mjhjmtu

The logical volume(aka LV), centos-home is created automatically when installing CentOS 7 by default, but I didn't use it manually.

Now, I have mounted an empty directory, work to centos-home.

/home/anselmo/work ==> /dev/mapper/centos-home

The following are the results of df -h after mount.

[anselmo@anselmo-centos7 ~]$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/centos_anselmo--centos7-root 50G 45G 5.2G 90% /
devtmpfs 63G 0 63G 0% /dev
tmpfs 63G 302M 63G 1% /dev/shm
tmpfs 63G 43M 63G 1% /run
tmpfs 63G 0 63G 0% /sys/fs/cgroup
/dev/sdb3 1014M 358M 657M 36% /boot
/dev/sdc1 200M 12M 189M 6% /boot/efi
/dev/mapper/centos_anselmo--centos7-home 2.6T 1.7T 948G 65% /home
tmpfs 13G 92K 13G 1% /run/user/1000
/dev/mapper/centos-home 65G 8.8G 56G 14% /home/anselmo/work

Though I mounted an empty directory, the LV had already used space 8.8G.

How can I find what uses this space?

According to this Q/A this is preallocated diskspace and it is related to "XFS Dynamic Speculative EOF Preallocation"

This is a move to reduce file fragmentation during streaming writes by
speculatively allocating space as file sizes increase. The amount of
space preallocated per file is dynamic and is primarily a function of
the free space available on the filesystem (to preclude running out of
space entirely).

It follows this schedule:

freespace max prealloc size

5% full extent (8GB)

4-5% 2GB (8GB >> 2)

3-4% 1GB (8GB >> 3)

2-3% 512MB (8GB >> 4)

1-2% 256MB (8GB >> 5)

<1% 128MB (8GB >> 6)

This is an interesting addition to the filesystem as it may help with
some of the massively fragmented files I deal with.