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Under the hood: volumes

You should have one volume attached to your instance:

$ nova volume-list
+--------------------------------------+--------+--------------+------+-------------+----------+--------------------------------------+
|                  ID                  | Status | Display Name | Size | Volume Type | Bootable |             Attached to              |
+--------------------------------------+--------+--------------+------+-------------+----------+--------------------------------------+
| 16150e7c-92b1-4bfd-a425-450bf4de6f36 | in-use |   myvolume   |  1   |     None    |  false   | 291574d9-7445-4231-b248-bafbf3b116ec |
+--------------------------------------+--------+--------------+------+-------------+----------+--------------------------------------+

LVM

The default DevStack configuration uses LVM to implement volumes. DevStack uses an LVM volume group caled stack-volumes, which you can list using the vgs command:

$ sudo vgs
  VG            #PV #LV #SN Attr   VSize  VFree
  stack-volumes   1   1   0 wz--n- 10.01g 9.01g

We can get a detailed display of all of the logical volumes:

$ sudo lvdisplay
  --- Logical volume ---
  LV Name                /dev/stack-volumes/volume-16150e7c-92b1-4bfd-a425-450bf4de6f36
  VG Name                stack-volumes
  LV UUID                UgFMV0-3uBM-r4p8-LCla-Kuk2-0Fvj-YFxCyB
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                1.00 GiB
  Current LE             256
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           252:0

Note how the the volume name volume-16150e7c-92b1-4bfd-a425-450bf4de6f36 matches the OpenStack volume ID 16150e7c-92b1-4bfd-a425-450bf4de6f36.

In a DevStack deployment, the physical volumes are backed by a file /opt/stack/data/stack-volumes-backing-file that is exposed as a loop device. Try running the following commands:

$ file /opt/stack/data/stack-volumes-backing-file
$ sudo losetup -a

In a production OpenStack production that used LVM, the physical volumes would be actually disk partitions instead of files mounted as loopback devices.

iscsi

OpenStack uses iSCSI to allow virtual machines to connect to LVM volumes on remote systems.

In a DevStack deployment, the same system is both the iSCSI server and iSCSI client. In a production environment the LVM volumes would be located on a different physical machine than the one running the hypervisor.

Try running the following command to view information about volumes that are being exported over iSCSI.

$ sudo tgtadm --lld iscsi --op show --mode target

The next exercise is Under the hood: networking.