PureFS

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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:

1. PureFsHeader
2. PureFsVolumeHeader
3. PureFsFileMap
4. PureFsDataMap
5. PureFsFileTable
6. PureFsDataTable

Advantages(+):

1. Easy to implement.
2. Easy to understand.
3. Supports nested directories.
4. Maximum available 2^64 - 1 sectors.

Disadvantages(-):

1. It can be journalable, provided that the OS takes care of this task itself.
2. The maximum length of the file name is 255 characters.
3. The maximum number of volumes is 40.
4. Does not support Unicode names.
5. 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.

See Also

PureFS V1.1.0
Murmur v3 32-bit hash