man8/sync.8

   1 .\" Reboot/halt and Linux information extracted from Rick Faith's original
   2 .\" sync(8) manpage, dating back to the Linux 0.99 days.  The Linux-specific
   3 .\" information is attributed to Linus Torvalds
   4 .\" Copyright 1992, 1993 Rickard E. Faith (faith@cs.unc.edu)
   5 .\" May be distributed under the GNU General Public License
   6 .TH SYNC 8 1998-11-01 "GNU" "Linux Programmer's Manual"
   7 .SH NAME
   8 sync \- synchronize data on disk with memory
   9 .SH SYNOPSIS
  10 .B "sync [\-\-help] [\-\-version]"
  11 .SH DESCRIPTION
  12 .B sync
  13 writes any data buffered in memory out to disk.
  14 This can include (but is not limited to) modified superblocks,
  15 modified inodes, and delayed reads and writes.
  16 This must be implemented by the kernel;
  17 The
  18 .B sync
  19 program does nothing but exercise the
  20 .BR sync (2)
  21 system call.
  22 .PP
  23 The kernel keeps data in memory to avoid doing (relatively slow) disk
  24 reads and writes.
  25 This improves performance, but if the computer
  26 crashes, data may be lost or the file system corrupted as a result.
  27 .B sync
  28 ensures that everything in memory is written to disk.
  29 .PP
  30 .B sync
  31 should be called before the processor is halted in an unusual manner
  32 (e.g., before causing a kernel panic when debugging new kernel code).
  33 In general, the processor should be halted using the
  34 .BR shutdown (8)
  35 or
  36 .BR reboot (8)
  37 or
  38 .BR halt (8)
  39 commands, which will attempt to put the system in a quiescent state
  40 before calling
  41 .BR sync (2).
  42 (Various implementations of these commands exist; consult your
  43 documentation; on some systems one should not call
  44 .BR reboot (8)
  45 and
  46 .BR halt (8)
  47 directly.)
  48 .SH "OPTIONS"
  49 .TP
  50 .B "\-\-help"
  51 Print a usage message on standard output and exit successfully.
  52 .TP
  53 .B "\-\-version"
  54 Print version information on standard output, then exit successfully.
  55 .TP
  56 .B "\-\-"
  57 Terminate option list.
  58 .SH ENVIRONMENT
  59 The variables \fBLANG\fP, \fBLC_ALL\fP, \fBLC_CTYPE\fP,
  60 and \fBLC_MESSAGES\fP have the usual meaning.
  61 .SH "CONFORMING TO"
  62 POSIX.2
  63 .SH NOTES
  64 On Linux,
  65 .B sync
  66 is only guaranteed to schedule the dirty blocks for writing; it can
  67 actually take a short time before all the blocks are finally written.
  68 The
  69 .BR reboot (8)
  70 and
  71 .BR halt (8)
  72 commands take this into account by sleeping for a few seconds after
  73 calling
  74 .BR sync (2).
  75 .PP
  76 This page describes
  77 .B sync
  78 as found in the file\%utils-4.0 package;
  79 other versions may differ slightly.
  80 .SH "SEE ALSO"
  81 .BR sync (2),
  82 .BR halt (8),
  83 .BR reboot (8),
  84 .BR update (8)