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Recently I installed Suse Linux and Sled. After the installation, I found out that more memory is allocated for mappers; I don't know what is the use of these things. Will they be used once the main physical volume is filled?

I guess it is better for me to put my fdisk and df results, here they are:

Disk /dev/sda: 596.2 GiB, 640135028736 bytes, 1250263728 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 8503B13D-9FCF-4B8B-A05D-3D3C9E423393

Device      Start        End    Sectors  Size Type
/dev/sda1    2048     321535     319488  156M EFI System
/dev/sda2  321536 1250263039 1249941504  596G Linux LVM




Disk /dev/mapper/system-root: 40 GiB, 42949672960 bytes, 83886080 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes


Disk /dev/mapper/system-swap: 2 GiB, 2147483648 bytes, 4194304 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes


Disk /dev/mapper/system-home: 100 GiB, 107374182400 bytes, 209715200 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes


Disk /dev/sdb: 931.5 GiB, 1000204883968 bytes, 1953525164 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xdbbfb035

Device     Boot Start        End    Sectors   Size Id Type
/dev/sdb1          63 1953520064 1953520002 931.5G  b W95 FAT32


linux-c3of:~ # df -h
Filesystem               Size  Used Avail Use% Mounted on
devtmpfs                 1.9G  8.0K  1.9G   1% /dev
tmpfs                    1.9G  184K  1.9G   1% /dev/shm
tmpfs                    1.9G   11M  1.9G   1% /run
tmpfs                    1.9G     0  1.9G   0% /sys/fs/cgroup
/dev/mapper/system-root   40G   11G   29G  27% /
/dev/mapper/system-root   40G   11G   29G  27% /.snapshots
/dev/mapper/system-root   40G   11G   29G  27% /srv
/dev/mapper/system-root   40G   11G   29G  27% /var/cache
/dev/mapper/system-root   40G   11G   29G  27% /tmp
/dev/mapper/system-root   40G   11G   29G  27% /var/tmp
/dev/mapper/system-root   40G   11G   29G  27% /boot/grub2/x86_64-efi
/dev/mapper/system-root   40G   11G   29G  27% /usr/local
/dev/mapper/system-root   40G   11G   29G  27% /opt
/dev/mapper/system-root   40G   11G   29G  27% /boot/grub2/i386-pc
/dev/mapper/system-root   40G   11G   29G  27% /var/lib/mysql
/dev/mapper/system-root   40G   11G   29G  27% /var/lib/named
/dev/mapper/system-root   40G   11G   29G  27% /var/lib/machines
/dev/mapper/system-root   40G   11G   29G  27% /var/opt
/dev/mapper/system-root   40G   11G   29G  27% /var/spool
/dev/mapper/system-root   40G   11G   29G  27% /var/log
/dev/mapper/system-root   40G   11G   29G  27% /var/crash
/dev/mapper/system-root   40G   11G   29G  27% /var/lib/pgsql
/dev/mapper/system-root   40G   11G   29G  27% /var/lib/mailman
/dev/mapper/system-root   40G   11G   29G  27% /var/lib/mariadb
/dev/mapper/system-root   40G   11G   29G  27% /var/lib/libvirt/images
/dev/sda1                156M  4.6M  152M   3% /boot/efi
/dev/mapper/system-home  100G   29G   72G  29% /home
tmpfs                    385M   60K  385M   1% /run/user/1000
/dev/sdb1                932G  189G  743G  21% /run/media/ari/New Volume

And that 932G volume is of external hard disk.

I would like to understand the logic behind these allocations.

  • It looks like you only have three: root, home, and swap. vgs and/or pvs should show you how much free space you have (not allocated to a logical volume, the "mapper" stuff). – derobert Nov 29 '16 at 18:32
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I think you will find the answer to your question researching the BTRFS file system. It appears that your SUSE installation was left to use default settings and it did an automatic partitioning of your hard disk using BTRFS.

I had evaluated BTRFS on my SLES 12 system a 1-2 years ago, and was not impressed. I do not understand the logic behind the numerous mappings either. In evaluating any possible performance increase using the BTRFS file system, I found none only increased complexity.

My preference has been to format the operating system hard drive or volume of /dev/sda with /dev/sda1 as FAT32 for booting using ELILO. Then /dev/sda2 is the only other partition on the disk, formatted as EXT3. I do not use SWAP as my systems have more than 100GB of RAM, and this method has served me well for many years. In my opinion the only file systems that are good,reliable, and simple for the root partition are EXT3, EXT4, and XFS.

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despite the flame mail response.

since btrfs provides COW and system/file rollbacks, /var etc need to be separated so as not to backup junk. you can see with sudo btrfs subvolume list / the mapper/* stuff may be because your using LVM? which is superfluous with btrfs. btrfs has undergone much development recently and I use it without issue (just keep an eye on snapper filling your disk)

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