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[Marco CONSONNI <mcocmo62@xxxxxxxxx>]

Hello,


When you use a flavor with an ephemeral disk size different from zero, the
instance is booted with an extra virtual disk whose size is indicated by
the ephemeral value (in GB).

Using cirros image, try a flavor with ephemeral disk size different from
zero (you need to create one yourself because the "standard" flavors have
ephemeral size equal to 0), then log into the just booted instance and type:


*$ ls /dev/vd**

/dev/vda   /dev/vda1  /dev/vdb



Disk /dev/vdb is a (virtual) disk, automatically created at boot time,
corresponding to the ephemeral disk space indicated by the flavor . Please
note that /dev/vda, mounted as /dev/vda1, is the boot disk, always created
when you boot an instance.

Verify the size of the available disks; more specifically, verify /dev/vdb:

*
*

*$ sudo fdisk -l*

Disk /dev/vda: 1073 MB, 1073741824 bytes

255 heads, 63 sectors/track, 130 cylinders, total 2097152 sectors

Units = sectors of 1 * 512 = 512 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk identifier: 0x00000000



   Device Boot      Start         End      Blocks   Id  System

/dev/vda1   *       16065     2088449     1036192+  83  Linux



Disk /dev/vdb: 1073 MB, 1073741824 bytes

16 heads, 63 sectors/track, 2080 cylinders, total 2097152 sectors

Units = sectors of 1 * 512 = 512 bytes

Sector size (logical/physical): 512 bytes / 512 bytes

I/O size (minimum/optimal): 512 bytes / 512 bytes

Disk identifier: 0x00000000



Disk /dev/vdb doesn't contain a valid partition table



Please note that /dev/vdb is made available as raw device, meaning that you
need to partition and format it before using.

You can find instructions on how to do that here
http://docs.openstack.org/folsom/openstack-compute/admin/content/configure-nova-volume.html(search
for command fdisk)

Also note that this disk, being ephemeral, disappears when you terminate
the VM. If you want to keep the data you produce with a VM that is destined
to be terminated, you need to use Volumes that you explicitly create and
attach using the services implemented by Cinder (former nova-volume).


For what concerns the size you define for the boot disk, try and lunch two
instances: one with flavor m1.tiny the other with m1.small:


-- tiny --

*$ sudo fdisk -l
*
Disk /dev/vda: 41 MB, 41126400 bytes
255 heads, 63 sectors/track, 5 cylinders, total 80325 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1   *       16065       80324       32130   83  Linux


-- small --


*$ sudo fdisk -l
*
Disk /dev/vda: 21.5 GB, 21474836480 bytes
255 heads, 63 sectors/track, 2610 cylinders, total 41943040 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1   *       16065    41929649    20956792+  83  Linux


As you notice, the size indicated by the flavor has effects on the size of
the boot disk.


Hope it helps,
Marco.



On Mon, Dec 3, 2012 at 7:03 PM, Ahmed Al-Mehdi <ahmedalmehdi@xxxxxxxxx>wrote:

> Hello,
>
> I instantiated a VM using the cirros image, using the pre-defined
> "m1.small" (1 VCPU, 2 GB Ram, 20G boot disk, 0 Ephemeral disk).  I then
> logged into the console of the VM to view some system stats.  The num of
> CPU and memory makes sense, but I am a bit confused on the storage aspect.
>  I see the output of "df -h " as following:
>
> $ df -h
> Filesystem              Size         Used            Available      Use%
>   Mounted on
> /dev                  1001.1M              0              1001.1M
> 0%         /dev
> /dev/vda1             23.2M        12.9M                   9.1M       59%
>         /
> tmpfs                1004.1M              0              1004.1M
> 0%         /dev/shm
> tmpfs                  200.0K        20.0K                180.0K       10%
>         /run
>
>
> What is the difference between Boot disk and Ephemeral disk?
>
> How can I correlate the 20G boot disk to the output of "df -h".
>
>
> Regards,
> Ahmed.
>
>
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