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Chapter 3: Introduction to File Systems

• Ensuring users have access to data. This means that systems are up and operational, file permissions are set up property, and data is accessible.

• Securing file systems against unauthorized access. Only authorized users should have access to them. The data must be protected from intruders.

  This chapter discusses the basic structures that make up the file system, the utility that creates file systems, and how Solaris accesses the file system.

  A File System Defined

  A file system is a structure of directories used to organize and store files on disk. It is a collection of files and directories stored on disk in a standard UNIX file system format. You'll see the term "file system" used in several ways. Usually file system describes a particular type of file system (disk-based, network-based, or pseudo file system). It might also describe the entire file tree from the root directory downward. In another context, the term "file system" might be used to describe the structure of a disk slice, described later in this chapter.

  The Solaris system software uses the virtual file system (VFS) architecture, which provides a standard interface for different file system types. The VFS architecture enables the kernel to handle basic operations, such as reading, writing, and listing files, without requiring the user or program to know about the underlying file system type. Furthermore, Solaris provides file-system administrative commands that enable you to maintain file systems.

  Defining a Disk's Geometry

  Before creating a file system on a disk, you need to understand the basic geometry of a disk drive. Disks come in many shapes and sizes. The number of heads, tracks, and sectors and the disk capacity vary from one model to another.

  A hard disk consists of several separate disks mounted on a common spindle. Data stored on each disk surface is written and read by disk heads. The circular path a disk head traces over a spinning disk is called a track.

  Each track is made up of a number of sectors laid end to end. A sector consists of a header, a trailer, and 512 bytes of data. The header and trailer contain error-checking information to help ensure the accuracy of the data. Taken together, the set of tracks traced across all of the individual disk surfaces for a single position of the heads is called a cylinder.

  Disk Controller

  Associated with every disk is a controller, an intelligent device responsible for organizing data on the disk. Some disk controllers are located on a separate circuit board and some are embedded in the disk drive.

  Defect List

  Disks might contain areas where data cannot be written and retrieved reliably. These areas are called defects. The controller uses the error-checking information in each disk block's trailer to determine whether a defect is present in that block. When a block is found to be defective, the controller can be instructed to add it to a defect list and avoid using that block in the future. The last two cylinders are set aside for diagnostic use and for storing the disk defect list.

  Disk Labels

  A special area of every disk is set aside for storing information about the disk's controller, geometry, and slices. This information is called the disk's label or Volume Table of Contents (VTOC). To label a disk means to write slice information onto the disk. You usually label a disk after defining its slices. If you fail to label a disk after creating slices, the slices will be unavailable because the operating system has no way of "knowing" about the slices.

  Partition Table

  An important part of the disk label is the partition table that identifies a disk's slices, the slice boundaries (in cylinders), and the total size of the slices. A disk's partition table can be displayed by using the format utility.

  Solaris File System Types

  Solaris file systems can be put into three categories: disk-based, network-based, and pseudo.

  Disk-Based File Systems

  Disk-based file systems reside on the system's local disk. The four types of disk file systems are

  Network-Based File Systems

  Network-based file systems are file systems accessed over the network. Typically, networkbased file systems reside on one system and are accessed by other systems across the network.

  The Network File System (NFS) or remote file systems are systems made available from remote systems. NFS is the only available network-based file system.

  Pseudo File Systems

  Pseudo file systems are virtual or memory-based file systems that provide access to special kernel information and facilities. Most pseudo file systems do not use file-system disk space, although a few exceptions exist. Cache File Systems, for example, use a file system to contain the cache. Some pseudo file systems, such as the temporary file system, might use the swap space on a physical disk.

  • SWAPFS--A file system or one used by the kernel for swapping.

  • LOFS--The Loopback File System enables you to create a new virtual file system. You can access files by using an alternative path name. The entire file system hierarchy looks as though it is duplicated under /tmp/newroot, including any file systems mounted from NFS servers. All files are accessible with either a pathname starting from / or a pathname starting from /tmp/newroot.

  • CacheFS--The Cache File System enables you to use disk drives on local workstations to store frequently used data from a remote file system or CD-ROM. The data stored on the local disk is the cache.

  • TMPFS--The temporary file system uses local memory for file system reads and writes. Because TMPFS uses physical memory and not the disk, access to files in a TMPFS file system is typically much faster than to files in a UFS file system. Files in the temporary file system are not permanent; they are deleted when the file system is unmounted and when the system is shut down or rebooted. TMPFS is the default file system type for the /tmp directory in the SunOS system software. You can copy or move files into or out of the /tmp directory just as you would in a UFS /tmp file system. The TMPFS file system uses swap space as a temporary backing store as long as adequate swap space is present...