Re: [RFC] fsblock

[Chris Mason <chris.mason@oracle.com>]

On Tue, Jun 26, 2007 at 07:23:09PM +1000, David Chinner wrote:
> On Tue, Jun 26, 2007 at 01:55:11PM +1000, Nick Piggin wrote:

[ ... fsblocks vs extent range mapping ]

> iomaps can double as range locks simply because iomaps are
> expressions of ranges within the file.  Seeing as you can only
> access a given range exclusively to modify it, inserting an empty
> mapping into the tree as a range lock gives an effective method of
> allowing safe parallel reads, writes and allocation into the file.
> 
> The fsblocks and the vm page cache interface cannot be used to
> facilitate this because a radix tree is the wrong type of tree to
> store this information in. A sparse, range based tree (e.g. btree)
> is the right way to do this and it matches very well with
> a range based API.

I'm really not against the extent based page cache idea, but I kind of
assumed it would be too big a change for this kind of generic setup.  At
any rate, if we'd like to do it, it may be best to ditch the idea of
"attach mapping information to a page", and switch to "lookup mapping
information and range locking for a page".

A btree could be used to hold the range mapping and locking, but it
could just as easily be a radix tree where you do a gang lookup for the
end of the range (the same way my placeholder patch did).  It'll still
find intersecting range locks but is much faster for random
insertion/deletion than the btrees.

-chris
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