Expert Day: HPVM hosts and cDSFs?

[Joe Ledesma]

Hello,

Can HPVM Virtual Disks use cluster-wide device special files (cDSFs) on the hosts?

Already using Agile devices as Virtual Disks with AVIO.

Found no mention of cDSFs in recent HPVM release notes.

I realize HPVM already looks at the WWIDs and handles automatically how the storage device names can be different on each host. For example, if a SAN LUN is seen as disk450 on one host and disk451 on another host, hpvmstatus for the same VM on each host will show the different device names but it all just works.

cDSFs would help avoid errors when adding Virtual Disks to VMs and especially when removing Virtual Disks from VMs and then un-presenting the LUNs from the hosts.

Background: Customer on SX2000-based midrange server npars using HPVM for database and app workloads, in production on HPVM 4.2 with VMs as Serviceguard A.11.19 packages configuration on Virtual Disk SAN storage. Planning update to HPVM 4.3 and A.11.20. (Started with HPVM 2.0 in 2007.) Test/dev VMs are on an i2 blade using HPVM 4.3 on Virtual FileDisks on NFS.

Joe

[Alan_153]

No. HPVM V4.3 doesn’t support cDSFs.

Alan

[Dave Olker]

Hi Joe,

One other tip to be aware of regarding HPVM storage and keeping track of which VMs are using which disks: hpvmdevinfo(1M) is your friend.  :)

You can run hpvmdevinfo either on the VM Host or inside a VM Guest and it will tell you which storage devices are in use and which device files they map to.  Since cDSFs are not supported, this tool can be a big help in keeping things straight on different VM guests.

Dave

[Joe Ledesma]

Hi Dave,

Yes, we use hpvmdevinfo(1M) to map between host and guest storage devices and it is quite helpful.

Also helpful is how in the HPVM storage implementation for Virtual Disks the real WWID is passed through, so the WWID one sees in the guest is the real WWID. This helps when manually mapping directly between guest devices and the storage array.

However, while hpvmdevinfo(1M) helps with mapping between host and guest, it does not help with mapping across hosts. This is where cDSFs would help.

Looking at this example to explain:

> For example, if a SAN LUN is seen as disk450 on one host and disk451 on another host, hpvmstatus for the same VM on each host will show the different device names but it all just works.

Imagine there is also a different SAN LUN that is seen as disk451 and disk452 on the respective hosts and both the disk450/disk451 and the disk451/disk452 are Virtual Disks in the same guest.

Now if you want to plan for moving off the first SAN LUN:

With hpvmdevinfo(1M) you determine that this disk450|disk451 is disk4 in the guest. So inside the guest you do the file system or volume manager work to move off disk4. Now you want to remove the storage device from the VM and then un-present the LUN from the HPVM multiserver environment hosts/SG cluster nodes.

In planning the steps, you have to say:

If you are on hostA, hpvmmodify the guest to remove disk450 or if you are on hostB, …remove disk451.

(If you are on hostA don’t want to remove disk451 from the guest there by accident since is still in use in the guest as disk5…)

Or:

Make sure you are on hostA, then remove disk450 from the guest.

Then after the SAN change, you want to double-check that the correct LUN was un-presented:

ioscan should now report NO_HW for disk450 if you are on hostA….disk451 if you are on hostB.

Basically, in your procedures you either have to anticipate how things may look different depending what host an admin might be logged into, or establish a policy that you always do certain changes from a certain host. It might vary per guest. For example your Serviceguard VM packages might have different primary nodes in the node switching parameters and this primary node could be considered the “home base” for that VM so you do all changes on it from there.

Joe

[Joe Ledesma]

Alan,

It would be helpful if the HPVM release notes could be updated to say this. I realize one could just assume correctly that since cDSFs are not mentioned in the release notes that they were not supported.

See my other reply for an example of how cDSFs would be useful in HPVM Virtual Disk SAN environment.

Joe

[Dave Olker]

Hi Joe,

Thanks for the detailed example.  I'll pass this along as another example of a customer requesting cDSF support. 

In the mean time, I know there are scripts floating around that allow you to rename device files so that you could have the same disks use the same device files on multiple hosts in a cluster. 

I've attached a sample script that I know a lot of people have used successfully to do exactly that.  I just ran it on my box and was able to very easily rename a DVD disk device file to a new name.  Worked like a charm.

Hope this helps,

Dave

[Joe Ledesma]

Hi Dave,

Thanks for posting the script.

To add to the suggestion that HP add cDSF support to HPVM:

It would be helpful if a utility like hpvmdevtranslate(1M) were provided, which used the WWIDs to produce a script /var/opt/hpvm/common/hpvm_dev_convert to update the VM config when upgrading the host from 11.23 to 11.31.

Joe

[Dave Olker · Accepted solution]

Thanks Joe. Noted!