Open Stack

David is right, Ceph implements volume snapshotting at the RBD level,
not even RADOS level: whole 2 levels of abstraction above file system.
It doesn't matter if it's XFS, BtrFS, Ext4, or VFAT (if Ceph supported
VFAT): Ceph RBD takes care of it before individual chunks of an RBD
volume are passed to RADOS as objects and get written into the file
system as files by an OSD process.

The reason Fuel documentation recommends to disable QCOW2 format for
images is that RBD does not support QCOW2 disks directly, so Nova and
Cinder have to _convert_ a QCOW2 image into RAW format before passing
it to QEMU's RBD driver. This means that you end up downloading the
QCOW2 image from Ceph to a nova-compute node (first full copy),
converting it (second full copy), and uploading the resultant RAW
image back to Ceph (third full copy) just to launch a VM or create a
volume from an image.


On Thu, May 28, 2015 at 8:33 AM, David Medberry <openstack at medberry.net> wrote:
> yep. It's at the CEPH level (not the XFS level.)
>
> On Thu, May 28, 2015 at 8:40 AM, Stephen Cousins <steve.cousins at maine.edu>
> wrote:
>>
>> Hi David,
>>
>> So Ceph will use Copy-on-write even with XFS?
>>
>> Thanks,
>>
>> Steve
>>
>> On Thu, May 28, 2015 at 10:36 AM, David Medberry <openstack at medberry.net>
>> wrote:
>>>
>>> This isn't remotely related to btrfs. It works fine with XFS. Not sure
>>> how that works in Fuel, never used it.
>>>
>>> On Thu, May 28, 2015 at 8:01 AM, Forrest Flagg <fostro.flagg at gmail.com>
>>> wrote:
>>>>
>>>> I'm also curious about this.  Here are some other pieces of information
>>>> relevant to the discussion.  Maybe someone here can clear this up for me as
>>>> well.  The documentation for Fuel 6.0, not sure what they changed for 6.1,
>>>> [1] states that when using Ceph one should disable qcow2 so that images are
>>>> stored in raw format.  This is due to the fact that Ceph includes its own
>>>> mechanisms for copy-on-write and snapshots.  According to the Ceph
>>>> documentation [2], this is true only when using a BTRFS file system, but in
>>>> Fuel 6.0 Ceph uses XFS which doesn't provide this functionality.  Also, [2]
>>>> recommends not using BTRFS for production as it isn't considered fully
>>>> mature.  In addition, Fuel 6.0 [3] states that OpenStack with raw images
>>>> doesn't support snapshotting.
>>>>
>>>> Given this, why does Fuel suggest not using qcow2 with Ceph?  How can
>>>> Ceph be useful if snapshotting isn't an option with raw images and qcow2
>>>> isn't recommended?  Are there other factors to take into consideration that
>>>> I'm missing?
>>>>
>>>> [1]
>>>> https://docs.mirantis.com/openstack/fuel/fuel-6.0/terminology.html#qcow2
>>>> [2]
>>>> http://ceph.com/docs/master/rados/configuration/filesystem-recommendations/
>>>> [3]
>>>> https://docs.mirantis.com/openstack/fuel/fuel-6.0/user-guide.html#qcow-format-ug
>>>>
>>>> Thanks,
>>>>
>>>> Forrest
>>>>
>>>> On Thu, May 28, 2015 at 8:02 AM, David Medberry <openstack at medberry.net>
>>>> wrote:
>>>>>
>>>>> and better explained here:
>>>>> http://ceph.com/docs/master/rbd/qemu-rbd/
>>>>>
>>>>> On Thu, May 28, 2015 at 6:02 AM, David Medberry
>>>>> <openstack at medberry.net> wrote:
>>>>>>
>>>>>> The primary difference is the ability for CEPH to make zero byte
>>>>>> copies. When you use qcow2, ceph must actually create a complete copy
>>>>>> instead of a zero byte copy as it cannot do its own copy-on-write tricks
>>>>>> with a qcow2 image.
>>>>>>
>>>>>> So, yes, it will work fine with qcow2 images but it won't be as
>>>>>> performant as it is with RAW. Also, it will actually use more of the native
>>>>>> underlying storage.
>>>>>>
>>>>>> This is also shown as an Important Note in the CEPH docs:
>>>>>> http://ceph.com/docs/master/rbd/rbd-openstack/
>>>>>>
>>>>>> On Thu, May 28, 2015 at 4:12 AM, Shake Chen <shake.chen at gmail.com>
>>>>>> wrote:
>>>>>>>
>>>>>>> Hi
>>>>>>>
>>>>>>> Now I try to use Fuel 6.1 deploy openstack Juno, use Ceph as cinder,
>>>>>>> nova and glance backend.
>>>>>>>
>>>>>>> In Fuel document suggest if use ceph, suggest use RAW format image.
>>>>>>>
>>>>>>> but if I upload qcow2 image, seem working well.
>>>>>>>
>>>>>>> what is the different use qcow2 and RAW in Ceph?
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> Shake Chen
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> OpenStack-operators mailing list
>>>>>>> OpenStack-operators at lists.openstack.org
>>>>>>>
>>>>>>> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-operators
>>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> OpenStack-operators mailing list
>>>>> OpenStack-operators at lists.openstack.org
>>>>> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-operators
>>>>>
>>>>
>>>
>>>
>>> _______________________________________________
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>>> OpenStack-operators at lists.openstack.org
>>> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-operators
>>>
>>
>>
>>
>> --
>> ________________________________________________________________
>>  Steve Cousins             Supercomputer Engineer/Administrator
>>  Advanced Computing Group            University of Maine System
>>  244 Neville Hall (UMS Data Center)              (207) 561-3574
>>  Orono ME 04469                      steve.cousins at maine.edu
>>
>
>
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-- 
Dmitry Borodaenko