Bart Samwel writes:

This looks like it's a generic property of syncing an ext3 file system.
Try turning off laptop_mode and then running "sync". You will probably
see the same behaviour.

yep, every sync() causes a gratuitous write to disk N seconds later
even when no data has been read or written since the last sync().

$ while sleep 5; do sync; done
^C
$ 

Jan  1 14:09:31 gw kernel: kjournald(760): WRITE block 2662 on hda1 
Jan  1 14:09:36 gw kernel: kjournald(760): WRITE block 2664 on hda1 
Jan  1 14:09:42 gw kernel: kjournald(760): WRITE block 2666 on hda1 
Jan  1 14:09:47 gw kernel: kjournald(760): WRITE block 2668 on hda1 
Jan  1 14:09:52 gw kernel: kjournald(760): WRITE block 2670 on hda1 

quoted

Note, the reason I ask is this is a SSD so just because a physical
read has taken place recently unneeded writes should be avoided.

Turning laptop_mode to 0, but leaving other settings the same
resolves the uneeded write:

For your SSD I guess you need to get rid of the
sync-after-disk-activity, but keep the other VM behaviours of
laptop_mode (such as avoiding swapping out pages / writing back dirty
pages in order to free memory as long as it is also possible to just
drop pages that are not dirty).

You can probably achieve this by:
- having a large commit interval etc., like you have now
- setting laptop_mode to a very large value, e.g. a couple of hours.
That will trigger a sync if and only if there has been *no* disk
activity at all for hours on end -- i.e., pretty much never. And the
other write-reducing VM features of laptop_mode will still be enabled.

It would perhaps be a good thing to split these mechanisms into separate
knobs. Write batching (the sync-after-disk-activity stuff and also the
dirty_ratio / dirty_background_ratio changes) are a completely separate
mechanism from write avoidance (the other mechanism I mentioned).

I'm going to run without laptop mode for a while to see how this
goes.  This system is a small router/firewall with most writes going
to a tmpfs so the number of writes to the ext3 fs is only a few a
month (provided no one logs into the system to do a config change).

Thanks for the help.

-- 
Dave