[Jdbm-general] Re: Another confusing thing

[<cg...@cd...>]

ji...@ex... said:
> The other thing that is confusing me is the row-id concept. I can't
> seem to be grasping it. Can you elaborate here so I can get the mental
> picture of what is actually happening. 

That's easy. On the user level, you want rowids. Furthermore, you want as 
little hassle as possible mapping rowids to actual storage locations, so you 
cook up a scheme like I did with the <block,offset> encoded in a single bunch 
of bytes, represented externally as an opaque integer.

However, on the storage level, you don't want such a scheme, because it 
prevents you from moving stuff around in order to optimize storage.

So you introduce indirection: the user gets a logical rowid, which points to a 
translation page and an offset within the translation pages - at this 
location, you store the actual physical rowid, which is the physical 
page/offset location that contains the actual data. When you shuffle the data 
around because of storage needs (JDBM currently only does this when expanding 
allocation, but it could do all sorts of stuff), the new location is recorded 
by rewriting the physical rowid in the logical rowid location. A bit like the 
MacOS memory allocation stuff.


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