32-bit block addresses in ext4 inode struct

Question

In the Linux kernel source, the block numbers in an on-disk inode struct are 32-bit. Why? Surely Linux can support more than 2^32 blocks...

score 4 · Accepted Answer · edited Nov 21 '19 at 05:32

4

The interpretation of the array inode.i_block is different in Ext4 compared to previous on-disk filesystem formats. In Ext4, when the inode has the EXT4_EXTENT_FL set in i_flags this array stores the root of the extent tree and up to four extent descriptors (struct ext4_extent or struct ext4_extent_idx). You will notice that in the extent descriptor there are 48 bits for the block address. For older on-disk formats, e.g. Ext3, the maximum number of block does indeed fit in 32 bits.

See Ext4 data structures and algorithms, section 4.2 The Contents of inode.i_block.

edited Nov 21 '19 at 05:32

LustreOne

1,774

answered Nov 17 '19 at 21:40

AlexP

10,455

Why isn't it declared as a union? – extremeaxe5 Nov 21 '19 at 03:53
@extremeaxe5: I have no idea. – AlexP Nov 21 '19 at 05:37

32-bit block addresses in ext4 inode struct

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