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I'm learning how to work with loop devices and noticed that Docker uses them by default on CentOS. I can mount loop devices that I created, but when I try to mount Docker's devices I get error:

$ mount /dev/mapper/docker-8\:1-3146939-pool /mnt/docker-data/
mount: wrong fs type, bad option, bad superblock on /dev/mapper/docker-8:1-3146939-pool,
       missing codepage or helper program, or other error

I can't even see what FS it uses: 

$ lsblk --fs
loop1
└─docker-8:1-3146939-pool
loop2
└─docker-8:1-3146939-pool

I'd like to understand how can Docker work with such device without FS and why it's not possible for me to mount it?

Stanislav Bashkyrtsev
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1 Answers1

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After some research it appeared that those were Mapped Devices. Those are created by device-mapper and come in different flavours. One of them is Thin Pool + Thin Volumes. While Thin Volumes can be mounted, Thin Pool is not an ordinary device that can be. It's used only to provide metadata for Thin Volumes about where to store data saved on them.

Stanislav Bashkyrtsev
  • 124
  • 11