Does sync(2) block writes until completed? Post: 302239838

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Top Forums UNIX for Advanced & Expert Users Does sync(2) block writes until completed? Post 302239838 by mschwage on Wednesday 24th of September 2008 01:59:56 PM

09-24-2008

Registered User

Does sync(2) block writes until completed?

Gentle readers,
I am trying to observe system behavior on our RHEL 5.2 machines. I notice that, it appears to me, based on random iterations of

Code:

dd if=/dev/zero of=/var/tmp/bigfile bs=1024 count=20000

...that if the pdflush is flushing buffers at the time of my write, my write will take about as long as

Code:

dd if=/dev/zero of=/var/tmp/bigfile bs=1024 count=20000 ; sync

This indicates to me that pdflush blocks all write()'s until the flush to disk is complete. The question is: am I correct?

Most of the time, the first dd (without the sync) is much faster than the second. By watching /proc/meminfo, I am able to see approximately when the sync initiated by pdflush takes place. This would be every 30 seconds, as my machine is not filling the disk buffers very quickly. We're talking only a few hundred kilobytes every 30 seconds.

Thanks.
-Mike

mschwage

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sync

sync(2) 							   System Calls 							   sync(2)

NAME

       sync - update super block

SYNOPSIS

       #include <unistd.h>

       void sync(void);

DESCRIPTION

       The  sync() function writes all information in memory that should be on disk, including modified super blocks, modified inodes, and delayed
       block I/O.

       Unlike fsync(3C), which completes the writing before it returns, sync() schedules but does not  necessarily  complete  the  writing  before
       returning.

USAGE

       The  sync()  function  should  be  used	by  applications that examine a file system, such as fsck(1M), and df(1M), and is mandatory before
       rebooting.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       df(1M), fsck(1M), fsync(3C), attributes(5), standards(5)

SunOS 5.11							    5 Jul 1990								   sync(2)