[Samba] Avoiding constant HDD access

[Andrew Bartlett]

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. 

The cancellation might be the mutex being unlocked again.

> > 
> > 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. 

Thanks,

Andrew Bartlett

-- 
Andrew Bartlett                       http://samba.org/~abartlet/
Authentication Developer, Samba Team  http://samba.org
Samba Developer, Catalyst IT          http://catalyst.net.nz/services/samba