Using Storage and Compute Resources Together

Having the ability to use persistent storage on a virtual machine allows you to keep the results of an analysis or to separate the service state (e.g. a database) from the service installation.

Persistent storage can be attached to a virtual machine when it is deployed or dynamically after it has started.

Attach a Disk at Deployment Time

The easiest way to attach a persistent disk to machine is to use the option --persistent-disk when launching a virtual machine. The command for doing this looks like:

$ stratus-run-instance --persistent-disk=${VOLUME_UUID} ${APP_ID}

Attaching a disk at deployment time has a couple of limitations. First, only one persistent disk can be attached to the machine. Second, the persistent disk remains attached to the virtual machine for the entire lifetime of the virtual machine.

Initializing the Disk

A new persistent disk is an empty, uninitialized block device. Consequently, the first time the disk is used, it must be formatted.

When the virtual machine starts the persistent disk will be attached to the machine. Logging into the virtual machine, you can find the device name for the disk by using the command fdisk -l, looking for a disk without a partition table and with the expected size.

A new physical disk is usually partitioned so that the space can be used for different purposes (file storage, swap, etc.). For persistent disks on the cloud, there is really no benefit in doing this unless there is a reason you need more than one file system on the disk. If you do want to partition the disk, you can use the command fdisk to do so.

You must however format the disk to make it useful. To format the disk, use the command mkfs. It is very strongly recommended that you provide a label for the file system.

$ mkfs --type ext4 -L MYLABEL /dev/sdc
mke2fs 1.42.9 (4-Feb-2014)
/dev/sdc is entire device, not just one partition!
Proceed anyway? (y,n) y
Filesystem label=MYLABEL
Writing superblocks and filesystem accounting information: done

If you did not partition the disk, you’ll be asked about formatting the entire device. Just answer yes (‘y’) in this case.


The formatting of the disk should be done only the first time you use the persistent disk. If you reformat the disk later, all of the existing data will be lost.

Mounting the Disk

To use the disk to store data, it must be mounted into the file system. From within the virtual machine do:

$ mkdir /mnt/pdisk
$ mount -L MYLABEL /mnt/pdisk
$ ls /mnt/pdisk
$ touch /mnt/pdisk/mydata.txt

This shows that you can use the disk normally as you would any other disk on the system.


The benefit in providing a label is that you don’t need to search for the device name of the disk. The device name may vary depending on when the disk is attached to the machine and different operating systems may use different naming conventions.

When the machine is shutdown, the persistent disk will be released and can then be mounted on another machine.


Repeat this procedure yourself. At the end, start up a separate virtual machine with this disk and verify that the file you initially stored is still there.

Attach a Disk Dynamically

Although using the --persistent-disk option of stratus-run-instance is easy; it isn’t always convenient. You can attach and detach disks dynamically from running virtual machines to provide more flexiblity and to overcome the limitations stated earlier.

The commands for attaching and detaching persistent disks dynamically are:

$ stratus-attach-volume --instance ${VM_ID} VOLUME_UUID
$ stratus-detach-volume --instance ${VM_ID} VOLUME_UUID

These can be used at any time while the virtual machine is running. Within the virtual machine, the volume must still be mounted from the file system in order to be accessible.

Even though modern file systems are tolerant of abrupt deconnections of devices, it is always a good idea to unmount the disk from the file system from within the virtual machine before detaching the disk with the stratus-detach-volume command.


Start a virtual machine, wait for it to come up, and log into it. From another terminal, dynamically attach a persistent disk to the machine. Verify that the disk is visible on the virtual machine (using blkid or fdisk -l). Mount the disk and verify that any previous data is visible. Unmount the disk and detach it. Verify that the disk is no longer visible on the virtual machine.


Attach and detach a persistent disk multiple times. Does the device name remain the same or change?


A volume can be mounted only on one virtual machine at a time. Try to mount a volume on more than one virtual machine to see what error message is given.