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Distributed File System
A) General principles
As most POSIX compliant operating systems, ALMOS-MKH sees the external storage device as an array of clusters, where each clusters can store one page of 4 Kbytes. Each cluster occupies eight 512 bytes physical sectors, and is indexed by a cluster identifier, calledd cluster_id.
To support various File System types, ALMOS-MKH defines a generic Virtual File System API defined in the almos-mkh/kernel/fs/vfs.c, and almos-mkh/kernel/fs/vfs.h files.
All supported fille systems are supposed to have a hierarchical tree_based structure, where some special directory files contain links to other files that can be directory files, or terminal files. Therefore, any file X in the file system can be unambiguously defined by a pathname describing the path from the VFS root to the X file. A directory file has only on single path name, but a terminal file can have several parent directories, defining several path names for one single file.
The directory file format is specific for each file system type.
Any cluster_id can be allocated to any file (terminal or directory), and for all supported file systems, the ordered set of clusters allocated to a given file is registered in a structure called File Allocation Table (FAT).
The FAT format is specific for each file system type.
Any file system stores three types of informations:
- some clusters are used to store the terminal files,
- other clusters are used to store the directory files,
- specific clusters are used to store the FAT.
As all UNIX or Microsoft operating systems ALMOS-MKH implements a File System Cache, that is a partial copy, in kernel memory, of the file system on device. The VFS is actually the implementation of this generic File System Cache.
The file systems currently supported by ALMOS-MKH are
- the FATFS respects the Microsoft FAT32 format,
- the DEVFS describes all peripheral devices available in the hardware architecture,
- the RAMFS isentirely implemented in physical memory, and does not require any access to an external block device.
B) VFS implementation
To reduce the memory footprint of this File System Cache, ALMOS-MKH uses two methods:
- For the directory files, only a subset or the directory entries contained is copied in the File System Cache.
- For the terminal files, only the pages that have been actually accessed are copied in the File System Cache.
This File System Cache is therefore dynamically extended by the OS to satisfy the user processes requests. The physical memory allocated to this cache is only released when a file is removed from the file system on device.
