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?

  • What filesystem you use for this LV? – Romeo Ninov Mar 4 at 7:00
  • What is the output of the following commands: ls -Rabl /home/anselmo/work and du -kx /home/anselmo/work ? – telcoM Mar 4 at 10:35
  • @RomeoNinov I'm using LVM2 and xfs. – Anselmo Park Mar 5 at 7:26
  • @telcoM It has another "anselmo" directory. It looks like my home directory at old time. Does it mean it is an used disk? I thought that I removed all partitions of the disk when I installed my OS, but the result seems that it is my misunderstanding. – Anselmo Park Mar 5 at 7:30
  • When you installed CentOS 7, it auto-created that LV as you said. It also created a home directory for any regular user(s) specified during the installation, and populated the home directory (or directories) with the contents of /etc/skel. That plus the preallocation feature described by @RomeoNinov probably explain it all. – telcoM Mar 5 at 9:25

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.

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