[glue-wg] Some doubts

[Burke, S (Stephen)]

Riccardo Zappi [mailto:riccardo.zappi at cnaf.infn.it] said:
> Within a storage area networks you can find several servers sharing 
> storage devices such as disk subsystem and tape libraries and 
> also data 
> stored upon them. SAN aggregate and share the storage devices 
> via volume 
> manager. You can deploy some different file systems upon the same SAN 
> infrastructure.

Maybe we should go back a bit. The purpose of the DataStore is *not* to
represent all the details of the hardware, or indeed any of the details!
In the past we have had no hardware description at all, and probably we
still don't need it. However, people tend to keep raising some questions
related to hardware, like how you know if the site uses tape storage. My
proposal was to have a *simple* description of the hardware that can
answer simple questions only - basically just whether the hardware is
tape or disk, and perhaps what the total size is. My hope was that by
doing that we could stop having discussions about hardware. If we are
actually going to have more discussions about all these details then I
think I want to withdraw the proposal completely, we would be better off
not to have it at all since we still have no use-cases for it.

> Now, if I've understood well, there are only two/three DataStores:
> - one to represents the "disk quality",
> - one to represents the "tape quality",
> - and, eventually, one to represents the "hierarchical-system" not 
> manageable by user.

I'm not quite sure what you mean by the third one, but yes, basically
the DataStore just divides the storage for a given Resource into tape
and disk and nothing more complex.

> But, at the same time, we have a set of StorageResources 
> corresponding to File Systems created..
> 
> Is it correct above description?

In general I wouldn't say that a StorageResource corresponds to a file
system the way I would normally think of it, e.g. ext3, it's intended
for some higher-level management software like Castor. GPFS may be a
special case here as it's more than just another file system. Anyway,
even if a file system is published as a Resource you won't usually have
one Resource per file system; you would only need multiple Resources if
they are really separarate instances, e.g. they are managed separately
and could be at different versions. Otherwise you would have only one
Resource for GPFS, with some version number, and one Datastore for all
the disk it manages.

> If yes, I cannot be able to assign Size properties to 
> DataStores.. vice 
> versa I can assign sizes to StorageResources.

I don't understand the point. A piece of software (castor, gpfs) doesn't
have any size, the hardware it manages (a set of disks) has a size.

> We could move Size properties to StorageResources and make a 
> relationship between StorageShare <-> StorageResource and between 
> StorageResource <-> DataStore (many-to-many). In this way we can 
> represents above example, I think.

No, that's much too complicated. All we need to represent is that a site
has a disk farm and a tape robot, and that there is some software layer
between the SRM and the hardware.

Stephen