On Thu, Jun 28, 2007 at 08:35:48AM +1000, David Chinner wrote: > On Wed, Jun 27, 2007 at 07:50:56AM -0400, Chris Mason wrote: > > Lets look at a typical example of how IO actually gets done today, > > starting with sys_write(): > > > > sys_write(file, buffer, 1MB) > > for each page: > > prepare_write() > > allocate contiguous chunks of disk > > attach buffers > > copy_from_user() > > commit_write() > > dirty buffers > > > > pdflush: > > writepages() > > find pages with contiguous chunks of disk > > build and submit large bios > > > > So, we replace prepare_write and commit_write with an extent based api, > > but we keep the dirty each buffer part. writepages has to turn that > > back into extents (bio sized), and the result is completely full of dark > > dark corner cases. That's true but I don't think an extent data structure means we can become too far divorced from the pagecache or the native block size -- what will end up happening is that often we'll need "stuff" to map between all those as well, even if it is only at IO-time. But the point is taken, and I do believe that at least for APIs, extent based seems like the best way to go. And that should allow fsblock to be replaced or augmented in future without _too_ much pain. > Yup - I've been on the painful end of those dark corner cases several > times in the last few months. > > It's also worth pointing out that mpage_readpages() already works on > an extent basis - it overloads bufferheads to provide a "map_bh" that > can point to a range of blocks in the same state. The code then iterates > the map_bh range a page at a time building bios (i.e. not even using > buffer heads) from that map...... One issue I have with the current nobh and mpage stuff is that it requires multiple calls into get_block (first to prepare write, then to writepage), it doesn't allow filesystems to attach resources required for writeout at prepare_write time, and it doesn't play nicely with buffers in general. (not to mention that nobh error handling is buggy). I haven't done any mpage-like code for fsblocks yet, but I think they wouldn't be too much trouble, and wouldn't have any of the above problems... - 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
