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 | Memory_MB | Disk | Ephemeral | Swap | VCPUs | RXTX_Factor | Is_Public | extra_specs |
+----+--------------+-----------+------+-----------+------+-------+-------------+-----------+-------------+
| 1 | m2.small | 1875 | 10 | 0 | | 1 | 1.0 | True | {} |
| 11 | sp1.medium | 8192 | 20 | 40 | | 4 | 1.0 | True | {} |
In this case, the sp1.medium 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.