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SubjectRe: Possible Idea with filesystem buffering.
Rik van Riel wrote:

>On Sun, 20 Jan 2002, Shawn Starr wrote:
>>But why should each filesystem have to have a different method of
>>buffering/caching? that just doesn't fit the layered model of the
>>kernel IMHO.
>I think Hans will give up the idea once he realises the
>performance implications. ;)

Rik, what reiser4 does is take a slum (a slum is a contiguous in the
tree order set of
dirty buffers), and just before flushing it to disk we squeeze the
entire slum as far
to the left as we can, and encrypt any parts of it that we need to
encrypt, and assign
block numbers to it.

Tree balancing normally has a tradeoff between memory copies performed on
average per insertion, and tightness in packing nodes. Squeezing in
response to
memory pressure greatly optimizes the the number of nodes we are packed
while only performing one memory copy just before flush time for that
It is MUCH more efficient. Block allocation ala XFS can be much more
optimal if
done just before flushing. Encryption just before flushing rather than
with every
modification to a file is also much more efficient. Committing
also have a complex need to be memory pressure driven (complex enough
that I won't describe it here).

So, really, memory pressure needs to push a whole set of events in a well
designed filesystem. Thinking that you can just pick a page and write it
and write no other pages, all without understanding the optimizations of
the filesystem you write to, is simplistic.

Suppose we do what you ask, and always write the page (as well as some
other pages) to disk. This will result in the filesystem cache as a whole
receiving more pressure than other caches that only write one page in
response to pressure. This is unbalanced, leads to some caches having
shorter average page lifetimes than others, and it is therefor
suboptimal. Yes?


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