently
235logfs uses the top half of each inode's address space. The low 8TB
236(on 32bit) are filled with file data, the high 8TB are used for
237indirect blocks.
238
239One problem is that 16TB files created on 64bit systems actually have
240data in the top 8TB. But files >16TB would cause problems anyway, so
241only the limit has changed.
242

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