Estudio

Introduction
Linux is a Unix-like operating system, which runs on PC-386 computers. It was implemented first as extension to the Minix operating system [Tanenbaum 1987] and its first versions included support for the Minix filesystem only. The Minix filesystem contains two serious limitations: block addresses are stored in 16 bit integers, thus the maximal filesystem size is restricted to 64mega bytes, and directories contain fixed-size entries and the maximal file name is 14 characters.
We have designed and implemented two new filesystems that are included in the standard Linux kernel. These filesystems, called ``Extended File System'' (Ext fs) and ``Second Extended File System'' (Ext2 fs) raise the limitations and add new features.
In this paper, we describe the history of Linuxfilesystems. We briefly introduce the fundamental concepts implemented in Unix filesystems. We present the implementation of the Virtual File System layer in Linux and we detail the Second Extended File System kernel code and user mode tools. Last, we present performance measurements made on Linux and BSD filesystems and we conclude with the current status of Ext2fs and the future directions.History of Linux filesystems
In its very early days, Linux was cross-developed under the Minix operating system. It was easier to share disks between the two systems than to design a new filesystem, so Linus Torvalds decided to implement support for the Minix filesystem in Linux. The Minix filesystem was an efficient and relatively bug-free piece of software.
However, the restrictions in the design ofthe Minix filesystem were too limiting, so people started thinking and working on the implementation of new filesystems in Linux.
In order to ease the addition of new filesystems into the Linux kernel, a Virtual File System (VFS) layer was developed. The VFS layer was initially written by Chris Provenzano, and later rewritten by Linus Torvalds before it was integrated into the Linux kernel. Itis described in The Virtual File System.
After the integration of the VFS in the kernel, a new filesystem, called the ``Extended File System'' was implemented in April 1992 and added to Linux 0.96c. This new filesystem removed the two big Minix limitations: its maximal size was 2 giga bytes and the maximal file name size was 255 characters. It was an improvement over the Minix filesystem but someproblems were still present in it. There was no support for the separate access, inode modification, and data modification timestamps. The filesystem used linked lists to keep track of free blocks and inodes and this produced bad performances: as the filesystem was used, the lists became unsorted and the filesystem became fragmented.
As a response to these problems, two new filesytems werereleased in Alpha version in January 1993: the Xia filesystem and the Second Extended File System. The Xia filesystem was heavily based on the Minix filesystem kernel code and only added a few improvements over this filesystem. Basically, it provided long file names, support for bigger partitions and support for the three timestamps. On the other hand, Ext2fs was based on the Extfs code with manyreorganizations and many improvements. It had been designed with evolution in mind and contained space for future improvements. It will be described with more details in The Second Extended File System
When the two new filesystems were first released, they provided essentially the same features. Due to its minimal design, Xia fs was more stable than Ext2fs. As the filesystems were used more widely,bugs were fixed in Ext2fs and lots of improvements and new features were integrated. Ext2fs is now very stable and has become the de-facto standard Linux filesystem.
This table contains a summary of the features provided by the different filesystems:
| |Minix FS |Ext FS |Ext2 FS |Xia FS |
|Max FS size |64 MB |2 GB |4 TB |2 GB |...