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[Ahmed Al-Mehdi <ahmedalmehdi@xxxxxxxxx>]

Hi Marco,

Thank you very much for the info, much much clearer.  I was looking for the
boot disk using "ls -l /dev/sd*", but the existence of /dev/vda1 should
have given me a clue.

A few follow up questions:

- I am assuming the space for the VM boot disk is allocated from the local
hard disk of the physical host on which the VM is instantiated, right?
- If Yes
   - How is the boot disk represented on the physical host.  Is it a file
on the local filesystem that represent the VM boot disk?
   - I am guessing there is some logic in nova-scheduler that checs
first if there is enough disk apace on the physical host for the VM (along
with RAM, VCPUs) before launching the VM on the host?
   - Is there any way to find out from Horizon how much disk space is
available on a (or each) physical host for VM boot disk allocation?

Thank you,
Ahmed.




On Tue, Dec 4, 2012 at 12:07 AM, Marco CONSONNI <mcocmo62@xxxxxxxxx> wrote:

> 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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>