[Beowulf] LSI Megaraid stalls system on very high IO?

[Mark Hahn]

> This system might have a decent amount of memory - but are the sysctl
> tunings for the dirty buffer sizes set small or something?

actually, my experience is the opposite: if sysctls permit large 
accumulation of dirty buffers, a system with big memory and slow
write speeds will feel unpleasantly choppy.  basically, I think 
you should set vm.dirty_ratio less than the amount your storage 
system can commit to disk in O(few seconds).  that's the synchronous form 
of writeback - I also like to make the async form more active as well.
(lower vm.dirty_background_ratio and vm.dirty_writeback_centisecs=100).

of course, this is only relevant to big-memory-slow-disk machines.
(but consider a hypothetical 1TB box with a single disk and default
settings: if it dirties 100G, the sync writeback will kick in and 
saturate the disk for up to 10 minutes...)

regards, mark hahn.