[Samba] Avoiding constant HDD access

[Rowland Penny]

On Fri, 09 Nov 2018 07:40:23 +1300
Andrew Bartlett <abartlet at samba.org> wrote:

> On Thu, 2018-11-08 at 09:29 +0000, Rowland Penny wrote:
> > On Thu, 08 Nov 2018 22:08:51 +1300
> > Andrew Bartlett <abartlet at samba.org> wrote:
> > 
> > > On Thu, 2018-11-08 at 08:48 +0000, Rowland Penny via samba wrote:
> > > >  
> > > > Yes, but it isn't doing a read, it is trying to do a write and
> > > > then being cancelled.
> > > 
> > > Rowland,
> > > 
> > > Can you point us as the evidence that leads you to that
> > > conclusion? 
> > 
> > OK, the OP posted lines like these:
> > 
> > 04:11:08 AM   UID  PID   kB_rd/s   kB_wr/s kB_ccwr/s iodelay
> > Command 04:11:28 AM     0  832      0.00      0.20      0.20
> > 0  /usr/sbin/smbd -D
> > 
> > And from 'man pidstat'
> > 
> > kB_rd/s
> >        Number of kilobytes the task has caused to be read from disk
> > per second.
> > 
> > kB_wr/s
> >       Number of kilobytes the task has caused, or shall cause to be
> > written to disk per second.
> > 
> > kB_ccwr/s
> >      Number  of kilobytes whose writing to disk has been cancelled
> > by the task. This may occur when the task truncates some dirty
> > pagecache. In this case, some IO which another task has been
> > accounted for will not be happening.
> > 
> > kB_rd/s is 0.00
> > kB_wr/s and kB_ccwr/s are both 0.20
> > 
> > So, from my reading, nothing is being read, but something is trying
> > to write, but being cancelled.
> > 
> > I could of course be totally wrong, but if I am the 'pidstat'
> > manpage needs rewriting.
> 
> It is speculation, but this might be due to a mutex-enabled TDB.  Each
> read from a tdb would now be a write while the mutex is set.  This
> might not have been seen in the past when fcntl() was in use. 

Yes, but surely the 'read' would be shown as a 'read' as well as a write

> 
> The cancellation might be the mutex being unlocked again.

The OP actually has some of the locks turned off:

locking = no
strict locking = no

> 
> > > 
> > > I think you are extrapolating too much from an smb.conf line, even
> > > when serving read only shares, there are a lot of things in Samba
> > > that can case both reads and writes.
> > 
> > Not really extrapolating anything from smb.conf, I only pointed out
> > that 'read only = no' and 'writable = no' are exact opposites of
> > each other and shouldn't be in the same share together, or do you
> > disagree ?
> 
> OK.  Either way I don't think it is relevant to the diagnosis.  The
> last statement unambiguously wins. 
> 

That is what I thought.
 
I have had 'pidstat' running on a Unix domain member, Samba
4.6.15-Debian, for quite sometime and I got two lines like this:

15:13:57        0      3571     -1.00     -1.00     -1.00
1  /usr/sbin/winbindd 

and this twice:

11:38:17    10000     27033     -1.00     -1.00     -1.00
108  /usr/sbin/smbd -D

Start time: 10:10:37
End time:   18:47:37

I have also run 'pidstat' on a Raspberrypi, Samba 4.5.12-Debian
and got similar lines to the above, just slightly more often, but no
where near as often as the OP.

If it helps, I can set up a standalone server on the rpi tomorrow and
see what I get.

Rowland