[Linux-cluster] io scheduler and gnbd

[Markus Hochholdinger]

hi,

i'm succesfully using gnbd as a single service for a long time. Now i 
discovered a weired problem with the gnbd devices with kernel 2.6.18. I build 
the gnbd.ko module out of the cvs tree.
All works fine if you don't do to much an the gnbds. But if you stress test 
the devices, the gnbds will hang, e.g. reads and writes hang. If you restart 
the gnbd server, the client will continue to read and write until the next 
hang.
So i first checked my gnbd servers and tried from 1.01 till 1.03 and the 
latest cvs. But the problem is still there. From another gnbd client i had no 
problem, with none of these gnbd server versions (i was impressed you can mix 
these versions). Also changing the kernel on the gnbd server didn't helped.

So i was stick to the gnbd client with kernel 2.6.18. I have to use this 
kernel because of the new hardware. So i tried a little and found out that 
changing the default io scheduler for the gnbd devices on the client makes 
the hanging write and reads resume. The default scheduler was cfq and with 
this i can easily reproduce this behavior. With the deadline scheduler it 
doesn't.

So i read a little about io scheduling on linux. And my assumption is a gnbd 
device shouldn't need any io scheduling, because the network has no latency 
when seeking like a hard disk. On the gnbd server there are getting request 
from more than one gnbd client, so scheduled io on the client would mix up 
the scheduling on the server. And also the server does its own io scheduling 
when writing to the real disk.
So i could use the noop scheduler or have i missed something?

Has anyone on the list more info about io scheduling and gnbd?


-- 
greetings

eMHa
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