resizepart command by default not modify or remove existing files on a partition? Furthermore, does it never modify or remove existing files on a partition (even by some option)?
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When shrinking a filesystem,
resize2fs first checks if the part of the filesystem that is going to be cut away is free. If not, it can try to move those files out of the area that will be cut away, if there is space to do so. If this cannot be done, it stops and reports an error without shrinking the filesystem.
resizepart does not care about the filesystem at all. It just changes the partition table to specify a new location where the partition now ends. It does not overwrite anything at or near that location. After modifying the partition table, it will signal the kernel that the partition table has been changed. The kernel will read the new table and apply it if possible.
But for the filesystem driver, the end of the partition will be a hard wall. If the filesystem was not shrunk before the partition was, or the partition was accidentally shrunk more than the partition was, a part of the filesystem will now be cut off from the rest.
The filesystem will assume that the cut-off space is still available, until it actually attempts to use it. At that point the part of the kernel that is responsible for mapping any partition-relative block numbers to actual whole-disk block numbers will return an error to the filesystem driver, as the filesystem is trying to access beyond the end of its partition. The filesystem driver will usually drop to read-only mode as such an error tends to indicate that the filesystem may be corrupted. At that point, the system administrator usually gets involved.
At this point, if the sysadmin realizes that the partition resize operation has cut off part of the filesystem, and undoes the partition resize operation, the filesystem can be fully accessed through the partition device again, and everything may still be just fine: the filesystem may need a
fsck to clear the error flag, but the files will still be there. After mounting the filesystem again, the files that were bisected by the partition resize operation will be fully accessible again.
But if the sysadmin simply runs a filesystem check on the partition in its shrunken state, the filesystem checker will see that there are files that appear to continue beyond the end of the partition, and say to itself: "Let's amputate". Since it takes the partition size as a solid fact, it has no choice but to truncate or delete the files that seem to go beyond the end of the partition. This is where the actual damage is done. The filesystem metadata will also need some adjustment to remove the space that is beyond the end of the partition from the "books".
After the filesystem checker is done, the cut-off parts of the files are still there on the physical disk, beyond the new end of the partition, unchanged... but the parts of the files still inside the filesystem are now truncated into stumps.