[Samba] Avoiding constant HDD access

[Jeremy Allison]

On Thu, Nov 08, 2018 at 09:29:26AM +0000, Rowland Penny via samba 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.

The housekeeping is doing an fstat(fd,..) on a file descriptor
that is attached to a log file. That is a meta-data fetch that
may wake up the disk. That's why syscall strace would be conclusive
here.