Re: [00/41] Large Blocksize Support V7 (adds memmap support)

[David Chinner]

On Tue, Sep 11, 2007 at 04:00:17PM +1000, Nick Piggin wrote:
> > > OTOH, I'm not sure how much buy-in there was from the filesystems guys.
> > > Particularly Christoph H and XFS (which is strange because they already
> > > do vmapping in places).
> >
> > I think they use vmapping because they have to, not because they want
> > to. They might be a lot happier with fsblock if it used contiguous pages
> > for large blocks whenever possible - I don't know for sure. The metadata
> > accessors they might be unhappy with because it's inconvenient but as
> > Christoph Hellwig pointed out at VM/FS, the filesystems who really care
> > will convert.
> 
> Sure, they would rather not to. But there are also a lot of ways you can
> improve vmap more than what XFS does (or probably what darwin does)
> (more persistence for cached objects, and batched invalidates for example).

XFS already has persistence across the object life time (which can be many
tens of seconds for a frequently used buffer) and it also does batched
unmapping of objects as well.

> There are also a lot of trivial things you can do to make a lot of those
> accesses not require vmaps (and less trivial things, but even such things
> as binary searches over multiple pages should be quite possible with a bit
> of logic).

Yes, we already do the many of these things (via xfs_buf_offset()), but
that is not good enough for something like a memcpy that spans multiple
pages in a large block (think btree block compaction, splits and recombines).

IOWs, we already play these vmap harm-minimisation games in the places
where we can, but still the overhead is high and something we'd prefer
to be able to avoid.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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