Page 456 - DCAP103_Principle of operating system

Unit 14: Case Study of Linux Operating System

The basic idea behind this type of file systems is to maintain a journal, which describes all file Notes
system operations in a sequential order. By sequentially writing out and changes to the file system
data or metadata (i-nodes, superblock, etc.), the operations do not suffer from the overheads
of disk head movement during random disk accesses. Eventually, the changes will be written
out, committed, to the appropriate disk location, and the corresponding journal entries can be
discarded.
If a system crash or power failure occurs before the changes are committed, during restart, the
system will detect that the file system was not unmounted properly, will traverse the journal,
and apply the file system changes described in the journal log.
Ext3 is designed to be highly compatible with ext2, and in fact, all core data structures and disk
layout are the same in both systems. Furthermore, a file system which has been unmounted as an
ext2 system, can be subsequently mounted as an ext3 system and offer the journaling capability.

The journal is a file used as a circular buffer. It may be stored on the same or separate
device from the main file system. Since the journal operations are not “journalled”
themselves, these are not handled by the same ext3 file system. Instead, a separate JBD
(Journaling Block Device) is used to perform the journal read/write operations. JBD
supports three main data structures: log record, atomic operation handle, and transaction.
A log record describes a low level file system operation, typically resulting in changes
within a block. Since a system call such as write includes changes at multiple places —
i-nodes, existing file blocks, new file blocks, list of free blocks, etc., related log records are
grouped in atomic operations. Ext3 notifies JBD of the start and end of a system call processing,
so that JBD can ensure that either all log records in an atomic operation are applied, or none
of them. Finally, primarily for efficiency reasons, JBD treats collections of atomic operations as
transactions. Log records are stored consecutively within a transaction. JBD will allow portions
of the journal file to be discarded only after all log records belonging to a transaction are safely
committed to disk.
Since writing out a jog entry for each disk change may be costly, ext3 may be configured to keep
a journal of all disk changes, or only of changes related to the file system metadata (the i-nodes,
superblocks, bitmaps, and so on). Journaling metadata only introduces fewer system overheads
and results in better performance, however does not make any guarantees against corruption
of file data. Several other journaling file systems maintain logs of only metadata operations:
(e.g., SGI’s XFS).
14.6.7 /proc File System

Another Linux file system is the /proc (process) file system, an idea originally devised in the 8th
edition of UNIX from Bell Labs and later copied in 4.4BSD and System V. However, Linux extends the
idea in several ways. The basic concept is that for every process in the system, a directory is created
in /proc. The name of the directory is the process PID expressed as a decimal number. for example,
/proc/619 is the directory corresponding to the process with PID 619. In this directory files that
appear to contain information about the process, such as its command line, environment strings,
and signal masks. In fact, these files does not exist on the disk. When they are read, the system
retrieves the information from the actual process as needed and returns it in a standard format.
Many of the Linux extensions relate to other files and directories located in /proc. They contain
a wide variety of information about the CPU, disk partitions, devices, interrupt vectors, kernel
counters, file systems, loaded modules, and much more. Unprivileged user programs may read
much of this information to learn about system behavior in a safe way. Some of these files may
be written to in order to change system parameters.

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