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fs/cromfs: Yet another update to the README file.

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Gregory Nutt 2018-03-22 13:35:27 -06:00
parent 1d72f7afea
commit 012cd8a07a
1 changed files with 46 additions and 14 deletions
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fs/cromfs/README.txt
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@ -26,7 +26,7 @@ accept that for now. That means, for example, that you could have a file
system with 512Kb of data in only 322Kb of FLASH, giving you 190Kb to do
other things with.
LZF compression is not known for its high compression rations, but rather
LZF compression is not known for its high compression ratios, but rather
for fast decompression. According to the author of the LZF decompression
routine, it is nearly as fast as a memcpy!
@ -138,7 +138,7 @@ Architecture
============
The CROMFS file system is represented by an in-memory data structure. This
structure is a tree. At the root of the tree is a "volume node" that
structure is a "tree." At the root of the tree is a "volume node" that
describes the overall operating system. Other entities within the file
system are presented by other types of nodes: hard links, directories, and
files. These nodes are all described in fs/cromfs/cromfs.h.
@ -146,22 +146,34 @@ files. These nodes are all described in fs/cromfs/cromfs.h.
In addition to general volume information, the volume node provides an
offset to the the "root directory". The root directory, like all other
CROMFS directories is simply a singly linked list of other nodes: hard link
nodes, directory nodes, and files. This list is managed by a "peer
offsets": Each node in the directory contains an offset to its peer in the
same directory. This directory list is terminated with a zero offset.
nodes, directory nodes, and files. This list is managed by "peer offsets":
Each node in the directory contains an offset to its peer in the same
directory. This directory list is terminated with a zero offset.
Hard link, directory, and file nodes all include such a "peer offset". Hard
link nodes simply refer to other others and are more or less contained.
Directory nodes contain, in addition, a "child offset" to the first entry in
another singly linked list of nodes comprising the sub-directory.
The volume header lies at offset zero. Hence, any offset to a node or data
block can be converted to an absolute address in the in-memory CROMFS image
by simply adding that offset to the well-known address of the volume header.
File nodes provide file data. They are followed by a variable length list
of compressed data blocks. Each compressed data block contains an LZF
header as described in include/lzf.h.
Each hard link, directory, and file node in the directory list includes
such a "peer offset" to to the next node in the list. Each node is followed
by the NUL-terminated name of the node. Each node also holds an additional
offset. Directory nodes contain a "child offset". That is, the offset to
the first entry in another singly linked list of nodes comprising the sub-
directory.
So, given this information, we could illustrate the sample CROMFS file
Hard link nodes hold the "link offset" to the node which is the target of
the link. The link offset may be an offset to another hard link node, to a
directory, or to a file node. The directory link offset would refer the
first node in singly linked directory list that represents the directory.
File nodes provide file data. The file name string is followed by a
variable length list of compressed data blocks. In this case each
compressed data block begins with an LZF header as described in
include/lzf.h.
So, given this description, we could illustrate the sample CROMFS file
system above with these nodes (where V=volume node, H=Hard link node,
D=directory node, F=file node, D=Data block:
D=directory node, F=file node, D=Data block):
V
`- +- H: .
@ -193,6 +205,26 @@ D=directory node, F=file node, D=Data block:
`- F: JackBeNimble.txt
`- D,D,D,...D
Where, for example:
H: ..
Represents a hard-link node with name ".."
|
+- D: testdir1
| |- H: .
Represents a directory node named "testdir1". The first node of the
directory list is a hard link with name "."
|
+- F: JackSprat.txt
| `- D,D,D,...D
Represents f file node named "JackSprat.txt" and is followed by some
sequence of compressed data blocks, D.
Configuration
=============