11.2 File-System Implementation
11.2.1 Overview
- A boot-control block, ( per volume ) a.k.a. the boot block in UNIX or the partition boot sector in Windows contains information about how to boot the system off of this disk. This will generally be the first sector of the volume if there is a bootable system loaded on that volume, or the block will be left vacant otherwise.
- A volume control block, ( per volume ) a.k.a. the master file table in UNIX or the superblock in Windows, which contains information such as the partition table, number of blocks on each filesystem, and pointers to free blocks and free FCB blocks.
- A directory structure ( per file system ), containing file names and pointers to corresponding FCBs. UNIX uses inode numbers, and NTFS uses a master file table.
- The File Control Block, FCB, ( per file ) containing details about ownership, size, permissions, dates, etc. UNIX stores this information in inodes, and NTFS in the master file table as a relational database structure.
- An in-memory mount table.
- An in-memory directory cache of recently accessed directory information.
- A system-wide open file table, containing a copy of the FCB for every currently open file in the system, as well as some other related information.
- A per-process open file table, containing a pointer to the system open file table as well as some other information. ( For example the current file position pointer may be either here or in the system file table, depending on the implementation and whether the file is being shared or not. )
- When a new file is created, a new FCB is allocated and filled out with important information regarding the new file. The appropriate directory is modified with the new file name and FCB information.
- When a file is accessed during a program, the open( ) system call reads in the FCB information from disk, and stores it in the system-wide open file table. An entry is added to the per-process open file table referencing the system-wide table, and an index into the per-process table is returned by the open( ) system call. UNIX refers to this index as a file descriptor, and Windows refers to it as a file handle.
- If another process already has a file open when a new request comes in for the same file, and it is sharable, then a counter in the system-wide table is incremented and the per-process table is adjusted to point to the existing entry in the system-wide table.
- When a file is closed, the per-process table entry is freed, and the counter in the system-wide table is decremented. If that counter reaches zero, then the system wide table is also freed. Any data currently stored in memory cache for this file is written out to disk if necessary.
11.2.2 Partitions and Mounting
- Physical disks are commonly divided into smaller units called partitions. They can also be combined into larger units, but that is most commonly done for RAID installations and is left for later chapters.
- Partitions can either be used as raw devices ( with no structure imposed upon them ), or they can be formatted to hold a filesystem ( i.e. populated with FCBs and initial directory structures as appropriate. ) Raw partitions are generally used for swap space, and may also be used for certain programs such as databases that choose to manage their own disk storage system. Partitions containing filesystems can generally only be accessed using the file system structure by ordinary users, but can often be accessed as a raw device also by root.
- The boot block is accessed as part of a raw partition, by the boot program prior to any operating system being loaded. Modern boot programs understand multiple OSes and filesystem formats, and can give the user a choice of which of several available systems to boot.
- The root partition contains the OS kernel and at least the key portions of the OS needed to complete the boot process. At boot time the root partition is mounted, and control is transferred from the boot program to the kernel found there. ( Older systems required that the root partition lie completely within the first 1024 cylinders of the disk, because that was as far as the boot program could reach. Once the kernel had control, then it could access partitions beyond the 1024 cylinder boundary. )
- Continuing with the boot process, additional filesystems get mounted, adding their information into the appropriate mount table structure. As a part of the mounting process the file systems may be checked for errors or inconsistencies, either because they are flagged as not having been closed properly the last time they were used, or just for general principals. Filesystems may be mounted either automatically or manually. In UNIX a mount point is indicated by setting a flag in the in-memory copy of the inode, so all future references to that inode get re-directed to the root directory of the mounted filesystem.
11.2.3 Virtual File Systems
- Virtual File Systems, VFS, provide a common interface to multiple different filesystem types. In addition, it provides for a unique identifier ( vnode ) for files across the entire space, including across all filesystems of different types. ( UNIX inodes are unique only across a single filesystem, and certainly do not carry across networked file systems. )
- The inode object, representing an individual file
- The file object, representing an open file.
- The superblock object, representing a filesystem.
- The dentry object, representing a directory entry.
- Linux VFS provides a set of common functionalities for each filesystem, using function pointers accessed through a table. The same functionality is accessed through the same table position for all filesystem types, though the actual functions pointed to by the pointers may be filesystem-specific. See /usr/include/linux/fs.h for full details. Common operations provided include open( ), read( ), write( ), and mmap( ).
