[Xen-bugs] [Bug 743] Unable to start more than a few domU's on RHEL4 amd64 (Error: Backend device not found)

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http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=743





------- Comment #7 from tejasvia@xxxxxxxxx  2006-08-28 16:42 -------
I tried using the blktap driver. First, it is slow; maybe because the
user-space tools spew a lot of messages to syslog by default. 

With the blktap driver, after I start around 5 VM's (3 vbd's each), I get
random failures:
"Error: Device 0 (vif) could not be connected. Hotplug scripts not working"
OR
"Kernel panic - not syncing: VFS: Unable to mount root fs on
unknown-block(0,0)"


>From syslog:
---
Aug 28 19:32:26 compute-5-24 BLKTAPCTRL:        PID: [1135]
Aug 28 19:32:26 compute-5-24 logger: /etc/xen/scripts/vif-bridge: Successful
vif-bridge offline for vif38.0, bridge xenbr0.
Aug 28 19:32:26 compute-5-24 BLKTAPCTRL: Write_msg called: CTLMSG_PARAMS,
sending [aio:/usr0/VM/vm7/vm7-fs.img, /usr0/VM/vm7/vm7-fs.img]
Aug 28 19:32:26 compute-5-24 BLKTAPCTRL: Generated cookie, 40944
Aug 28 19:32:26 compute-5-24 BLKTAPCTRL: Received CTLMSG_PID_RSP
Aug 28 19:32:26 compute-5-24 TAPDISK: Tapdisk: Received msg, len 59, type 1,
UID 40944, drivertype 0
Aug 28 19:32:26 compute-5-24 BLKTAPCTRL: Read_msg failure - CTLMSG_IMG
Aug 28 19:32:26 compute-5-24 TAPDISK: Received CTLMSG_PARAMS:
[/usr0/VM/vm7/vm7-fs.img]
Aug 28 19:32:26 compute-5-24 BLKTAPCTRL: Unmapping vbd
Aug 28 19:32:26 compute-5-24 TAPDISK: Loaded driver: name [tapdisk_aio], type
[0]
Aug 28 19:32:26 compute-5-24 BLKTAPCTRL: Write_msg called: CTLMSG_CLOSE
Aug 28 19:32:26 compute-5-24 TAPDISK: XXX: block-aio
open('/usr0/VM/vm7/vm7-fs.img')
Aug 28 19:32:26 compute-5-24 BLKTAPCTRL: DEL_DISKTYPE: Freeing entry
Aug 28 19:32:26 compute-5-24 TAPDISK: Couldn't get fd for AIO poll support. 
This is probably because your kernel does not have the aio-poll patch applied.
--

I have EPOLL compiled into the kernel. I am not sure what other "aio-poll
patch" the user-space tool is complaining about. I am not sure if this is the
cause for the failures.


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