2 . 6
S U M M A R Y
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the event of failures. All of these approaches must check the disk at boot
time, a potentially lengthy operation (especially as disk sizes increase). Fur-
ther, should a crash happen at an inopportune time, the file system may still
be corrupt.
A more modern approach to avoiding corruption is journaling. Journaling,
a technique borrowed from the database world, avoids corruption by batching
groups of changes and committing them all at once to a transaction log. The
batched changes guarantee the atomicity of multiple changes. That atomicity
guarantee allows the file system to guarantee that operations either happen
completely or not at all. Further, if a crash does happen, the system need only
replay the transaction log to recover the system to a known state. Replaying
the log is an operation that takes at most a few seconds, which is considerably
faster than the file system check that nonjournaled file systems must make.
Guaranteed Bandwidth/Bandwidth Reservation
The desire to guarantee high-bandwidth I/O for multimedia applications
drives some file system designers to provide special hooks that allow applica-
tions to guarantee that they will receive a certain amount of I/O bandwidth
(within the limits of the hardware). To accomplish this the file system needs
a great deal of knowledge about the capabilities of the underlying hardware it
uses and must schedule I/O requests. This problem is nontrivial and still an
area of research.
Access Control Lists
Access control lists (ACLs) provide an extended mechanism for specifying
who may access a file and how they may access it. The traditional POSIX
approach of three sets of permissions--for the owner of a file, the group that
the owner is in, and everyone else--is not sufficient in some settings. An
access control list specifies the exact level of access that any person may
have to a file. This allows for fine-grained control over the access to a file in
comparison to the broad divisions defined in the POSIX security model.
2.6
Summary
This chapter introduced and explained the basics of what a file system is,
what it does, and what additional features a file system may choose to imple-
ment. At the simplest level a file system provides a way to store and retrieve
data in a hierarchical organization. The two fundamental concepts of any file
system are files and directories.
In addition to the basics, a file system may choose to implement a variety
of additional features that enable users to more easily manage, navigate, and
Practical File System Design:The Be File System
, Dominic Giampaolo
page 31