Skip to content

Latest commit

 

History

History
67 lines (52 loc) · 2.56 KB

README.md

File metadata and controls

67 lines (52 loc) · 2.56 KB
Raw

GotFS

GotFS is a filesystem implemented on top of a key-value store. The key value store is GotKV, which is an immutable key-value store, sitting on top of a content-addressed data store. Every operation on a GotKV store returns a new reference to a new store with the change. GotFS inherits this property. All operations return a reference to a new filesystem. A GotFS filesystem is just a GotKV store with a specific key structure used to represent files and directories.

Objects

Ref

A reference to data in the content-addressed store. This type is provided by GotKV.

Extent

A part of a file. It includes a Ref, an offset, and a length. It is basically an instruction to:

  1. Get the referenced data from content-addressed storage
  2. Only include the data from offset to offset+length.

This enables packing small files into the same blob.

Info

Information about a file or directory. Most importantly the permissions and type of file.

Key Layout

All objects are represented by an Info entry at a specific key, and content stored under keys prefixed with the metadata key.

File data is stored in a content-addressed store, and references to the data are stored in GotKV.

For example: The file "test.txt" with 10B of data in it would produce the following key value pairs.

/                                -> Info (dir)
/test.txt/                       -> Info (file)
/test.txt/<NULL>< 64 bit: 10 >   -> Extent

A directory is stored as a metadata object.

/                                         -> Info (dir)
/mydir/                                   -> Info (dir)
/mydir/myfile.txt/                        -> Info (file)
/mydir/myfile.txt/<NULL><64 bit offset>   -> Part

It is possible for a file to be at the root

/                            -> Info (file)
/<NULL><64 bits      >       -> Extent
/<NULL>< next offset >       -> Extent

All Info keys end in a trailing /, including regular files. Keys for Info objects contain no NULL characters. Keys for extents contain exactly 1 NULL character 9 bytes from the end of key, separating the path from the offset. Extent keys are always prefixed by the Info key for the file they are part of.

Reading A File

To read from a file in GotFS you first lookup the Info entry for the path of the file. If there is not an entry at the path or the entry is not for a regular file, then return an error.

Then convert the offset to read from to a key. Seek to the first entry after that key. The extent referenced by that entry will end at the offset in the key, and will contain data overlapping the target range.