[How To] RDM mapping of local SATA storage for ESXi

RDM mapping of local SATA storage for ESXi

25 10 2010

This post has been sat in my WordPress Drafts folder for sometime since I no longer use local storage this way. I decided to post it however as (a) it’s a good learning curve for ESXi work and (b) others may have more luck that me.

I recently acquired three 1TB drives and decided to do something about my lack of storage at home. Always trying to make best use of existing kit (and save money) I decided to stick the drives in to my HP ML110 and try something in a VM instead of doing the sensible thing of lobbing them in to a dedicated NAS box.

After wasting a few hours I realised that the onboard SATA RAID controller of the ML110 just can’t do RAID5 and to make matters worse, when I gave up and created a RAID1 array with a hot spare, vSphere 4.1 didn’t recognise the array and instead saw the drives as 3 individual drives. I saw this as a chance to try out the WAFL-alike ZFS file system. FreeNAS had been my NAS of choice recently so I chose to try ZFS in that.

I point blank refused to create 3 1TB VMDK’s (one of each of the three drives) so I set about figuring out how to create Raw Device Mappings (RDMs) of the local SATA drives. There were a couple of posts on the net that got me a little closer, but no guide/article had the whole thing down, so that’s my aim with this blog post.

Step 1

Once you had your drives installed, SSH to your ESXi box (now even easier in vSphere 4.1) and go to the /dev/disks directory. There, if you perform a ls -l, you’ll see your drives listed:

Ignore the instances of your drives which show them as VM stores (vm1.*****). We want to look at the raw devices.

Step 2

Now move to the /vmfs/volumes folder. Here you can see your existing local datastore(s). If, like me, you had a solitary hard-drive, you’ll just see localdisk01 or whatever you chose to name the local datastore:

Step 3

Now we are going to use the vmkfstools utility to create our RDM’s. Remember that a RDM is just another VMDK, but instead of the VMDK pointing to a xxx-flat.vmdk file (which is the actual virtual hard disk), the VMDK points to our physical device. Being as we still need to create this VMDK file we need to save it somewhere. Since we just have the one local datastore, we are going to create the RDM VMDK files in it’s root.

The following command creates the RDM VMDK for us:

vmkfstools -z /vmfs/devices/disks/<name of RAW device from Step 1> <location to store VMDK>/<RDM name>.vmdk

In my personal example below, I am creating an RDM called rdm_WD2DWCAVU0477582.vmdk and it is being stored in the location /vmfs/volumes/localdisk01/ I chose the name of the VMDK to match the name of the serial number of the physical drive (and what is shown in Step 1) to help with troubleshooting in the future when I get an inevitable drive failure). You can call your RDM’s whatever you wish.

The name of the RAW device (t10.ATA____WDC_WD10EARS2D00Z5B1__________________________WD2DCAVU0477582 in my example) you will have noted from Step 1 when you listed all local devices attached to your ESXi host. This is why the tech Gods created Copy n Paste! You will want to copy the full device name as shown in Step 1 in to the vmkfstools command.

Step 4

Once you have repeated the steps for all of your local SATA drives, you can navigate to where you created the RDM’s (in my case /vmfs/volumes/localdisk01) and perform an ls -l *.vmdk to see the new VMDK’s you have created:

Don’t panic – the xxx-rdmp.vmdk files will reflect the size of the RAW devices they are mapping to, but rest assured it will be taking no more space than a few bytes on your local disk!

Step 5

You can now add your RDM’s to an existing VM. vSphere doesn’t recognise this as a true RDM (to a SAN) so you just browse the local disk datastore for the VMDK files that we created.

Edit the properties of an existing VM and click Add…

Step 6

Select Use an existing virtual disk and click Next >

Step 7

Click Browse. You now need to navigate your local datastore and select the VMDK’s that we created in Step 3).

Once complete you will be shown a confirmation window. Repeat Steps 5 through 7 to add additional RDM’s to your VM.

Step 8

You should now see your new Hard Disk’s in your VM and vSphere will correctly identify them as Mapped Raw LUN.

NOTE: One thing I forgot to show in the screen shots, is that you should create your RDM’s on a new SCSI controller! You do this by simply selecting a new SCSI ID starting with 1:x instead of 0:x. Existing VMDK’s should be on SCSI Controller 0. Your RDM’s should be on SCSI controller 1. Although my screenshot shows 0:3 this should read 1:3.

You can now save your VM configuration. Your VM will now access the RAW SATA drives  and be able to use things like SMART to monitor its health.

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