On Mon, Jun 15, 2009 at 5:29 PM, Richard Davies <richard@daviesmail.org> wrote:

There's a strong argument for clouds to support both. Persistent storage is
typically on SAN where it can be accessed from all virtualization hosts when
the server is stopped and restarted. Ephemeral storage is typically on local
disk, which is much faster but unavailable after the server is stopped and
restarted on a different machine.

Right, there are many different possibilities for storage that should arguably be specified by user requirement ("what" e.g. "geographically redundant") rather than technology ("how" e.g. "RAID 10"). The three main options I see for "local" storage are:

Ephemeral which is lost when the machine stops (thus simplifying infrastructure and reducing costs)
Local storage which may or may not exist next time you need it (good for caching configurations, calculations and so on but not much else)
Redundant storage which will probably survive failures (e.g. SAN)
HA storage which is geographically distributed, backed up, etc. and which you can rely on.

Of course the different levels would carry different costs and be useful for different purposes. We just need to find a simple way for users to get and set their preferences (whether expressed as "what" or "how").

Sam