On Wed, Feb 27, 2019 at 07:45:50AM +1100, Dave Chinner wrote: > On Tue, Feb 26, 2019 at 05:33:04PM +0800, Ming Lei wrote: > > On Tue, Feb 26, 2019 at 03:58:26PM +1100, Dave Chinner wrote: > > > On Mon, Feb 25, 2019 at 07:27:37PM -0800, Matthew Wilcox wrote: > > > > On Tue, Feb 26, 2019 at 02:02:14PM +1100, Dave Chinner wrote: > > > > > > Or what is the exact size of sub-page IO in xfs most of time? For > > > > > > > > > > Determined by mkfs parameters. Any power of 2 between 512 bytes and > > > > > 64kB needs to be supported. e.g: > > > > > > > > > > # mkfs.xfs -s size=512 -b size=1k -i size=2k -n size=8k .... > > > > > > > > > > will have metadata that is sector sized (512 bytes), filesystem > > > > > block sized (1k), directory block sized (8k) and inode cluster sized > > > > > (32k), and will use all of them in large quantities. > > > > > > > > If XFS is going to use each of these in large quantities, then it doesn't > > > > seem unreasonable for XFS to create a slab for each type of metadata? > > > > > > > > > Well, that is the question, isn't it? How many other filesystems > > > will want to make similar "don't use entire pages just for 4k of > > > metadata" optimisations as 64k page size machines become more > > > common? There are others that have the same "use slab for sector > > > aligned IO" which will fall foul of the same problem that has been > > > reported for XFS.... > > > > > > If nobody else cares/wants it, then it can be XFS only. But it's > > > only fair we address the "will it be useful to others" question > > > first..... > > > > This kind of slab cache should have been global, just like interface of > > kmalloc(size). > > > > However, the alignment requirement depends on block device's block size, > > then it becomes hard to implement as genera interface, for example: > > > > block size: 512, 1024, 2048, 4096 > > slab size: 512*N, 0 < N < PAGE_SIZE/512 > > > > For 4k page size, 28(7*4) slabs need to be created, and 64k page size > > needs to create 127*4 slabs. > > IDGI. Where's the 7/127 come from? > > We only require sector alignment at most, so as long as each slab > object is aligned to it's size, we only need one slab for each block > size. Each slab has fixed size, I remembered that you mentioned that the meta data size can be 512 * N (1 <= N <= PAGE_SIZE / 512). https://marc.info/?l=linux-fsdevel&m=155115014513355&w=2 Thanks, Ming
