Draft:Ul2fs
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Comment: Zero independent sources that discus UL2FS; no indication of meeting Wikipedia's notability criteria. Helpful Raccoon (talk) 05:29, 3 October 2026 (UTC)
| Full name | Unix friendly Linked List File System[1] |
|---|---|
| Introduced | September 17, 2026[2] with FUSE driver |
| Partition IDs | EFI: Linux System Partition |
| Structures | |
| Directory contents | Table |
| File allocation | Linked list |
| Bad blocks | Not implemented |
| Bootable | yes[3] |
| Limits | |
| Min volume size | 1408 bytes with 128 byte blocks, 28672 bytes with 4KiB blocks |
| Max volume size | 1 EiB for 4kiB blocks, 16EiB for 64KiB blocks, 2^112 bytes for 16EiB blocks |
| Max file size | 18,446,744,073,709,551,999 bytes (16 EiB-1) |
| File size granularity | 1 byte |
| Max no. of files | 4,294,967,294 (inodes) |
| Max filename length | 122 or 123 bytes |
| Max dirname length | Implementation dependent |
| Max directory depth | unlimited |
| Allowed filename characters | any byte except 0x0 or / |
| Features | |
| Dates recorded |
|
| Date range | 1970-01-01 to 2106-02-07 (36812-02-20 if using extra fields in the inode) |
| Date resolution | 1 second |
| Attributes | link count |
| File system permissions | POSIX bits + UID + GID |
| Transparent compression | only at the block level |
| Transparent encryption | only at the block level |
| Copy-on-write | no |
| Other | |
| Supported operating systems | Hardcore Linux (replaced ext4 in September 28, 2026[4]) |
| Website | https://github.com/AMAZING2545/HardcoreLinux/ (source at tools/src/axfs) |
UL2FS(Unix friendly Linked List FileSystem) is a FUSE filesystem for Linux and other Unix operating system that support FUSE.
UL2FS was designed to be a simple filesystem that could replace Ext4 in embedded systems, since it has full POSIX permissions, Hard link and Symbolic link support, while having higher metadata density[5]. Its FAT-like design makes it easy to implement in Firmware, which might be appealing for low power devices.
Features
[edit]- Inodes
- POSIX bits
- Low metadata overhead
- Hard links
- Symbolic links
Boot sector
[edit]UL2FS reserves the first block to the Superblock, which contains all informations to mount and use the filesystem.
| parameter | size | comment |
|---|---|---|
| JMP instruction | 8 bytes | X86 needs 3 bytes, RISC architectures generally need more[7] |
| block size | 1 byte | expressed as 2^x |
| volume name | 32 bytes | any byte except 0x0 |
| reserved blocks | 2 bytes | does not include the superblock's block |
| FAT size | 4 bytes | the size of the allocation table in blocks |
| size | 8 bytes | size of the volume in blocks |
| inodes | 4 bytes | amount of inodes in blocks |
| root directory size | 1 byte | deprecated, must be set to zero |
| cylinders | 2 bytes | used only in legacy systems |
| heads | 2 bytes | used only in legacy systems |
| sectors-per-track | 2 bytes | used only in legacy systems |
| extended FAT size | 2 bytes | upper part of FAT size, generally ignored |
| used blocks | 8 bytes | generally ignored |
UL2FS usually keeps only one copy of the superblock, but a implementation can store a copy in a reserved sector.citation needed
File Allocation Table
[edit]main article: Design of the FAT file system#FAT.
UL2FS' FAT region behaves the same as other FAT filesystems, except that it uses 48-bit pointers instead of 28/32-bit of FAT32 or ExFAT. 48 bit pointers allow for very large volumes that can exceed Exabytes and Zettabytes.[8]
UL2FS only uses 0(free block) and -1(end of chain) as reserved entries in the FAT.[6]
Inode Region
[edit]the inode table contains the inodes, which are structures that store file metadata and location on disk.[9]
| name | size | comment |
|---|---|---|
| POSIX bits | 2 bytes | octal POSIX bits |
| user | 2 bytes | owner of the inode |
| group | 2 bytes | group owner of the inode |
| era | 2 bytes | now unused, can be split
in two to extend timestamps |
| created | 4 bytes | unsigned UNIX timestamp |
| modified | 4 bytes | unsigned UNIX timestamp |
| links | 2 bytes | link count |
| size | 8 bytes | file size in bytes |
| start | 6 bytes | first block in the chain |
note: the inode does not store the file type (regular, directory, link).[10]
Directory Table
[edit]Directories are stored as entries in special inodes that do not contain plain data.
each directory has at least two entries:
- "." is a hard link to itself
- ".." is a hard link to the parent (itself if it is the root directory)
each entry is stored in a 128-byte entry.
| name | size | comment |
|---|---|---|
| filename | 123 bytes | any byte except 0x0 and / |
| attributes | 1 byte | 0 = regular, 1 = directory, 2 = symlink |
| inode | 4 bytes | pointer in the inode table |
Data region
[edit]the data region lies after the inode table and contains file data, directories and symlinks.
Performance
[edit]No formal comparison is available right now
As for all FAT-like filesystems random seek is O(n) and sequential seek is O(1)citation needed
Support
[edit]UL2FS can be built from source on any Linux distro that has FUSE support.[11]
UL2FS is the native filesystem of the Hardcore Linux project, as it is a subproject of it[12]
See also
[edit]References
[edit]- ↑ https://github.com/AMAZING2545/HardcoreLinux/issues/6
- ↑ https://github.com/AMAZING2545/HardcoreLinux/commit/9530fe1b978e88791d6ad9fd42988c93cff40ec2
- ↑ the superblock only occupies 76 bytes according to the reference implementation
- ↑ https://github.com/AMAZING2545/HardcoreLinux/releases/tag/v1.3.1
- ↑ UL2FS has 32 byte inodes, while Ext4 uses 128/256 byte inodes. source: https://archive.kernel.org/oldwiki/ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout.html (2016)
- 1 2 "Spec.md". Github.
{{cite web}}: CS1 maint: url-status (link) - ↑ https://support.arm.com/documentation/den0013/0400/Introduction-to-Assembly-Language/The-ARM-instruction-sets
- ↑ like FAT12/16/32/ex ul2fs uses the same fundamental linked list structure
- ↑ Tanenbaum, Andrew (2008). Modern Operating Systems, 3rd edition (3rd ed.). Pearson Education.
- ↑ there are not enough fields in the inode to store such info.
- ↑ the source code is plain POSIX C and depends only on libfuse
- ↑ AMAZING2545. "Release 1.3.1 · AMAZING2545/HardcoreLinux". GitHub. Retrieved 2026-10-01.
{{cite web}}: CS1 maint: numeric names: authors list (link)
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