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Subject[PATCH v2] erofs: update documentation
Date
From: Gao Xiang <gaoxiang25@huawei.com>

Some on-disk structures, fields have been renamed in v5.4,
the corresponding document should be updated as well.

Also fix misrespresentation of file time and words about
fixed-sized output compression, data inline, etc.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
---
changes since v1:
- mark that "fixed-sized output compression (with compacted indexes)"
was firstly introduced in v5.3 in order to minimumize metadata;
- reduce whitespace between on-disk values and words about data mapping
details;

Documentation/filesystems/erofs.txt | 27 ++++++++++++++-------------
1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/Documentation/filesystems/erofs.txt b/Documentation/filesystems/erofs.txt
index b0c085326e2e..db6d39c3ae71 100644
--- a/Documentation/filesystems/erofs.txt
+++ b/Documentation/filesystems/erofs.txt
@@ -24,11 +24,11 @@ Here is the main features of EROFS:
- Metadata & data could be mixed by design;

- 2 inode versions for different requirements:
- v1 v2
+ compact (v1) extended (v2)
Inode metadata size: 32 bytes 64 bytes
Max file size: 4 GB 16 EB (also limited by max. vol size)
Max uids/gids: 65536 4294967296
- File creation time: no yes (64 + 32-bit timestamp)
+ File change time: no yes (64 + 32-bit timestamp)
Max hardlinks: 65536 4294967296
Metadata reserved: 4 bytes 14 bytes

@@ -39,7 +39,7 @@ Here is the main features of EROFS:
- Support POSIX.1e ACLs by using xattrs;

- Support transparent file compression as an option:
- LZ4 algorithm with 4 KB fixed-output compression for high performance;
+ LZ4 algorithm with 4 KB fixed-sized output compression for high performance.

The following git tree provides the file system user-space tools under
development (ex, formatting tool mkfs.erofs):
@@ -85,7 +85,7 @@ All data areas should be aligned with the block size, but metadata areas
may not. All metadatas can be now observed in two different spaces (views):
1. Inode metadata space
Each valid inode should be aligned with an inode slot, which is a fixed
- value (32 bytes) and designed to be kept in line with v1 inode size.
+ value (32 bytes) and designed to be kept in line with compact inode size.

Each inode can be directly found with the following formula:
inode offset = meta_blkaddr * block_size + 32 * nid
@@ -117,10 +117,10 @@ may not. All metadatas can be now observed in two different spaces (views):
|-> aligned with 4B

Inode could be 32 or 64 bytes, which can be distinguished from a common
- field which all inode versions have -- i_advise:
+ field which all inode versions have -- i_format:

__________________ __________________
- | i_advise | | i_advise |
+ | i_format | | i_format |
|__________________| |__________________|
| ... | | ... |
| | | |
@@ -129,12 +129,13 @@ may not. All metadatas can be now observed in two different spaces (views):
|__________________| 64 bytes

Xattrs, extents, data inline are followed by the corresponding inode with
- proper alignes, and they could be optional for different data mappings,
- _currently_ there are totally 3 valid data mappings supported:
+ proper alignment, and they could be optional for different data mappings.
+ _currently_ total 4 valid data mappings are supported:

- 1) flat file data without data inline (no extent);
- 2) fixed-output size data compression (must have extents);
- 3) flat file data with tail-end data inline (no extent);
+ 0 flat file data without data inline (no extent);
+ 1 fixed-sized output data compression (with non-compacted indexes);
+ 2 flat file data with tail packing data inline (no extent);
+ 3 fixed-sized output data compression (with compacted indexes, v5.3+).

The size of the optional xattrs is indicated by i_xattr_count in inode
header. Large xattrs or xattrs shared by many different files can be
@@ -182,8 +183,8 @@ introduce another on-disk field at all.

Compression
-----------
-Currently, EROFS supports 4KB fixed-output clustersize transparent file
-compression, as illustrated below:
+Currently, EROFS supports 4KB fixed-sized output transparent file compression,
+as illustrated below:

|---- Variant-Length Extent ----|-------- VLE --------|----- VLE -----
clusterofs clusterofs clusterofs
--
2.20.1
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