在创建之前如何计算 ext4/etx3 FS 上的 inode 数量？

正如手册页中所写，该值基于块数和每个 inode 的字节数比率 bytes-per-inode = (blocks/inode) * block_size。

-N number-of-inodes
      Overrides the default calculation of the number of inodes that should be reserved for
      the filesystem (which is based on the number of blocks and the bytes per-inode ratio). 
      This allows the user to specify the number of desired inodes directly.

要检查我们将拥有多少个 inode，请在 mkfs.ext3/4 中使用 -n 选项。

# mkfs.ext4 -n /dev/sda
mke2fs 1.42.9 (28-Dec-2013)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
Stride=0 blocks, Stripe width=512 blocks
32768 inodes, 131072 blocks
6553 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=33685504
16 block groups
8192 blocks per group, 8192 fragments per group
2048 inodes per group
Superblock backups stored on blocks: 
    8193, 24577, 40961, 57345, 73729

通过使用带有选项 -l 的 tune2fs 工具，可以看到文件系统的 inode 大小。
使用相同的选项，还可以看到文件系统超级块的其他信息。
超级块包含有关文件系统的信息，例如可用的空闲块数和挂载数，这些信息可能对调优有用。

以下是用户如何指定 -l 选项的示例。
这里使用的示例是 RHEL 7 系统：

# tune2fs -l /dev/sda1 
tune2fs 1.42.9 (28-Dec-2013)
Filesystem volume name:   [none]
Last mounted on:          /boot
Filesystem UUID:          1a678f4f-85e4-43c6-b4b3-af1a732510ac
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery extent 64bit flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
Filesystem flags:         signed_directory_hash 
Default mount options:    user_xattr acl
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              65536
Block count:              262144
Reserved block count:     13107
Free blocks:              228732
Free inodes:              65200
First block:              0
Block size:               4096
Fragment size:            4096
Group descriptor size:    64
Reserved GDT blocks:      127
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         8192
Inode blocks per group:   512
RAID stride:              64
RAID stripe width:        64
Flex block group size:    16
Filesystem created:       Tue Jan 26 18:26:17 2016
Last mount time:          Sun Jun 26 00:03:14 2016
Last write time:          Sun Jun 26 00:03:14 2016
Mount count:              5
Maximum mount count:      -1
Last checked:             Tue Jan 26 18:26:17 2016
Check interval:           0 ([none])
Lifetime writes:          176 MB
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:	          256
Required extra isize:     28
Desired extra isize:      28
Journal inode:            8
Default directory hash:   half_md4
Directory Hash Seed:      27bb8d92-09a4-4734-89b8-f767a5f34fb3
Journal backup:           inode blocks

要获取 inode 的大小，请使用：

# tune2fs -l /dev/sda1 | grep -i inode
Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery extent 64bit flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
Inode count:              65536
Free inodes:              65200
Inodes per group:         8192
Inode blocks per group:   512
First inode:              11
Inode size:	          256
Journal inode:            8
Journal backup:           inode blocks

如上所示，ext4 系统上的 RHEL 7 中的 inode 大小为 256 字节。

我们还可以使用 df -i 命令来获取可用的 inode 总数以及已使用的 inode 数。

# df -i /dev/sda1
Filesystem     Inodes IUsed IFree IUse% Mounted on
/dev/sda1       65536  1919 63617    3% /boot