Extreme console latency during disk IO (8.0-RC1, previous releases also affected according to others)

[Svein Skogen (listmail account)]

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Ivan Voras wrote:
> Thomas Backman wrote:
>> I'm copying this over from the freebsd-performance list, as I'm
>> looking for a few more opinions - not on the problems *I* am having,
>> but rather to check whether the problem is universal or not, and if
>> not, find a possible common factor.
>> In other words: I want to hear about your experiences, *good or bad*!
>>
>> Here's the original thread (not from the beginning, though):
>> http://lists.freebsd.org/pipermail/freebsd-performance/2009-October/003843.html
>>
>>
>> Long story short, my version: when the disk is stressed hard enough,
>> console IO becomes COMPLETELY unbearable. 10+ seconds to switch
>> between windows in screen(1), running (or even typing) simple
>> commands, etc. This happens both via SSH and the serial console.
> 
> Hmm, this looks familiar - I've noticed it before on the physical (VGA)
> console and I notice it all the time under VMWare. It sort of looks like
> disk IO really blocks network IO in this case - I use the VMs over ssh.

I've seen some similar behaviour here with my zfs/istgt backend. I
"resolved" the issue by reducing the arc_max so that each disk-io cycle
would be "short enough" not to cause the istgt to generate timeouts
towards my two vmware hosts. The io-backend for the box is a megaraid
8308 (mfi) card with 8 disks on it. This box is running RELENG_7 now (I
tried running it with RELENG_8 but it had a nasty tendency of simply
locking up, dragging down both my ESXi hosts with it).

//Svein

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