|Linux & Unix Commands - Search Man Pages
SYNC(2) BSD System Calls Manual SYNC(2)
sync -- synchronize disk block in-core status with that on disk
Standard C Library (libc, -lc)
The sync() function forces a write of dirty (modified) buffers in the block buffer cache out
to disk. The kernel keeps this information in core to reduce the number of disk I/O trans-
fers required by the system. As information in the cache is lost after a system crash, ker-
nel thread ioflush ensures that dirty buffers are synced to disk eventually. By default, a
dirty buffer is synced after 30 seconds, but some filesystems exploit ioflush features to
sync directory data and metadata faster (after 15 and 10 seconds, respectively).
The function fsync(2) may be used to synchronize individual file descriptor attributes.
Many modern disks contain write-back caches. In theory sync() flushes these. In practice
there are many possible ways for this mechanism to go astray. It is prudent (where possi-
ble) to allow a few seconds after syncing for everything to settle before e.g. turning off
It may also be desirable to use dkctl(8) or scsictl(8) to disable the write-back cache
fsync(2), dkctl(8), scsictl(8), sync(8)
A sync() function call appeared in Version 6 AT&T UNIX.
Historically, sync() would schedule buffers for writing but not actually wait for the writes
to finish. It was necessary to issue a second or sometimes a third call to ensure that all
buffers had in fact been written out. In NetBSD, sync() does not return until all buffers
have been written.
BSD March 25, 2009 BSD
All times are GMT -4. The time now is 11:23 PM.