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 - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
