On Wed, 2014-01-22 at 09:21 -0800, James Bottomley wrote: > On Wed, 2014-01-22 at 17:02 +0000, Chris Mason wrote:
[ I like big sectors and I cannot lie ] > > > I really think that if we want to make progress on this one, we need > > code and someone that owns it. Nick's work was impressive, but it was > > mostly there for getting rid of buffer heads. If we have a device that > > needs it and someone working to enable that device, we'll go forward > > much faster. > > Do we even need to do that (eliminate buffer heads)? We cope with 4k > sector only devices just fine today because the bh mechanisms now > operate on top of the page cache and can do the RMW necessary to update > a bh in the page cache itself which allows us to do only 4k chunked > writes, so we could keep the bh system and just alter the granularity of > the page cache. > We're likely to have people mixing 4K drives and <fill in some other size here> on the same box. We could just go with the biggest size and use the existing bh code for the sub-pagesized blocks, but I really hesitate to change VM fundamentals for this. >From a pure code point of view, it may be less work to change it once in the VM. But from an overall system impact point of view, it's a big change in how the system behaves just for filesystem metadata. > The other question is if the drive does RMW between 4k and whatever its > physical sector size, do we need to do anything to take advantage of > it ... as in what would altering the granularity of the page cache buy > us? The real benefit is when and how the reads get scheduled. We're able to do a much better job pipelining the reads, controlling our caches and reducing write latency by having the reads done up in the OS instead of the drive. -chris -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
