VMware Communities: VCB in SCSI hot-add mode

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31 Replies Last post: Aug 28, 2009 2:32 PM by markdjones82

VCB in SCSI hot-add mode

Aug 13, 2008 6:18 AM

alex0 29 posts since
Aug 13, 2008

Using this mode allows VCB to run as a VM.

However the documentation (vi3_35_25_u2_vm_backup.pdf) states:

"Creating a VCB Helper Virtual Machine
If you use Consolidated Backup in the Hot‐Add mode, you need to create a shadow
virtual machine for Consolidated Backup to use internally. The shadow virtual machine
has the same name as your virtual VCB proxy with the VCB-HELPER suffix added. For
example, if the name of your VCB proxy virtual machine is BackupProxy, the shadow
virtual machine, should be named BackupProxy(VCB-HELPER)."

Whiskey Tango Foxtrot.

What's a VCB Helper VM? How do I create it? What software goes on it? What resource specs for the helper?
The documentation is woeful.

Furthermore the doco goes on to say:

"When using Consolidated Backup in the Hot‐Add mode, the VCB user must have an
ability to perform additional tasks, such as adding and removing virtual disks, on the
VCB proxy and VCB helper virtual machines."

Ie it says "VCB helper virtual machines" PLURAL... so now we need MORE THAN ONE VCB helper VM?
How many do I need? What rules of thumb?

Also, what specs is the main VCB proxy (either VM or physical). ?? The doco makes absolutely no mention of minimum specs.

WOEFUL. How did this doc ever make it through QA? I guess the same way ESX 3.5U2 got released with a timebomb.

Any light would be appreciated.

Tags: vcb, scsi, hot-add, mode, helper

Reply Re: VCB in SCSI hot-add mode Aug 14, 2008 2:09 AM

alex0 29 posts since
Aug 13, 2008

Can someone shed some light on this?

Reply Re: VCB in SCSI hot-add mode Aug 14, 2008 6:41 AM

dconvery 1,865 posts since
May 10, 2006

Alex -

First off - Welcome to the forums!

Several new things have been addeed to VCB 1.5 and everyone is still figuring it out... From what I understand, Hot-add mode allows you to use VCB in a VM. What happens is that the "helper" VM is running Windows and is used to mount the VMDK files as if they were hot-added disks. This is useful for file-levelbackups.VCB will direct the helper VM to mount the disk. You PROBABLY don't need VCB framework installed on the VM, but I would, just in case. Actually makeing a cone and renaming the hostname may expedite things.

The extra permissions in Virtual Center allow the VCB user to direct the adding and deleting of these disks to the helper VM. As for the plural "helper VMs", I would expect multiple proxies and helpers in large installations with hundreds of VMs and multiple clusters. Obviously, you would need at least one proxy and helper in each cluster. Since VCB backups are very I/O intensive, they will need plenty of RAM. In a physical world, I recommend 4GB with the /PAE switch inboot.ini.

Interstingly, VCB was originally intended to take some load off of the ESX hosts and the VMs and put it on a separate machine utilizing the SAN. Thus, you would have a truly LAN-free backup mechanism assuming the proxy was also a media server. Version 1.1 introduced "NBD" mode, allowing backups over the LAN and now with version 1.5, we have hot-add mode. I think this is just VMware's answer to what someone is asking for. I still "preach" that VCB should be used as originally intended when planning for backups. Using hot-add mode places additional strain on the ESX hosts. I have done plenty of implementations and I always say this:

"Just because you CAN do something, doesn't mean you SHOULD..."

This is all my opinion and I am sure someone will chime in to make arguments against my opinion...

Dave

PS - if you are going to VMworld, check out my breakout session: BC2497 - VMware Consolidated Backup- Report from the Trenches

Reply Re: VCB in SCSI hot-add mode Aug 14, 2008 4:10 PM

alex0 29 posts since
Aug 13, 2008

Dave,

Thanks for your response.

"Several new things have been addeed to VCB 1.5 and everyone is still figuring it out" .. if the VMware documentation was decent there would be nothing to figure out.
I still have no idea what specs this "helper VM" should be, i assume if you append the precise notation "(VCB-HELPER)" after the helper VM name then VCB just knows to use it.

Also, it says using SCSI HOT-add it reads the disks via the ESX I/O stack.

I assume this means
a) CPU of the ESX host will be used while this happens
b) the I/O stack will be used, thus possibly impacting I/O performance of other VMs

Do you agree with a) and b) ?

Also, any idea HOW MUCH CPU overhead and I/O overhead is involved with this?
How much of the I/O stack does it use?

I agree that running VCB as a VM may not be ideal, however for some customers, our hands may be forced, ie they don't want to buy a dedicated hardware for VCB, and they want to use VCB.
Either way, I need to know exactly how VCB SCSI-hot add and these mystery helper VMs need to be configured and also the impact to the ESX host.

The documentation falls WELL SHORT of the mark.

Any further thoughts from your perspective would be appreciated.

Regards
alex

Reply Re: VCB in SCSI hot-add mode Aug 14, 2008 5:26 PM

in response to: alex0

dconvery 1,865 posts since
May 10, 2006

A VCB Helper VM will use the ESX I/O stack instead of the SAN I/O. So, yes there will be added strain to the ESX server. Obviously, if you plan for this I/O and additional CPU and memory resource utilization, it should not be a problem. I still like a dedicated VCB because it can be directly attached to a tape library without worry about weather it can be virtualized. Remember, the ultimate goal is to have something off site in case of a complete disaster. Tape is usually cheaper than SAN replication. As for the documentation, I agree with you for some of the docs. It seems that there is a rush to get something out the door and hopefully we will just figure it out.

