Re: Q: Priority of md_thread()

["Jakob Østergaard" <jakob@unthought.net>]

On Wed, May 22, 2002 at 01:59:57PM +0200, Potthoff, Kay wrote:
> I have a question regarding the priority of the md_thread() function
> (defined in md.c).
> 
> I have seen that the priority of this thread is set to -20 (parameter
> current->nice), which means that this thread takes the highest priority. The
> problem I have is that this thread blocks some other task that is answering
> keep alive network pings from another (sibling) node. Because the keep alive
> answers are blocked the other node decides to take-over the RAID. This
> situation leeds to the fact that two nodes try to start the RAID. MD don't
> like this. The RAID is considered to be faulty and both nodes try to repair
> it, that is a problem.

That system cannot be stable.

Remember the -20 is only for the CPU scheduler, and the md_thread is not CPU
bound (unless you have ISA controllers, or *very* special disks).  What I am
trying to say is, I doubt much would change if the nice level was changed to
20 instead...

I think your problem is that something (and probably as you state, md_thread)
is causing fluctuations in the ping replies, causing your other nodes to take
an action which they shouldn't take based on a ping fluctuation (that could
just as easily be caused by a packet loss or some other random event in the
whole computer-network-computer system).

If you want multiple nodes mounting the same array, you need some consistency
protocol to be able to *guarantee* that no two nodes will try to master the
same array simultaneously. Ping fluctuations do not constitute a consistency
protocol.


> I figuered out that the cause of this problem is the high priority of the
> md_thread() function. I feel tempted to decrease this priority, but I feel
> some upcoming ache in my belly because I don't know the exact reason why the
> priority had been set to -20. I hope that somebody can explain me why! The
> comments in the sources seem to regard to RAID-5 personality. If so that
> would be no problem, I intend only to use RAID-1.

Please let me know if I completely misunderstood the situation   :)

-- 
................................................................
:   jakob@unthought.net   : And I see the elder races,         :
:.........................: putrid forms of man                :
:   Jakob Østergaard      : See him rise and claim the earth,  :
:        OZ9ABN           : his downfall is at hand.           :
:.........................:............{Konkhra}...............:
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