03-03-2013
Unless you have replaced the standard UNIX sync command with something else, it will not start any database operations. The only thing the standard sync command does is schedule flushing of file system buffers to disk. On a large server with a huge amount of memory, flushing those buffers to disk may literally take an hour depending on the disk speeds and I/O data paths. While scheduled flushes are being processed, the system may indeed be sluggish.
Many database systems perform synchronous writes to be sure that data for completed transactions is safely stored on the underlying files rather than just sitting in buffers to be flushed later. It doesn't sound like your database software does this. If transactions are flushed when they are completed, there will be a small performance penalty at the end of each transaction (and you data would be safe if a power failure or other calamity occurs later), but you wouldn't suffer the big hits you're seeing now. Note that the sync command does not wait for data to be flushed before it returns; it just schedules the flush and returns letting the OS perform the flush to disk while it continues to process other requests.
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LEARN ABOUT XFREE86
fflush
FFLUSH(3) Linux Programmer's Manual FFLUSH(3)
NAME
fflush - flush a stream
SYNOPSIS
#include <stdio.h>
int fflush(FILE *stream);
DESCRIPTION
For output streams, fflush() forces a write of all user-space buffered data for the given output or update stream via the stream's underly-
ing write function.
For input streams associated with seekable files (e.g., disk files, but not pipes or terminals), fflush() discards any buffered data that
has been fetched from the underlying file, but has not been consumed by the application.
The open status of the stream is unaffected.
If the stream argument is NULL, fflush() flushes all open output streams.
For a nonlocking counterpart, see unlocked_stdio(3).
RETURN VALUE
Upon successful completion 0 is returned. Otherwise, EOF is returned and errno is set to indicate the error.
ERRORS
EBADF stream is not an open stream, or is not open for writing.
The function fflush() may also fail and set errno for any of the errors specified for write(2).
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
+----------+---------------+---------+
|Interface | Attribute | Value |
+----------+---------------+---------+
|fflush() | Thread safety | MT-Safe |
+----------+---------------+---------+
CONFORMING TO
C89, C99, POSIX.1-2001, POSIX.1-2008.
POSIX.1-2001 did not specify the behavior for flushing of input streams, but the behavior is specified in POSIX.1-2008.
NOTES
Note that fflush() flushes only the user-space buffers provided by the C library. To ensure that the data is physically stored on disk the
kernel buffers must be flushed too, for example, with sync(2) or fsync(2).
SEE ALSO
fsync(2), sync(2), write(2), fclose(3), fileno(3), fopen(3), setbuf(3), unlocked_stdio(3)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the
latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
GNU 2017-09-15 FFLUSH(3)