bug#34143: [coreutils 8.28] du -x is reporting a lower disk usage for /mnt when partitions are mounted

[Assaf Gordon]

tags 34143 notabug
close 34143
stop

Hello,

On 2019-01-19 3:11 p.m., Joseph Paul wrote:
>     It may not be a bug at all, but I was surprised to find out that 'du
>     -x' is reporting a lower disk usage on /mnt when partitions are
>     mounted.

This is not a bug.

Technically, as you wrote below, du simply skips (and does not count)
any directory that is not on the same filesystem.

[...]
>     linux$ du -x /mnt
>     4    /mnt/data
>     4    /mnt/VL1800
>     4    /mnt/nfs/nas
>     8    /mnt/nfs
>     20    /mnt
>
>     /mnt is now bigger.
>
>     Is this a normal result, because even when mounted, physically, the
>     directories '/mnt/VL1800' and '/mnt/data' still  exist on the '/'
>     filesystem, or not ?
>     Shouldn't they still occupy 4Kb of disk space each on the '/'
>     filesystem when partitions are mounted ?

They do occupy as much disk space as before,
but du has no way to know how much they occupy,
because the kernel reports that they are on a different device
and you requested -x/--one-file-system.

We can even take it a step further, and mount a new filesyetem
on a non-empty directory - all the directory's content won't be counted:

As root, create the directory structure:

    cd /tmp
    mkdir -p a a/b a/c a/d

Now fill the "b" directory with a large file:

    dd if=/dev/zero of=a/b/bigfile bs=1M count=1

Before any mounts, "b" is counted:

    # du -x a
    4     a/c
    1028  a/b
    4     a/d
    1040  a

Now create a temporary file system loop file, and mount it over "b":

    dd if=/dev/zero of=disk.img bs=1M count=10
    mkfs.ext3 disk.img
    mount -o loop disk.img a/b

Re-checking disk-usage, "b" is not even listed,
and its content (1MB) is not counted:

   # du -x a
   4   a/c
   4   a/d
   12  a

---

To see why du skips it, you can check the Device-ID associated with each 
directory:

   # stat -c "%n   Device-ID: %D   Mount-Point: %m" a a/b a/c a/d
   a   Device-ID: 812   Mount-Point: /tmp
   a/b   Device-ID: 700   Mount-Point: /tmp/a/b
   a/c   Device-ID: 812   Mount-Point: /tmp
   a/d   Device-ID: 812   Mount-Point: /tmp

Your device numbers will differ, but the number for "a/b" will not be
the same as for the rest.

When du sees a different device number, it simply skips the directory.

Once unmounted, the device-id returns to the old value,
and "a/b" will be counted with its content:

   # umount a/b
   # stat -c "%n   Device-ID: %D   Mount-Point: %m" a a/b a/c a/d
   a   Device-ID: 812   Mount-Point: /tmp
   a/b   Device-ID: 812   Mount-Point: /tmp
   a/c   Device-ID: 812   Mount-Point: /tmp
   a/d   Device-ID: 812   Mount-Point: /tmp


As such, I'm closing this as "not a bug",
but discussion can continue by replying to this thread.


regards,
 - assaf