Qnx4fs
Encyclopedia
QNX4FS is an extent-based file system
used by the QNX4
and QNX6 operating systems.
As the file system uses soft updates
, it remains consistent even after a power failure, without using journaling
. Instead, the writes are carefully ordered and flushed to disk
at appropriate intervals so that the on-disk structure always remains consistent, no matter if the operation is interrupted. However, unflushed changes to the file system are nevertheless lost, as the disk cache
is typically stored in volatile memory
. This design has a considerable performance gain over journaling, by just bypassing that step.
Another notable property of this file system is that its actual metadata, like inode
information and disk bitmaps, are accessible in the same way as any other file on the file system (as /.inodes and /.bitmap, respectively). This is consistent with QNX's (in fact, Plan 9 from Bell Labs
's, or historically Unix
's) philosophy that "everything is a file."
File system
A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...
used by the QNX4
QNX
QNX is a commercial Unix-like real-time operating system, aimed primarily at the embedded systems market. The product was originally developed by Canadian company, QNX Software Systems, which was later acquired by Canadian BlackBerry-producer Research In Motion.-Description:As a microkernel-based...
and QNX6 operating systems.
As the file system uses soft updates
Soft updates
In computer file systems, soft updates is an approach to maintaining disk integrity after a crash or power outage. They are an alternative to journaling file systems....
, it remains consistent even after a power failure, without using journaling
Journaling file system
A journaling file system is a file system that keeps track of the changes that will be made in a journal before committing them to the main file system...
. Instead, the writes are carefully ordered and flushed to disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
at appropriate intervals so that the on-disk structure always remains consistent, no matter if the operation is interrupted. However, unflushed changes to the file system are nevertheless lost, as the disk cache
Disk cache
Disk cache may refer to:* Disk buffer, the small amount of buffer memory present on a hard drive.* Page cache, the cache of disk pages kept by the operating systems, stored in unused main memory.* General application-level caching of data stored on the disk....
is typically stored in volatile memory
Volatile memory
Volatile memory, also known as volatile storage, is computer memory that requires power to maintain the stored information, unlike non-volatile memory which does not require a maintained power supply...
. This design has a considerable performance gain over journaling, by just bypassing that step.
Another notable property of this file system is that its actual metadata, like inode
Inode
In computing, an inode is a data structure on a traditional Unix-style file system such as UFS. An inode stores all the information about a regular file, directory, or other file system object, except its data and name....
information and disk bitmaps, are accessible in the same way as any other file on the file system (as /.inodes and /.bitmap, respectively). This is consistent with QNX's (in fact, Plan 9 from Bell Labs
Plan 9 from Bell Labs
Plan 9 from Bell Labs is a distributed operating system. It was developed primarily for research purposes as the successor to Unix by the Computing Sciences Research Center at Bell Labs between the mid-1980s and 2002...
's, or historically Unix
Unix philosophy
The Unix philosophy is a set of cultural norms and philosophical approaches to developing software based on the experience of leading developers of the Unix operating system.-McIlroy: A Quarter Century of Unix:...
's) philosophy that "everything is a file."