On 08/01/2017 11:14 AM, John Petrini wrote:
Just my two cents here but we started out using mostly Ephemeral storage
in our builds and looking back I wish we hadn't. Note we're using Ceph
as a backend so my response is tailored towards Ceph's behavior.
The major pain point is snapshots. When you snapshot an nova volume an
RBD snapshot occurs and is very quick and uses very little additional
storage, however the snapshot is then copied into the images pool and in
the process is converted from a snapshot to a full size image. This
takes a long time because you have to copy a lot of data and it takes up
a lot of space. It also causes a great deal of IO on the storage and
means you end up with a bunch of "snapshot images" creating clutter. On
the other hand volume snapshots are near instantaneous without the other
drawbacks I've mentioned.
On the plus side for ephemeral storage; resizing the root disk of images
works better. As long as your image is configured properly it's just a
matter of initiating a resize and letting the instance reboot to grow
the root disk. When using volumes as your root disk you instead have to
shutdown the instance, grow the volume and boot.
I hope this help! If anyone on the list knows something I don't know
regarding these issues please chime in. I'd love to know if there's a
better way.
I'd just like to point out that the above is exactly the right way to
think about things.
Don't boot from volume (i.e. don't use a volume as your root disk).
Instead, separate the operating system from your application data. Put
the operating system on a small disk image (small == fast boot times),
use a config drive for injectable configuration and create Cinder
volumes for your application data.
Detach and attach the application data Cinder volume as needed to your
server instance. Make your life easier by not coupling application data
and the operating system together.
Best,
-jay
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