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SubjectRe: [RFC] VM: I have a dream...
>Are there any Linux file systems that work by mmapping the entire
>drive and using the paging system to do the read/writes? With 64 bits
>there's enough address space to do that now. How does this perform
>compared to a traditional block based scheme?

They pretty much all do that. A filesystem driver doesn't actually map
the whole drive into memory addresses all at once and generate page faults
by referencing memory -- instead, it generates the page faults explicitly,
which it can do more efficiently, and sets up the mappings in smaller
pieces as needed (also more efficient). But the code that reads the pages
into the file cache and cleans dirty file cache pages out to the disk is
the same paging code that responds to page faults on malloc'ed pages and
writes such pages out to swap space when their page frames are needed for
other things.

>With the IBM 128b address space aren't the devices vulnerable to an
>errant program spraying garbage into the address space? Is it better
>to map each device into it's own address space?

Partitioning your storage space along device lines and making someone who
wants to store something identify a device for it is a pretty primitive
way of limiting errant programs. Something like Linux disk quota and
rlimit (ulimit) is more appropriate to the task, and systems that gather
all their disk storage (even if separate from main memory) into a single
automated pool do have such quota systems.

Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems

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