Hello everyone,
Current host build is RHEL 6.2, soon to be upgrading.
I'm in the process of mapping out a KVM/RHEV topology. I have questions about
the landscape of storage pools and the instantiation of specific scsi_host IDs
using virsh nodedev-create and some magic XML definitions. I'm grouping these
questions together because the answer to one may impact the other.
High-level goals:
- fence off I/O by providing NPIV paths to VMs individually
- provide each VM with a pair of paths to the same storage for redundancy
Problem 1 - Storage Pools:
I've successfully created a storage pool by starting with an initial SCSI host
based off one of my virtual HBAs. I see my LUNs. I performed a virsh
pool-dumpxml and captured the configuration of the storage pool using just one
SCSI host. I then modified the XML file to include a 2nd SCSI host, stopped &
deleted the original storage pool, then created & started a new storage pool.
In the GUI, I am able to see the same LUNs, listed twice (once for each path).
Here's my XML file:
<pool type='scsi'>
<name>VM1POOL</name>
<uuid>f0465f3f-3c9c-766c-3c96-678a99ec81cb</uuid>
<capacity>144813588480</capacity>
<allocation>144813588480</allocation>
<available>0</available>
<source>
<adapter name='host5'/>
</source>
<source>
<adapter name='host6'/>
</source>
<target>
<path>/dev/disk/by-path</path>
<permissions>
<mode>0700</mode>
<owner>-1</owner>
<group>-1</group>
</permissions>
</target>
</pool>
Question 1 - Is this the correct approach for presenting a VM with SAN storage
in a redundant fashion? Or should I instead use the mpath approach and find
some way with the storage pool XML file to restrict which mpath devices are
utilized (thereby restricting which LUNs the storage pool sees?).
It seems my current approach provides each LUN twice, however a LUN has two
different volume identifiers based on its path:
UNIT:0:0:13
UNIT:0:1:13
Problem 2 - Specifying scsi_host# in a device XML file
As I mentioned, I've successfully created a virtual HBA using virsh
nodedev-create and an XML config. Here's a sample config:
[root@host ~]# cat vm1vhba1.xml
<device>
<parent>scsi_host3</parent>
<capability type='scsi_host'>
<capability type='fc_host'>
<wwnn>a97a3b9220010002</wwnn>
<wwpn>a97a3b9220010001</wwpn>
</capability>
<capability type='vport_ops' />
</capability>
</device>
Pretty straightforward. The parent is physical SCSI host 3. The resulting
SCSI host is 5. This is one of the SCSI hosts used in the storage pool above.
So, I wanted to test what would happen to my VM if I ran virsh nodedev-destroy
scsi_host5. Essentially, testing the redundancy of the paths to the VM. The
device was successfully deleted, however a few other things happened, both
expected and unexpected.
- Since I only associated one of the two LUN units from my storage pool with
the VM, I expected this to cause the VM to fail. It did fail, however in a
headless fashion. Unexpected. The VM is still up and running in memory. Any
effort to write out to the disks results in failures like "read-only
filesystem". Expected.
- The Storage Pool entered into an inactive state. Unexpected. Efforts to
restart the storage pool using virsh pool-start failed because one of the
scsi_host devices was not present. Expected.
I tried to recreate the device using virsh nodedev-create vm1vhba1.xml, and the
device was created successfully but as scsi_host9 (the next available scsi_host
beyond the original scsi_host5).
So, unless I can recreate this scsi_host5, I won't be able to restart my
storage pool and restore I/O to my VM.
Question 2: Is there a definition I can use in the XML config to specify the
resulting scsi_host #?
I've been told that RHEL 6.3 is supposed to provide some methodology of
auto-instantiating my virtual HBAs upon the reboot of a host so that the VMs on
the will see their storage and can start automatically on boot. I have a
feeling that the answer to my question may be brushing up against this
functionality.
I am also assuming that if it's possible to specify the scsi_host# in my XML
config, that I'll be able to restart my storage pool successfully and that will
enable my VM to reestablish the connection to their storage.
Thanks in advance,
MD
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