[Openstack] Ceph.

Bruno L teolupus.ext at gmail.com
Sat Oct 4 20:04:25 UTC 2014

Hi Marcus,

RADOS, Ceph's object store foundation, is strongly consistent (CP).

The RADOS Gateway, the system that exposes RADOS via the Swift and S3 APIs,
can be configured for async replication of objects across sites (so you are
effectively overlaying an AP system on top of a CP system).

Add CephFS to the mix (in the light of providing shared file systems via
Manila) and you have a single storage solution backing up all your storage
needs for OpenStack. This is the beauty and flexibility that attracted us
to it. When there is so much to manage in OpenStack, the last thing we
wanted was to have three disparate storage systems.

The thing to be aware of is that currently the radosgw replaces Swift
completely in the architecture (historically it was not easy to implement a
different back end for swift, like swapping the hypervisor you use to back

Some engineers from RedHat recently added the ability (or made it easier)
to use other object store systems with the the swift API (originally to
plug in GlusterFS). I believe eNovance/RedHat are working on having Ceph as
a backend for Swift.

On 1/10/2014 10:26 AM, "Marcus White" <roastedseaweed.k at gmail.com> wrote:

> Hello,
> Had some basic Ceph questions.
> a. Ceph is strongly consistent and different from usual object, does
> that mean all metadata also, container and account etc is all
> consistent and everything is updated in the path of the client
> operation itself, even for a single site? Is it the same for block and
> file?
> b. If it is strongly consistent, is that the case across sites also?
> How can it be performant across geo sites if that is the case? If its
> choosing consistency over partitioning and availability...For block, I
> read somewhere that it is now eventually consistent(AP vs CP) via DR.
> Gets a bit confusing with all the literature out there. If it is DR,
> isnt that slightly different from the Swift case?
> c. For block, is it CP on a single site and then usual DR to another
> site using snapshotting?
> d. For block, is it just a linux block device or is it SCSI?
> TIA,
> MW
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