[sldev] Changes to eliminate storage wasted by single-use, temporary assets.

[Joshua Bell]

Thordain Curtis wrote:
> The Second Life 1.22 Server Update blog post lists "Changes to 
> eliminate storage wasted by single-use, temporary assets." as a change 
> to the simulator.  I cannot seem to find out anything regarding the 
> feature itself, what it effects, etc.  The only reference I was 
> pointed to is an extremely obscure mention in another post on SLDev.
>
This is what's internally known as "single reference attachments" 
work... I thought it had some mention on sldev, but I'm not finding it now.

On logoff, attachment state is serialized as assets. On login it is 
reconstituted into the in-memory representation of the sim. At this 
point, the serialized copy of the data is unnecessary and will never be 
used again (the single reference to it is now discarded), so it can be 
purged from the asset store and save us precious storage space.

Due to the complexity of the system (e.g. you can attach items from the 
library, different code paths, etc) it's required several iterations, 
and not all the kinks are out yet. The assets are NOT currently being 
purged - instead they're moved into a holding area that is checked as a 
fallback if a read from the primary store fails. We'll know the system 
is working correctly when there are no reads from the holding area. 
After the most recent update we're watching carefully to see if we've 
caught all of the cases. Eventually we'll be able to flip the code from 
"move" to "delete" mode and purge the holding area, but of course we're 
paranoid.

Joshua