I have several questions on how inode number is used in linux filesystem. These questions may be related but for clearness I will ask them one by one.
Q1: Take EXT2/3 for example, from "Design and Implementation of the Second Extended Filesystem" I know that there are inode tables in block groups, and I know inode presents file, but what if there are too many files but the inode numbers are not enough? My best guess is that filesystem does nothing if all its inode numbers were exhausted. And if some file get deleted later, its inode may be recycled.
Q2: I run "df -i" on my Linux server and I get this:
Filesystem Inodes IUsed IFree IUse% Mounted on
/dev/sda1 1313280 7853 1305427 1% /
devtmpfs 0 0 0 - /dev
tmpfs 525298 4 525294 1% /dev/shm
/dev/sda2 65808 50 65758 1% /boot
/dev/sda5 1313280 146 1313134 1% /opt
/dev/sda6 655776 37 655739 1% /tmp
/dev/sda7 655776 5219 650557 1% /home
/dev/sda8 1313280 840 1312440 1% /var
/dev/sda9 655776 36529 619247 6% /usr
/dev/sda10 6432 11 6421 1% /crash
/dev/sda12 135488 11 135477 1% /usr/local/instances
tmpfs 525298 3 525295 1% /var/run/xenstored
tmpfs 525298 3 525295 1% /var/lib/xenstored
tmpfs 525298 3 525295 1% /var/lib/xend/socket
tmpfs 517536 3 517533 1% /var/run/libvirt/socket
I see that each filesystem has its own inode counts and these filesystems (like devtmpfs and tmpfs) are all mounted to root file system. I guess each filesystem have their own inode tables, but are inode numbers of different filesystems distributed in different ranges, like root filesystem is [0, N] and tmpfs is [N+1, M]?
If inode numbers of different filesystems are in different ranges (which I don't think so), how filesystems make an agreement on range division?
If inode numbers of different filesystems are generated based on the same rule (like all start from 0), can different filesystems conflict if we mount them together? As I know, inode presents files in running OS, what the OS will do if a file in root filesystem has inode number N and another file in another filesystem (like tmpfs) also has the inode number N?
PS. I'm kind of new to Linux and I'm not coming from a English-speaking country, sorry for if I didnot make clear expression. Thanks in advance.
The full unique identifier for a file is (st_dev, st_ino). This means that inode numbers do not have to be unique across filesystems and every file is still uniquely identified.
This is why you cannot have hardlinks that cross devices. Directory entries only contain an inode number, not a device number. So directory entries can only refer to files on the same filesystem as the directory is on.
Even if you added a device field to the directory entry, it would be meaningless and device numbers can change when you add or remove drives and only refer to a particular device while a system is running. Also, how would you uniquely number all of the devices in the entire world. With USB drives, for example, you can take a given device from machine to machine. How would you ensure the device numbers stayed the same so the links still worked?
Lastly, not all filesystem types have a fixed quantity of inode numbers. reiserfs and (I believe) btrfs dynamically allocate new inodes as needed, and so the total inodes field of the filesystem doesn't really reflect any sort of useful value.