zfs discuss - Jun 2010 - zfs periodic writes on idle system [Re: Getting desktop to auto sleep]

Why does zfs produce a batch of writes every 30 seconds on opensolaris b134
(5 seconds on a post b142 kernel), when the system is idle?

On an idle OpenSolaris 2009.06 (b111) system,  /usr/demo/dtrace/iosnoop.d
shows no i/o activity for at least 15 minutes.

The same dtrace test on an idle b134 system shows a batch of writes every 30
seconds.

And on current opensolaris bits, on an idle system, I see writes every 5
seconds.


The periodic writes prevents that the disk can enter power save mode.
And this breaks the /etc/power.conf autoS3 feature.  Why does zfs have
to write something to disk when the system is idle?


> > Putting the flag does not seem to do anything to the
> > system. Here is my power.conf file: 
> ...
> > autopm		enable
> > autoS3			enable
> > S3-support		enable
> 
> Problem seems to be that all power managed devices
> must be at their lowest power level, otherwise autoS3
> won''t suspend the system.  And somehow one or more
> device does not reach the lowest power level.
...
> The laptop still does not power down, because every
> 30 seconds there is a batch of writes to the hdd drive,
> apparently from zfs, and that keeps the hdd powered
> up.
> 
> The periodic writes can be monitored with:
> 
>     dtrace -s /usr/demo/dtrace/iosnoop.d
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> Why does zfs produce a batch of writes every 30 seconds on opensolaris b134
> (5 seconds on a post b142 kernel), when the system is idle?
It was caused by b134 gnome-terminal. I had an iostat
running in a gnome-terminal window, and the periodic
iostat output is written to a temporary file by gnome-terminal.
This kept the hdd busy. Older gnome-terminals (b111)
didn''t write terminal output to a disk file. Workaround is
to use xterm instead of b134 gnome-terminal. for a
command that periodically produces output
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