It a little frustrating for me because I am speaking about VCB at VMworld. I am changing my preso now to incorporate VCB 1.5 as much as I can.

Dave

Reply Re: VCB in SCSI hot-add mode Sep 3, 2008 8:10 PM

soliton 4 posts since
May 20, 2007

I am trying to set up vcbMounter to run in a hotadd mode using vcb proxy and helper VMs. Both proxy and the helper VMs are using local vmfs store on esx 3.5.upd2 server.
VC is running ver. 2.5 and ecb 1.5 is installed on the proxy. The helper machine is a clone of the proxy and the vm being backed up is hosted on the same local vmfs.
The file system on the proxy is prepared as described in "Virtual Machine Backup Guide Update 2 Release for ESX Server 3.5, ESX Server 3i version 3.5, VirtualCenter 2.5."
The command I run from the proxy is:
vcbMounter -h -u -p -a name:test -r d:\mnt\test -t fullvm -m hotadd
it fails with the message:
"Error: Failed to hot remove SCSI disk: scsi0:0 is not present. Failed to remove disk scsi0:0."
It also unregisters all disks on the helper machine (and sometimes completely deletes those from the store).
I wonder if someone got any other reference to configuration information for the hotadd feature apart from the paper I mentioned, which is nothing more than just an announcement of the new product features and is not much help at all.

Reply Re: VCB in SCSI hot-add mode Sep 3, 2008 8:44 PM

in response to: soliton

alex0 29 posts since
Aug 13, 2008

Here is some more info I came across:

My VCB Proxy in a VM is called 1.5_proxy.

I want to backup the VM cor_W2K8. This is on a completely different datastore. There is no SAN connectivity. I could use the -m nbd option to backup over the network, but since VCB 1.5 I can use the new hot-add mechanism to back it up.

To use hot-add, I create a VM called 1.5_proxy(VCB_HELPER) which resides on the same datastore as the target VM (cor_W2K8).

I gave the VCB_HELPER VM all the same settings as my proxy (I don't think it matters), but the thing is that the VCB_HELPER VM does not have any disks of its own.

The sole purpose of this VM is for tracking linked clones - once you managed to create a linked clone of the target VM for backing up, it gets associated to the VCB_HELPER. Once the VCB-HELPER has the 'references' for the linked clones, the real VCB proxy can use those references to do disk-to-disk copy via the VMkernel rather than copying over the network, which gives the improved performance.

My understanding is that you would need to have a VCB_HELPER on each datastore which contains VMs that you want to backup (but I'll leave someone with more expertise with using hot-add to clarify that).

Reply Re: VCB in SCSI hot-add mode Sep 7, 2008 7:27 PM

soliton 4 posts since
May 20, 2007

Alex,
Thanks for the tip.I can advance a little bit further now but the mounting process still failes.
I see the following steps implemented:
1. Create virtual machine snapshot - completed
2. Create linked clone - completed
3. Reload virtual machine - completed
4. Reconfigure virtual machine - completed
5. Remove snapshot - completed
The process appears to fail on step 4 with the error:
"Error: Cannot obtain clone information."
and rolls back in step 5.
Any help is appreciated.

Reply Re: VCB in SCSI hot-add mode Sep 8, 2008 2:57 AM

in response to: alex0

dconvery 1,865 posts since
May 10, 2006

Hey Alex -

Are you saying that the helper VM does NOT need even an OS? It just needs to be a blank VM?

Reply Re: VCB in SCSI hot-add mode Sep 8, 2008 3:04 AM

in response to: dconvery

depping 2,992 posts since
Jan 17, 2005 VMware

He's right, just a blank VM. no OS...

Duncan
My virtualisation blog:
http://www.yellow-bricks.com

If you find this information useful, please award points for "correct" or "helpful".

Reply Re: VCB in SCSI hot-add mode Sep 8, 2008 3:07 AM

in response to: depping

dconvery 1,865 posts since
May 10, 2006

Thanks Duncan.

Reply Re: VCB in SCSI hot-add mode Sep 8, 2008 3:08 AM

soliton 4 posts since
May 20, 2007

It doesnt look like the actual os is requiered. However, a SCSI controller is apparently needed.

Reply Re: VCB in SCSI hot-add mode Sep 8, 2008 3:09 AM

in response to: dconvery

alex0 29 posts since
Aug 13, 2008

Dave,

Yes the info I have been provided from VMware (unofficial) has the helper VM setup with NO disks.

I'm still wondering if we need one helper PER datastore?
And what exactly does the helper do?
When is it required, when is it NOT required?

Image below:

Attachments:

$113FDFFC3411679.jpg (39.6 K)

Reply Re: VCB in SCSI hot-add mode Sep 8, 2008 3:13 AM

in response to: alex0

dconvery 1,865 posts since
May 10, 2006

Well, Soiton's comment about needing a SCSI adapter makes sense. It is hot-adding disks to the helper VM. I believe you really only need one helper VM per cluster, because it would have access to all of the datastores in that cluster.

Dave

Reply Re: VCB in SCSI hot-add mode Sep 8, 2008 3:32 AM

in response to: dconvery

alex0 29 posts since
Aug 13, 2008

Possibly the helper VM is required if the main VCB_PROXY is sitting on a LUN presented by a different SAN to the LUN the VM you want to backup is sitting on or something.

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