On 2019/6/10 11:39, Theodore Ts'o wrote: > On Mon, Jun 10, 2019 at 11:25:47AM +0800, Jianchao Wang wrote: >> Hi Artem >> >> Thanks so much for your help. >> >> On 2019/6/6 20:32, Artem Blagodarenko wrote: >>> Hello Jianchao, >>> >>> Not enought input data to give an answer. It depends on mkfs options. For example, if flex_bg option is enabled, then several block groups are tied together as one logical block group; the bitmap spaces and the inode table space in the first block group, so some groups are not totally free just after FS creating. >> >> In my environment, there are 16 bgs per flex_bg. >> The bitmaps and inode table .etc should lay on the first bg of every flex_bg. >> So I can see there are about 8223 blocks allocated in the 1st bg of every flex_bg. >> >> But as you can see in the output of mb_groups, there are some bgs >> which get allocated about 1024 blocks. >> >> I have out figured out what are they for. > > The best way to understand what the blocks are used for is to use the > dumpe2fs program, e.g: > > % mke2fs -t ext4 -Fq /tmp/foo.img 1T > % dumpe2fs /tmp/foo.img | more > ... > Group 0: (Blocks 0-32767) csum 0xe6f6 [ITABLE_ZEROED] > Primary superblock at 0, Group descriptors at 1-128 > Reserved GDT blocks at 129-1152 > Block bitmap at 1153 (+1153), csum 0xb033995a > Inode bitmap at 1169 (+1169), csum 0x108d3d73 > Inode table at 1185-1696 (+1185) > 23385 free blocks, 8181 free inodes, 2 directories, 8181 unused inodes > Free blocks: 9383-32767 > Free inodes: 12-8192 > Group 1: (Blocks 32768-65535) csum 0xe50b [INODE_UNINIT, BLOCK_UNINIT, ITABLE_ZEROED] > Backup superblock at 32768, Group descriptors at 32769-32896 > Reserved GDT blocks at 32897-33920 > Block bitmap at 1154 (bg #0 + 1154), csum 0x00000000 > Inode bitmap at 1170 (bg #0 + 1170), csum 0x00000000 > Inode table at 1697-2208 (bg #0 + 1697) > 31615 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes > Free blocks: 33921-65535 > Free inodes: 8193-16384 > ... > Hi Ted Many thanks for your help These allocated blocks should be the reserved GDT blocks. Many thanks again. Jianchao
