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
