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update(8) [bsd man page]

UPDATE(8)						      System Manager's Manual							 UPDATE(8)

NAME
update - periodically update the super block SYNOPSIS
update DESCRIPTION
Update is a program that executes the sync(2) primitive every 30 seconds. This insures that the file system is fairly up to date in case of a crash. This command should not be executed directly, but should be executed out of the initialization shell command file. SEE ALSO
sync(2), sync(8), init(8), rc(8) BUGS
With update running, if the CPU is halted just as the sync is executed, a file system can be damaged. This is partially due to DEC hard- ware that writes zeros when NPR requests fail. A fix would be to have sync(8) temporarily increment the system time by at least 30 seconds to trigger the execution of update. This would give 30 seconds grace to halt the CPU. 7th Edition November 17, 1996 UPDATE(8)

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SYNC(8) 						      System Manager's Manual							   SYNC(8)

NAME
sync - synchronize data on disk with memory SYNOPSYS
sync [--help] [--version] DESCRIPTION
sync writes any data buffered in memory out to disk. This can include (but is not limited to) modified superblocks, modified inodes, and delayed reads and writes. This must be implemented by the kernel; The sync program does nothing but exercise the sync(2) system call. The kernel keeps data in memory to avoid doing (relatively slow) disk reads and writes. This improves performance, but if the computer crashes, data may be lost or the filesystem corrupted as a result. sync ensures that everything in memory is written to disk. sync should be called before the processor is halted in an unusual manner (e.g., before causing a kernel panic when debugging new kernel code). In general, the processor should be halted using the shutdown(8) or reboot(8) or halt(8) commands, which will attempt to put the system in a quiescent state before calling sync(2). (Various implementations of these commands exist; consult your documentation; on some systems one should not call reboot(8) and halt(8) directly.) GNU STANDARD OPTIONS
--help Print a usage message on standard output and exit successfully. --version Print version information on standard output, then exit successfully. -- Terminate option list. ENVIRONMENT
The variables LANG, LC_ALL, LC_CTYPE, and LC_MESSAGES have the usual meaning. CONFORMING TO
POSIX 1003.2 NOTES
On Linux, sync is only guaranteed to schedule the dirty blocks for writing; it can actually take a short time before all the blocks are finally written. The reboot(8) and halt(8) commands take this into account by sleeping for a few seconds after calling sync(2). This page describes sync as found in the fileutils-4.0 package; other versions may differ slightly. Mail corrections and additions to aeb@cwi.nl. Report bugs in the program to fileutils-bugs@gnu.ai.mit.edu. SEE ALSO
sync(2), halt(8), reboot(8), update(8) GNU fileutils 4.0 1998-11 SYNC(8)

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