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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LEARN ABOUT LINUX

syncer

SYNCER(4)                                                  BSD Kernel Interfaces Manual                                                  SYNCER(4)

NAME

     syncer -- file system synchronizer kernel process

SYNOPSIS

     syncer

DESCRIPTION

     The syncer kernel process helps protect the integrity of disk volumes by flushing volatile cached file system data to disk.

     The kernel places all vnode(9)'s in a number of queues.  The syncer process works through the queues in a round-robin fashion, usually pro-
     cessing one queue per second.  For each vnode(9) on that queue, the syncer process forces a write out to disk of its dirty buffers.

     The usual delay between the time buffers are dirtied and the time they are synced is controlled by the following sysctl(8) tunable variables:

     Variable         Default      Description
     kern.filedelay   30           time to delay syncing files
     kern.dirdelay    29           time to delay syncing directories
     kern.metadelay   28           time to delay syncing metadata

SEE ALSO

     sync(2), fsck(8), sync(8), sysctl(8)

HISTORY

     The syncer process is a descendant of the 'update' command, which appeared in Version 6 AT&T UNIX, and was usually started by /etc/rc when
     the system went multi-user.  A kernel initiated 'update' process first appeared in FreeBSD 2.0.

BUGS

     It is possible on some systems that a sync(2) occurring simultaneously with a crash may cause file system damage.  See fsck(8).

BSD                                                                July 14, 2000                                                               BSD