[rrd-developers] rrdcached contention when flushing

[kevin brintnall]

On Mon, Nov 03, 2008 at 05:35:50PM -0000, Daniel.Pocock at barclayscapital.com wrote:
> They all become un-stuck at the same time, maybe 20 seconds later, and
> then the graphs appear very quickly.

Daniel,

The FLUSH commands are waiting to be notified that the file has been
written out to disk.  They block on pthread_cond_wait() and don't return
until the queue thread has written the file out to disk.

What is happening on your system at that time?  Are there other events
which may slow down the I/O?

I have seen this behavior before on one of my Linux 2.6.x machines.  When
it has dirtied too many pages, all I/O on the system pauses until it has
flushed the "write-back" pages out to disk.  What kind of system are you
running?

> I'm using r1621 + the patch adding pthread_cond_init(&ci->flushed,
> NULL);

You should upgrade to at least r1626.  Otherwise, you may notice some
files that are getting flushed by the flush proces (corresponding to the
-f timer) are hanging around in queue forever.  The bug was introduced in
r1588, resolved in r1626.

-- 
 kevin brintnall =~ /kbrint at rufus.net/