[RFC] Add new extent structure in ext4

[Robin Dong <hao.bigrat@gmail.com>]

Hi Ted, Andreas and the list,

After the bigalloc-feature is completed in ext4, we could have much more
big size of block-group (also bigger continuous space), but the extent
structure of files now limit the extent size below 128MB, which is not
optimal.

We could solve the problem by creating a new extent format to support
larger extent size, which looks like this:

struct ext4_extent2 {
	__le64	ee_block;	/* first logical block extent covers */
	__le64	ee_start;	        /* starting physical block */
	__le32	ee_len;		/* number of blocks covered by extent */
	__le32	ee_flags;	/* flags and future extension */
};

struct ext4_extent2_idx {
	__le64	ei_block;	        /* index covers logical blocks from 'block' */
	__le64	ei_leaf;	        /* pointer to the physical block of the next level */
	__le32	ei_flags;	        /* flags and future extension */
	__le32	ei_unused;	/* padding */
};

I think we could keep the structure of ext4_extent_header and add new
imcompat flag EXT4_FEATURE_INCOMPAT_EXTENTS2.

The new extent format could support 16TB continuous space and larger volumes.

What's your opinion?

-- 
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Best Regard
Robin Dong