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© Rakhesh Sasidharan

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partedUtil and installing ESXi on a USB disk and using it as a datastore

Recently I wanted to install ESXi 6.5 on a USB disk and also use that disk as a datastore to store VM on. I couldn’t get any VMs to run off the USB disk but I spent some time getting the USB disk presented as a datastore so wanted to post that here.

Installing ESXi 6.5 to a USB is straight-forward.

And this blog post is a good reference on what to do so that a USB disk is visible as a datastore. This blog post is about presenting a USB disk without ESXi installed on it – i.e. you use the USB disk entirely as a datastore. In my case the disk already had partitions on it so I had to make some changes to the instructions in that blog post. This meant a bit of mucking about with partedUtil, which is the ESXi command line way of fiddling with partition tables. (fdisk while present is no longer supported as it doesn’t do GPT).

1. First, connect to the ESXi host via SSH.

2. Shutdown the USB arbitrator service (this is used to present a USB disk to a VM): /etc/init.d/usbarbitrator stop

3. Permanently disable this service too: chkconfig usbarbitrator off

4. Now find the USB disk device from /dev/disks. This can be done via an ls -al. In my case the device was called /dev/disks/t10.SanDisk00Cruzer_Switch0000004C531001441121115514.

So far so good?

To find the partitions on this device use the partedUtil getptbl command. Example output from my case:

The “gpt” indicates this is a GPT partition table. The four numbers after that give the number of cylinders (7625), heads (255), sectors per track (63), as well as the total number of sectors (122508544). Multiplying the cylinders x heads x sectors per head should give a similar figure too (122495625).

An entry such as 9 1843200 7086079 9D27538040AD11DBBF97000C2911D1B8 vmkDiagnostic 0 means the following:

  • partition number 9
  • starting at sector 1843200
  • ending at sector 7086079
  • of GUID 7086079 9D27538040AD11DBBF97000C2911D1B8, type vmkDiagnostic (you can get a list of all known GUIDs and type via the partedUtil showGuids command)
  • attribute 0

In my case since the total number of sectors is 122495625 (am taking the product of the CHS figures) and the last partition ends at sector 7086079 I have free space where I can create a new partition. This is what I’d like to expose to the ESX host.

There seems to be gap of 33 sectors between partitions (at least between 8 and 7, and 7 and 6 – I didn’t check them all :)). So my new partition should start at sector 7086112 (7086079 + 33) and end at 122495624 (122495625 -1) (we leave one sector in the end). The VMFS partition GUID is AA31E02A400F11DB9590000C2911D1B8, thus my entry would look something like this: 10 7086112 122495624 AA31E02A400F11DB9590000C2911D1B8 0.

But we can’t do that at the moment as the disk is read-only. If I try making any changes to the disk it will throw an error like this:

From a VMware forum post I learnt that this is because the disk has a coredump partition (the vmkDiagnostic partitions we saw above). We need to disable that first.

5. Disable the coredump partition: esxcli system coredump partition set --enable false

6. Delete the coredump partitions:

7. Output the partition table again:

So what I want to add above is partition 9. An entry such as 9 1843232 122495624 AA31E02A400F11DB9590000C2911D1B8 0.

8. Set the partition table. Take note to include the existing partitions as well as the command replaces everything.

That’s it. Now partition 9 will be created.

All the partitions also have direct entries under /dev/disks. Here’s the entries in my case after the above changes:

Not sure what the “vml” entries are.

9. Next step is to create the datastore.

That’s it! Now ESXi will see a datastore called “USB-Datastore” formatted with VMFS6. :)