Burke, S (Stephen) wrote:
Type (disk, tape, ... - open enumeration)
... which we would define the common types, right?
Yes - but do we have anything common apart from tape and disk? dvd?
I think we should at least foresee dvd, and keep it open-ended.
one Datastore can only be managed by one Resource - if there are e.g. multiple sets of disk servers managed by several different software systems
that would constitute multiple Datastores.
OK, I think, but it might depend on the "management" aspect of Datastores.
Can you elaborate? As I said earlier we have the situation at RAL that all the different Castor instances (Resources) share a common tape system, but I think that's too complicated to deal with, we should just treat it as multiple tape Datastores.
Agreed.
This link would be optional one-to-many: each StorageShare is associated with a single Datastore (0..1 multiplicity) and
I may have commented on that proposal already: I do not agree with it. A StorageShare can be associated with several (independent) Datastores, each providing a storage technology that is needed by the Share. For example, one Datastore for the disk and another for the tape.
each Datastore is associated with any number of StorageShares (0..* multiplicity).
Would that be acceptable?
Well ... rather than making Share <-> Datastore *..* you could consider defining three separate Share <-> Datastore relations, one for each of Online, Nearline and Offline, and each of which would be *..1. However, that would imply that the storage for a Share for a given Latency can only be provided by one Datastore, and I suspect that you can't be sure of that in all cases. An example could be LCG-style Online/Custodial D1T1 where you have three Datastores: tape, disk cache in front of the tape, and permanent disk (I think this is what CNAF has?).
For D1T1 we should not publish any hidden cache in front of the tape: that cache is an optimization that the client has no control over (one cannot pin files in it). Can we collapse the tape with the cache into a single Datastore that has both an Online and a Nearline component? If not, I suppose we can collapse cache with disk if we do not care about the semantic differences at that level.