Christopher Smith wrote:
stitch it together into a RAID1. So, it looks like this:
"Concentrator"
/dev/md0
/ \
GigE GigE
/ \
"Disk node 1" "Disk node 2"
So far I've tried using iSCSI and GNBD as the "back end" to make the
disk space in the nodes visible to the concentrator. I've had two
problems, one unique to using iSCSI and the other common to both.
Personally, I wouldn't mess with iSCSI or GNBD. You don't need GNBD in
this scenario anyway; simple nbd (which is in the mainline kernel...get
the userland tools at sourceforge.net/projects/nbd) will do just fine,
and I'd be willing to bet that it is more stable and faster...
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".
Well, that's the fault of either iSCSI or GNBD. md/raid1 over nbd works
flawlessly in this scenario on 2.6 kernels (for 2.4, you'll need a
special patch -- ask me for it, if you want).
--
Paul
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