Re: Compressed cache system [Re: Google Summer of Code project ideas]

[Hubert Feyrer <hubert%feyrer.de@localhost>]

On Sun, 23 Apr 2006, Matt Fleming wrote:
> Now this point has got me to thinking. Is the way that an object is written 
> to backing store dictated by its uvm_pagerops structure? And if so, would 
> creating a pseudo device with its own pager operations be a reasonable way to 
> go about handling writing to the compressed cache? Surely this way the pseudo 
> device would handle the compression and decompression of the pages for the 
> object.
>
> Although I suppose it would have to be in addition to the uvm_pagerops struct 
> that an object already had (or else how would the compressed cache know how 
> to write to the _real_ backing store?).
>
> Does this seem like a feasible idea?

I don't know zero about UVM, so I cannot comment.

But I guess you'll hit a problem that pops up with vnd(4) with 
VND_COMPRESSION, too: how are you going to manage your compressed storage?
"Input" data is fixed size (== pagesize), but "output" size depends on the 
compression factor, and thus you'll need some way to arrange things with a 
non-fixed offset. Worse, if you replace a page with something that 
compresses worse, you may not be able to use available space.


 - Hubert