[PATCH v6 24/24] erofs: add document

Gao Xiang
2019-08-02T12:54:52Z

This documents key features, usage, and
on-disk design of erofs.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
---
Documentation/filesystems/erofs.txt | 225 ++++++++++++++++++++++++++++
1 file changed, 225 insertions(+)
create mode 100644 Documentation/filesystems/erofs.txt

diff --git a/Documentation/filesystems/erofs.txt b/Documentation/filesystems/erofs.txt
new file mode 100644
index 000000000000..457e601e0467
--- /dev/null
+++ b/Documentation/filesystems/erofs.txt
@@ -0,0 +1,225 @@
+Overview
+========
+
+EROFS file-system stands for Enhanced Read-Only File System. Different
+from other read-only file systems, it aims to be designed for flexibility,
+scalability, but be kept simple and high performance.
+
+It is designed as a better filesystem solution for the following scenarios:
+ - read-only storage media or
+
+ - part of a fully trusted read-only solution, which means it needs to be
+ immutable and bit-for-bit identical to the official golden image for
+ their releases due to security and other considerations and
+
+ - hope to save some extra storage space with guaranteed end-to-end performance
+ by using reduced metadata and transparent file compression, especially
+ for those embedded devices with limited memory (ex, smartphone);
+
+Here is the main features of EROFS:
+ - Little endian on-disk design;
+
+ - Currently 4KB block size (nobh) and therefore maximum 16TB address space;
+
+ - Metadata & data could be mixed by design;
+
+ - 2 inode versions for different requirements:
+ v1 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)
+ Max hardlinks: 65536 4294967296
+ Metadata reserved: 4 bytes 14 bytes
+
+ - Support extended attributes (xattrs) as an option;
+
+ - Support xattr inline and tail-end data inline for all files;
+
+ - Support POSIX.1e ACLs by using xattrs;
+
+ - Support statx();
+
+ - Support transparent file compression as an option:
+ LZ4 algorithm with 4 KB fixed-output compression for high performance;
+
+The following git tree provides the file system user-space tools under
+development (ex, formatting tool mkfs.erofs):
+>> git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git
+
+Bugs and patches are welcome, please kindly help us and send to the following
+linux-erofs mailing list:
+>> linux-erofs mailing list <linux-erofs@lists.ozlabs.org>
+
+Note that EROFS is still working in progress as a Linux staging driver,
+Cc the staging mailing list as well is highly recommended:
+>> Linux Driver Project Developer List <devel@driverdev.osuosl.org>
+
+Mount options
+=============
+
+fault_injection=%d Enable fault injection in all supported types with
+ specified injection rate. Supported injection type:
+ Type_Name Type_Value
+ FAULT_KMALLOC 0x000000001
+ FAULT_READ_IO 0x000000002
+(no)user_xattr Setup Extended User Attributes. Note: xattr is enabled
+ by default if CONFIG_EROFS_FS_XATTR is selected.
+(no)acl Setup POSIX Access Control List. Note: acl is enabled
+ by default if CONFIG_EROFS_FS_POSIX_ACL is selected.
+cache_strategy=%s Select a strategy for cached decompression from now on:
+ disabled: In-place I/O decompression only;
+ readahead: Cache the last incomplete compressed physical
+ cluster for further reading. It still does
+ in-place I/O decompression for the rest
+ compressed physical clusters;
+ readaround: Cache the both ends of incomplete compressed
+ physical clusters for further reading.
+ It still does in-place I/O decompression
+ for the rest compressed physical clusters.
+
+Module parameters
+=================
+use_vmap=[0|1] Use vmap() instead of vm_map_ram() (default 0).
+
+On-disk details
+===============
+
+Summary
+-------
+Different from other read-only file systems, an EROFS volume is designed
+to be as simple as possible:
+
+ |-> aligned with the block size
+ ____________________________________________________________
+ | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data |
+ |_|__|_|_____|__________|_____|______|__________|_____|______|
+ 0 +1K
+
+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.
+
+ Each inode can be directly found with the following formula:
+ inode offset = meta_blkaddr * block_size + 32 * nid
+
+ |-> aligned with 8B
+ |-> followed closely
+ + meta_blkaddr blocks |-> another slot
+ _____________________________________________________________________
+ | ... | inode | xattrs | extents | data inline | ... | inode ...
+ |________|_______|(optional)|(optional)|__(optional)_|_____|__________
+ |-> aligned with the inode slot size
+ . .
+ . .
+ . .
+ . .
+ . .
+ . .
+ .____________________________________________________|-> aligned with 4B
+ | xattr_ibody_header | shared xattrs | inline xattrs |
+ |____________________|_______________|_______________|
+ |-> 12 bytes <-|->x * 4 bytes<-| .
+ . . .
+ . . .
+ . . .
+ ._______________________________.______________________.
+ | id | id | id | id | ... | id | ent | ... | ent| ... |
+ |____|____|____|____|______|____|_____|_____|____|_____|
+ |-> aligned with 4B
+ |-> 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:
+
+ __________________ __________________
+ | i_advise | | i_advise |
+ |__________________| |__________________|
+ | ... | | ... |
+ | | | |
+ |__________________| 32 bytes | |
+ | |
+ |__________________| 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:
+
+ 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);
+
+ 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
+ stored in shared xattrs metadata rather than inlined right after inode.
+
+ 2. Shared xattrs metadata space
+ Shared xattrs space is similar to the above inode space, started with
+ a specific block indicated by xattr_blkaddr, organized one by one with
+ proper align.
+
+ Each share xattr can also be directly found by the following formula:
+ xattr offset = xattr_blkaddr * block_size + 4 * xattr_id
+
+ |-> aligned by 4 bytes
+ + xattr_blkaddr blocks |-> aligned with 4 bytes
+ _________________________________________________________________________
+ | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ...
+ |________|_____________|_____________|_____|______________|_______________
+
+Directories
+-----------
+All directories are now organized in a compact on-disk format. Note that
+each directory block is divided into index and name areas in order to support
+random file lookup, and all directory entries are _strictly_ recorded in
+alphabetical order in order to support improved prefix binary search
+algorithm (could refer to the related source code).
+
+ ___________________________
+ / |
+ / ______________|________________
+ / / | nameoff1 | nameoffN-1
+ ____________.______________._______________v________________v__________
+| dirent | dirent | ... | dirent | filename | filename | ... | filename |
+|___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____|
+ \ ^
+ \ | * could have
+ \ | trailing '\0'
+ \________________________| nameoff0
+
+ Directory block
+
+Note that apart from the offset of the first filename, nameoff0 also indicates
+the total number of directory entries in this block since it is no need to
+introduce another on-disk field at all.
+
+Compression
+-----------
+Currently, EROFS supports 4KB fixed-output clustersize transparent file
+compression, as illustrated below:
+
+ |---- Variant-Length Extent ----|-------- VLE --------|----- VLE -----
+ clusterofs clusterofs clusterofs
+ | | | logical data
+_________v_______________________________v_____________________v_______________
+... | . | | . | | . | ...
+____|____.________|_____________|________.____|_____________|__.__________|____
+ |-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|
+ size size size size size
+ . . . .
+ . . . .
+ . . . .
+ _______._____________._____________._____________._____________________
+ ... | | | | ... physical data
+ _______|_____________|_____________|_____________|_____________________
+ |-> cluster <-|-> cluster <-|-> cluster <-|
+ size size size
+
+Currently each on-disk physical cluster can contain 4KB (un)compressed data
+at most. For each logical cluster, there is a corresponding on-disk index to
+describe its cluster type, physical cluster address, etc.
+
+See "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.
+
--
2.17.1