Yes. Really three(?) different beasts, here. Ephemeral storage is exactly that. It must contain nothing that needs to be preserved. So you can factor this out into three main cases:
1. Instance boots from ephemeral storage. All persistent state is not owned by the instance. 2. Instance boots from ephemeral storage. Some/all persistent state is in an additional attached Cinder volume. 3. Instance boots from a persistent Cinder volume, which contains persistent state. In (1) you are closest to a built-for-cloud application. The persistent state might be in cloud-object storage, or in some other net-accessible service. If the instance fails, simply boot another. No preservation of instance-state needed. In (2) you have a more traditional sort of application, deployed efficiently in the cloud. Failover consists of spinning up a new instance, attached to the volume with state. In (3) you have a traditional sort of application, simply deployed in the cloud. Less efficient, but less work needed. Failover consists of spinning up a new instance, booted off the Cinder volume. NOTE: In *no* case are you looking to preserve the state of the ephemeral volume!! If you need to preserve the volume, it must be in Cinder. It must *not* be ephemeral. Ephemeral means exactly that. > Excerpts from Adam Lawson's message of 2014-09-26 14:43:40 -0700: > \> I'm looking for discussions/plans re VM continuity. > > > > I.e. Protection for instances using ephemeral storage against host > failures > > or auto-failover capability for instances on hosts where the host suffers > > from an attitude problem? > > > > I know fail-overs are supported and I'm quite certain auto-fail-overs are > > possible in the event of a host failure (hosting instances not using > shared > > storage). I just can't find where this has been addressed/discussed. >
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