PureFS
| Filesystems |
|---|
| Virtual Filesystems |
| Disk Filesystems |
| CD/DVD Filesystems |
| Network Filesystems |
| Flash Filesystems |
Note: if you find a grammatical error in the article, please correct it (the author does not know English well, so there may be errors).
PureFS is a disk file system designed to simplify the interaction between the operating system and disks.
PureFS V1.2.0
Better than the previous version in terms of speed, but a bit more complicated.
PureFS V1.2.0 has a total of 6 main structures:
- PureFsHeader
- PureFsVolumeHeader
- PureFsFileMap
- PureFsDataMap
- PureFsFileTable
- PureFsDataTable
Advantages(+):
- Easy to implement.
- Easy to understand.
- Supports nested directories.
- Maximum available 2^64 - 1 sectors.
Disadvantages(-):
- It can be journalable, provided that the OS takes care of this task itself.
- The maximum length of the file name is 255 characters.
- The maximum number of volumes is 40.
- Does not support Unicode names.
- Takes up a lot of space.
Example Of Disk Space Allocation
Note: sector #1 is not the first sector on the disk, but the first sector of PureFS.
Partition with a single volume:
| Sector # | Structure |
|---|---|
| 1 | PureFS header |
| 2 | PureFS volume header |
| 3 | PureFS file map |
| 3+blocks/map | PureFS data map |
| 3+blocks/map*2 | PureFS file table |
| 3+blocks/map*2+32*blocks/map | PureFS data table |
Structures
PureFsHeader
Contains information about the file system.
| Offset | Size | Type | Name | Value |
|---|---|---|---|---|
| 0x00 | 0x08 | char[8] | Signature | `->PureFS` |
| 0x08 | 0x04 | u32 | Version | PureFS version number(1.2.0 = 0x010200) |
| 0x0C | 0x08 | u64 | NumVolumes | Number of PureFS volumes |
| 0x14 | 0x01 | u8 | Checksum | Complements byte sum of fields above to 0 |
| 0x15 | 0x0B | u8[11] | Reserved | Reserved for future use |
| 0x20 | 0x28*0x0C | PureFsVolumePtr[40] | Volumes | Pointers to PureFS volumes |
PureFsVolumePtr:
| Offset | Size | Type | Name | Value |
|---|---|---|---|---|
| 0x00 | 0x08 | u64 | Offset | Offset to PureFS volume (in sectors) |
| 0x08 | 0x04 | u32 | NameHash | Murmur v3 32-bit volume name hash |
PureFsVolumeHeader
Contains information about the PureFS volume.
| Offset | Size | Type | Name | Value |
|---|---|---|---|---|
| 0x00 | 0x37+0x01 | char[55+1] | Name | PureFS volume name (zero-terminated) |
| 0x38 | 0x08 | u64 | Attributes | PureFS volume attributes |
| 0x40 | 0x08 | u64 | BlocksPerMap | Maximum number of blocks(sectors) per map(FileMap/DataMap) |
| 0x48 | 0x08 | u64 | NumFiles | Current number of files |
| 0x50 | 0x08 | u64 | NumData | Current number of data blocks(sectors) |
| 0x58 | 0x1A8 | u8[424] | Reserved | Reserved for future use |
Attributes:
- BOOTABLE(0x01) - marks that there is a program (OS kernel, for example) on the partition that bootloader should load.
- READABLE(0x02)
- WRITEABLE(0x04)
PureFsFileMap
Consists of PureFsVolumeHeader::BlocksPerMap sectors. There are 32 PureFsFileMapEntry entries in each sector.
PureFsFileMapEntry:
| Offset | Size | Type | Name | Value |
|---|---|---|---|---|
| 0x00 | 0x08 | u64 | FileIndex | PureFsFile block(sector) index in the file table |
| 0x08 | 0x04 | u32 | PathHash |
Murmur v3 32-bit hash of file path on partition (without partition name!) |
| 0x0C | 0x04 | u32 | ParentHash | Murmur v3 32-bit hash of the path to the parent directory on the partition (without partition name!) |
PureFsDataMap
Consists of PureFsVolumeHeader::BlocksPerMap sectors. There are 32 PureFsDataMapEntry entries in each sector.
PureFsDataMapEntry:
| Offset | Size | Type | Name | Value |
|---|---|---|---|---|
| 0x00 | 0x08 | u64 | FileIndex | PureFsData block(sector) index in the data table |
| 0x08 | 0x08 | u8[8] | Reserved | Reserved for future use |
PureFsFileTable
Contains PureFsFile blocks.
PureFsFile:
| Offset | Size | Type | Name | Value |
|---|---|---|---|---|
| 0x00 | 0x100 | char[255+1] | Name | File name (ASCII) (zero-terminated) |
| 0x100 | 0x08 | u64 | Size | Original file size |
| 0x108 | 0x04 | u32 | Attributes | File attributes |
| 0x10C | 0x08 | u64 | ParentIndex | Index of the PureFsFile block(sector) of the parent directory in the file table |
| 0x114 | 0x08 | u64 | DataIndex | Index of the PureFsData block(sector) in the data table |
| 0x11C | 0xE4 | u8[228] | Reserved | Reserved for future use |
Attributes:
- EXECUTABLE(0x01) - marks the file as a program that can be executed by the OS.
- READABLE(0x02)
- WRITEABLE(0x04)
PureFsDataTable
Contains PureFsData blocks.
PureFsData:
| Offset | Size | Type | Name | Value |
|---|---|---|---|---|
| 0x00 | 0x1F8 | u8[504] | Data | Part of the file data |
| 0x1F8 | 0x08 | u64 | NextIndex | Index of the next data block(sector) (0 if last) |
FAQ
- How are directories stored? - Directories are the same as files, except that the PUREFS_FILE_ATR_DIRECTORY bit is set in PureFsFile::Attributes, and the data blocks(sectors) (PureFsData) contain the indexes of the files they contain.
- How are symbolic links stored? - Symbolic links differ from regular files only in that they have no data blocks, and the PureFsFile::DataIndex field contains the index of the file/directory in the PureFsFileTable table.
- How are hash collisions handled? - For this purpose, the PathHash and ParentHash fields are provided in the PureFsFileMapEntry structure to minimize the risk of finding the wrong file. When the OS reads the PureFsFile structure, it must check that the file name matches the file being searched for.