Problem 2: The system doesn't deal with failure very well.
Once I got the RAID1 up and running, I tried to simulate a node
failure by pulling the network cable from the node while disk activity
was taking place. I was hoping the concentrator would detect the
"disk" had failed and simply drop it from the array (so it could later
be simply re-added). Unfortunately that doesn't appear to happen.
What does happen is that all IO to the md device "hangs" (eg: disktest
throughput drops to 0M/sec), I am unable to either 'cat /prod/mdstat'
to see the md device's status or use mdadm to manually fail the device
- both simply result in the command "hanging".
I probably can't help much at the moment, but...
I didn't realize that NBD type things had advanced to even this level of
stability. This is good news. I've been wanting to do something like
what you're trying for some time to overcome the bounds of
power/busses/heat/space that limit you to a single machine when building
a large md or LVM. Looking at the GNBD project page, it still seems
pretty raw, although a related project DDRAID seems to hold some promise.
I'd be pretty timid about putting anything close to production on these
drivers, though.
What distro / kernel version / level of GNBD are you using?
Regards-
Michael Stumpf
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