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[Xen-devel] Remus blktap2 issue

To: Xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] Remus blktap2 issue
From: Jonathan Kirsch <kirsch.jonathan@xxxxxxxxx>
Date: Tue, 7 Sep 2010 15:28:32 -0700
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I have been playing around with Remus on Xen 4.0.1, attempting to fail-over for an HVM domU.

I've run into some problems that I think could be related to tapdisk2 and its interaction with how one sets up Remus disk replication in the domU config file.

A few things I've noticed:

-The tap:remus:backupHostIP:port|aio:imagePath notation does not work for me, although this is what is written in the Remus documentation.  However, I have found the following to work (i.e., not complain when starting domU), so this is what I've been using:


When I invoke the remus script, I do indeed see checkpoint traffic flowing between primary host and backup host.  However, disk replication does not actually seem to be working (the image on the backup host is never modified).  xend.log gives me the following error on the backup host:

[2010-09-07 11:55:41 2584] ERROR (XendDomainInfo:2244) Failed to restart domain 6.
Traceback (most recent call last):
  File "/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py", line 2227, in _restart
  File "/usr/lib64/python2.6/site-packages/xen/xend/XendDomain.py", line 998, in domain_create_from_dict
    dominfo = XendDomainInfo.create_from_dict(config_dict)
  File "/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py", line 126, in create_from_dict
  File "/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py", line 469, in start
    XendTask.log_progress(31, 60, self._initDomain)
  File "/usr/lib64/python2.6/site-packages/xen/xend/XendTask.py", line 209, in log_progress
    retval = func(*args, **kwds)
  File "/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py", line 2896, in _initDomain
  File "/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py", line 2374, in _createDevices
    devid = self._createDevice(devclass, config)
  File "/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py", line 2336, in _createDevice
    return self.getDeviceController(deviceClass).createDevice(devConfig)
  File "/usr/lib64/python2.6/site-packages/xen/xend/server/BlktapController.py", line 212, in createDevice
    raise Exception, 'Failed to create device.\n    stdout: %s\n    stderr: %s\nCheck that target \"%s\" exists and that blktap2 driver installed in dom0.' % (out.rstrip(), err.rstrip(), file);
Exception: Failed to create device.
    stdout: vbd open failed: -22
Check that target "|aio:/home/jak/remus/XenGuest1.img" exists and that blktap2 driver installed in dom0.

Thus, it seems like the backup host does not properly set up the tap disk for one of two reasons: either dom0 does not have blktap2 support, or there is something wrong with the path being passed to tapdisk2.  I do believe that dom0 on the backup host has blktap2 support, because I can launch the guest on the backup host (without using Remus) using the tap2 notation.  Further, the file /home/jak/remus/XenGuest1.img does in fact exist on the backup host.

I could be completely wrong about this, but it would seem to me that the backup host should be trying to set up the tap disk without any mention of the hostIP and port number, since (if I understand correctly) Remus does not replicate the backup's disk.

Does anyone know what might be going on here?  Am I using the right syntax in my configuration file?  Any help you could provide would be greatly appreciated.

Thanks so much,
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