On Mon, Jul 15, 2013 at 12:20:01PM +0200, Felipe Monteiro de Carvalho wrote: > On Sun, Jun 23, 2013 at 1:59 PM, Theodore Ts'o <tytso@xxxxxxx> wrote: > > This had two difficulties. The first is that every 128MB (assuming a > > 4k blocksize and the default/maximum number of blocks per block > > group), there would be file system metadata, thus breaking up the > > maximum contiguous free space available for data blocks. > > But the break up of contiguous free space still occurs, because each > group has a group descriptor in it's beginning, right? The sparse_super feature restricts the number of group descriptors to those block groups which are multiples of the nth root of 3, 5, or 7. So a 4T file system, only has 22 superblocks and group descriptors out of 32768 blok groups (at block groups: 1, 3, 5, 7, 9, 25, 27, 49, 81, 125, 243, 343, 625, 729, 2187, 2401, 3125, 6561, 15625, 16807, and 19683). There is also the meta_bg feature, which defines yet another concept, a meta_bg group, which is the set of block groups which can fit in a single block group descriptor block. So for a 64-bit file system with 64-byte block group descriptors, you can fit 64 descriptors in a 4k block. So the size of a meta_bg group is 64 block groups. (For a file system w/o the 64-bit feature, the size of a meta_bg group is 128 block groups for file systems with 4k blocks.) With a file system with a meta_bg feature enabled, starting with the nth s_first_meta_bg (i.e., block group s_first_meta_bg * bg_per_meta_bg), the block groups are located at the first, second, and last block groups in a meta_bg group. The reason for the meta_bg feature is the resize_inode uses indirect blocks, which only works for 32-bit block numbers. So when we are doing an online resize, once we expand the file system to the point where we've run out of reserved bg descriptor blocks, or when we expand beyond the 32-bit block number boundary, we enable the meta_bg feature. The advantage of the meta_bg feature is that it's no longer required to reserved bg descriptor blocks, or to move the inode table when doing off-line resizes. The disadvantage of the meta_bg feature is that the primary block groups are no longer contiguous, so it slows down the time to mount a file system with a large number of bg descriptor blocks which are laid out using the meta_bg feature. Regards, - Ted P.S. And all of this is automatically handled by the ext2fs library. :-P -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
