On Thu, Apr 26, 2007 at 07:53:57PM -0700, Andrew Morton wrote: > On Fri, 27 Apr 2007 12:27:31 +1000 David Chinner <[EMAIL PROTECTED]> wrote: > > On Thu, Apr 26, 2007 at 07:04:38PM -0700, Andrew Morton wrote: > > > On Tue, 24 Apr 2007 15:21:05 -0700 [EMAIL PROTECTED] wrote: > > > Also, afaict your important requirements would be met by retaining > > > PAGE_CACHE_SIZE=4k and simply ensuring that pagecache is populated by > > > physically contiguous pages > > > > Sure, that addresses the larger I/O side of things, but it doesn't address > > the large filesystem blocksize issues that can only be solved with some kind > > of page aggregation abstraction. > > a) That wasn't a part of Christoph's original rationale list, so forgive > me for thinking it is not so important and got snuck in post-facto when > things got tough.
I've been pushing christoph to do something like this for more than a year purely so we can support large block sizes in XFS. He's got other reasons for wanting to do this, but that doesn't mean that the large filesystem blocksize issue is any less important. > blocksizes via this scheme - instantiate and lock four pages and go for > it. So now how do you get block aligned writeback? Or make sure that truncate doesn't race on a partial *block* truncate? You basically have to jump through nasty, nasty hoops, to handle corner cases that are introduced because the generic code can no longer reliably lock out access to a filesystem block. Eventually you end up with something like fs/xfs/linux-2.6/xfs_buf.c and doing everything inside the filesystem because it's the only way sane way to serialise access to these aggregated structures. This is the way XFS used to work in it's data path, and we all know how long and loud people complained about that..... A filesystem specific aggregation mechanism is not a palatable solution here because it drives filesystems away from being able to use generic code. Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
