Changes between Version 1 and Version 2 of file_system

Timestamp:

Jan 11, 2020, 8:25:02 PM (4 years ago)

Author:

alain

Comment:

--

file_system

@@ -3 +3 @@
 [[PageOutline]]
 
-== A) General principles ==
+== __A) General principles__ ==
 
-TBD...
+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
+ [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/fs/vfs.c  almos-mkh/kernel/fs/vfs.c] et [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/fs/vfs.h 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:
+ 1. some clusters are used to store the '''terminal files''',
+ 1. other clusters are used to store the '''directory files''',
+ 1. 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.
+
+=== B.1 Inodes Tree ===
+
+
+=== B.2 File Mapper ===
+
+
+=== B.3 FIle Descriptor ===
+
+