Re: i_blksize

[Andrew Morton <akpm@osdl.org>]

Andries.Brouwer@cwi.nl wrote:
>
> > Looks like I got myself confused
> 
> Yes. But nevertheless, now that you brought this up,
> we might consider throwing out i_blksize.
> 
> I am not aware of anybody who actually uses this to give
> per-file advice. So, it could be in the superblock.

I suppose so.  reiserfs plays with it.

I can't really see that anyone would want to set the I/O size hint on a
per-inode basis, especially as the readahead and writebehind code will
cheerfully ignore it.

> Any objections?

I don't think it's worth fiddling with at this time, really.

> If sizeof(struct inode) decreases by 1% then we can keep 1% more inodes.
>
> That reminds me - I threw out i_dev and i_cdev, but Al reintroduced i_cdev.
> We should do as some comment says and make a union with i_bdev and i_pipe.
> Another 8 bytes gone.

Well all the inode slab caches are using SLAB_HWCACHE_ALIGN at present, so
it's a little moot.  Especially on a pentium4-compiled kernel.

But I expect most distributed 2.6 kernels will be pII or pIII-compiled. 
Let's look:

SMP:
	sizeof(struct ext2_inode_info) = 0x1d0
	sizeof(struct ext3_inode_info) = 0x1e0

Both of these pack eight-per-page.  Need to get them to 0x1c4 (and remove
SLAB_HWCACHE_ALIGN) to get to nine-per-page.

UP:
	sizeof(struct ext3_inode_info) = 0x1c4		(whew!)
	sizeof(struct ext2_inode_info) = 0x1b4

So for these filesystems at least, we need to remove SLAB_HWCACHE_ALIGN and
we will get a 12% improvement in packing density on uniprocessor.

unionification of i_[bcp]dev sounds like a good idea to give us a little
margin there.