On Tue, 3 Feb 2015 12:45:44 -0500 Weedy <weedy2887@xxxxxxxxx> wrote:
> Is there a kernel option or sysfs toggle that disables write caching?
> Or forces the kernel to commit everything constantly.
>
> ------
> I don't really want to join another ML, especially a higher traffic
> one just to ask this when it only bugs me sometimes. But I'll shut up
> if this is unwanted.
> ------
>
> I use a similar kernel config with respect to selected options on all
> my systems but this only effect my laptop.
Maybe it has something to do with "laptop_mode"
/proc/sys/vm/laptop_mode
https://www.kernel.org/doc/Documentation/laptops/laptop-mode.txt
> On any of my personal system or system I have remote access too
> nr_dirty drifts up and down and nr_writeback stays around 0 (assuming
> the system isn't working hard).
> On my laptop both nr_dirty and nr_writeback stay at 0. I can make them
> go up to 10ish if I untar something but then almost immediately go
> back to 0. If I didn't know better I would swear dirty_*_centisecs or
> something was set to a near instant commit interval but I haven't
> found evidence of that. The hard drive light blinks almost constantly
> once a second, even if I'm at a X login screen.
That doesn't quite sound like laptop mode...
laptop mode syncs things more aggressively when the disk is spinning, and
more lazily when it isn't ... or something like that.
So I can imagine nr_dirty staying low - any read will spin the disk, and that
will quickly flush out any dirty pages.
The once-a-second blink is harder to explain.
Still, it would be worth checking if laptop mode is enabled, and if so
disable it and see if anything changes.
NeilBrown
> As I said this doesn't bug me most of the time but if I let my FF
> session get too large or start multiple VMs anything that might make
> me swap a little, the machine pretty much dies from IOWAIT. Which I'm
> guessing is because it's trying to flush (syncfs?) imediately and
> constantly.
>
> You guys spend all day in the IO subsystem, any idea where I can keep
> looking? It has persisted across reboots and kernel updates.
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