Xen devel - Aug 2008 - Q on XendDomainInfo.py - _configBootloader for iSCSI booting (with sample patch)

Hello,

I''m still playing with getting block-iscsi working in a stock Xen 3.2.1
install on Centos. The stock code appears to not be working correctly, though
distributions (SuSE, at least) have their own patches to address this. I have
been having some difficulty getting the SuSE patches to work on my own, so I set
about to make the change in a minimal way for my own uses. (But, others are
welcome to find value in it.)

In short, I am changing _configureBootloader to notice that it is getting a
iscsi: parameter as a disk (in theory, this should be changed to any device type
that isn''t phy or tap) and then it creates a dummy VBD object. That
triggers the hotplug event and causes the disk to be added to the xenstore. I
then iterate over the VBDs in XenStore until I find the matching UUID, take the
''node'' parameter from there (which contains the real local
disk), and pass that to the bootloader. So far, this code works with "xm
create" but not with my Xen-API-based start script... but it''s as
likely to be a problem in the latter as the former.

Is there anything fundamentally wrong with this approach? Should I do it another
way? In specific, I don''t like creating the dummy VBD, but I
don''t know how to trigger the hotplug process without providing a
device node. (I''m using dom0.create_vbd() and that always assumes you
have the node already.)

(Should I overwrite the ''dev'' leaf in xenstore to match the
''node'' leaf? I suspect I should, but I don''t do this
yet.)

I''ve attached the patch and I look forward to any feedback you can
provide.

Joe Pranevich

--

--- original-XendDomainInfo.py	2008-08-21 16:40:40.000000000 -0400
+++ XendDomainInfo.py	2008-08-21 18:14:35.000000000 -0400
@@ -2113,7 +2113,50 @@
             fn = blkdev_uname_to_file(disk)
             taptype = blkdev_uname_to_taptype(disk)
             mounted = devtype == ''tap'' and taptype !=
''aio'' and taptype != ''sync'' and not
os.stat(fn).st_rdev
-            if mounted:
+
+            # Check for iSCSI first
+            match  = re.match(''^(.*?):(.*)$'', disk);
+            access_type = match.group(1)
+            access_location = match.group(2)
+
+            # Todo: Change this to access_type != known types (phy, etc)
+            if (access_type == ''iscsi''):
+                log.info("Mounting %s using %s.", (access_location,
access_type))
+
+                from xen.xend import XendDomain
+                dom0 = XendDomain.instance().privilegedDomain()
+
+                # We need a bogus VBD at this point...
+                vbd = {
+                   ''mode'': ''RO'',
+                   ''device'': ''/dev/null''
+                }
+
+                vbd_uuid = dom0.create_vbd(vbd, disk)
+          
+                # Now, we need to find out what the REAL device node is, to
pass to the bootloader
+                from xen.xend.XendDomain import DOM0_ID
+                from xen.xend.xenstore.xsutil import GetDomainPath
+                dom_path = GetDomainPath(DOM0_ID)
+
+                real_device = 0
+                vbds = xstransact.List("%s/backend/vbd/0" % dom_path)
+                for x in vbds:
+                    uuid = xstransact.Read("%s/backend/vbd/0/%s/uuid"
% (dom_path, x))
+                    if (uuid == vbd_uuid):
+                        found = 1
+                        real_device =
xstransact.Read("%s/backend/vbd/0/%s/node" % (dom_path, x))
+                        break
+                else:
+                   msg = "Unable to find VBD: %s" % vbd_uuid
+                   log.error(msg)
+                   raise VmError(msg)
+                        
+                msg = "ISCSI Device Detected: %s on %s" % (disk,
real_device)
+                fn = real_device
+                log.info(msg)
+               
+            elif mounted:
                 # This is a file, not a device.  pygrub can cope with a
                 # file if it''s raw, but if it''s QCOW or other
such formats
                 # used through blktap, then we need to mount it first.

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