Ephemeral

Ephemeral disks can be useful where you want to partition the second disk or have a specific disk configuration which is not possible within the system disk configuration.

Ephemeral storage is defined by the virtual machine flavor when the VM is created.

$ openstack flavor list
+-------+-----------+--------+------+-----------+-------+-----------+
| ID    | Name      |    RAM | Disk | Ephemeral | VCPUs | Is Public |
+-------+-----------+--------+------+-----------+-------+-----------+
| 12076 | m2.large  |   7500 |   40 |         0 |     4 | True      |
| 25274 | m9.eph    |   7500 |   40 |        40 |     4 | True      |
| 17895 | m2.small  |   1875 |   10 |         0 |     1 | True      |
| 38242 | m2.medium |   3750 |   20 |         0 |     2 | True      |
+-------+-----------+--------+------+-----------+-------+-----------+

In this case, the m9.eph flavor has an ephemeral disk of 40GB.

When an instance with ephemeral storage is created, a second disk is defined and mounted automatically on /dev/vdb (by default).

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
                       8547036   1691360   6421500  21% /
tmpfs                  4095860         0   4095860   0% /dev/shm
/dev/vda1               198337     28571    159526  16% /boot
/dev/vdb              41284928    180236  39007540   1% /mnt

Note: With the introduction of volumes, ephemeral disks are mostly obsolete.