Re: [dm-crypt] md-raid5+lvm+dm-crypt+kvm: one streaming write starves all reads

[Sven Eschenberg <sven@whgl.uni-frankfurt.de>]

You are not alone there.

But I fear there is no single flush timeout, I was trying to hunt
something like it down for a while without success.

The tuneable you gave determines how often pdflush daemons wake up,
another one (with centisecs in its name) gives a bound (or threshold)
after what time a dirty page is eligable at all (I'd assume that this
one needs to be smaller, to force all pages that went dirty since the
last pdflush wakeup, to be queued for a flush). Another tuneable gives
some sort of a filling ratio of dirty pages (in respect to whole system
memory) at which a flush is done (always? only when a daemon was woken
up beforehand?). Unfortunately each tuneable is documented on it's own,
there is no info, if all additional criteria is or-ed, xor-ed or and-ed
(or even a mixture?) and there is no single equation giving the whole
picture (well except somewhere in the source probably) - at least I
couldn't find one.

Regards

-Sven

P.S.: And I might be wrong, but I fear the I/O scheduling queue
discipline could influence real world disk I/O patterns as well (In
respect to the original post).

On Sat, 2009-09-19 at 02:45 +0200, Arno Wagner wrote:
> Hmm. It used to be that easy a long time ago. Seems the really 
> useful parameters are still hidden. I am more than a bit anoyed
> by this. Does anybody here know where the eraly flush timeout
> can be found? 
> 
> Arno
> 
> 
> On Fri, Sep 18, 2009 at 11:21:32PM +0200, Sven Eschenberg wrote:
> > Erh, no, this would come down to 0.1 seconds (since it's 100th - centi).
> > 
> > And now, it does not start a flush, it only wakes the daemon up for
> > analysis and consideration. If, and how much is flushed depends on other
> > tuneables.
> > Unfortunately it's not that straight forward.
> > 
> > Regards
> > 
> > -Sven
> > 
> > P.S.: Sorry for the duplicate post with the wrong e-mail addy.
> > 
> > On Fri, September 18, 2009 23:09, Arno Wagner wrote:
> > >
> > > A pity. Have you tried to set dirty_writeback_centisecs to
> > > something very low, e.g. 10? If I understand this correctly that would
> > cause regular flushes to start after 1 sec.
> > >
> > >
> > > Arno
> > >
> > >
> > 
> > 
> > 
> > 
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>