Back to basics – Hot add new disk to Linux VM

I recently showed this to a colleague who is new to Linux. It should be part of your minimum capabilities as an admin. Manipulating disks/volumes on a guest VM is a task we all have to do.

In this post I’m going to detail how to add a new disk to an Ubuntu VM, online. Call it a “back to basics” post.

First add another Hard-Disk to the guest as required. This will add a disk @ XX size, store it with the Guest VM files and it will be in persistent mode.

Logon to the guest and issue the lshw -C disk command to filter the list hardware output by disk;

As you can see, the new disk (16GB) does not show up, and will not until a scsci bus rescan is executed. The lazy way here would be to reboot, at which time a host rescan is done anyway.

A better way, is to force a scsi rescan. First, let’s determine which scsi host is in use

in this case it’s host2

oh noes, permission denied, even through I sudo’d !

If you examine the syntax, this is because although I used sudo, it is dropped after the “>”. the correct way is to run the echo through a pipeline then tee through.

the return to stdout is “- – -“. So what is this actually doing ?

The 3 “-” are acting as wildcards to represent the “Channel, SCSI ID and LUN” id’s available on a scsi host. So effectively “Scan all the things now!”

You could execute this against all the hosts by substituting the host id with a ? like this;

On a Red hat EL system (5.4 +) there is an included script /usr/bin/rescan-scsi-bus.sh which will achieve the same result.

From the host console, you should see a device discovery message; (or you could tail /var/log/dmesg )

running the  list hardware command again should now show the new Disk device;

fdisk sees it also, so we’re ready to partition and create the filesystem.

fidsk to partition

make the filesystem (format)

create label and manually mount

finally add to fstab so it automounts at boot time.

Next task will be to extend the partition, live. Coming soon.

Job done. Party on.