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Top Forums UNIX for Beginners Questions & Answers Linux Containers - /proc mounting and other queries Post 303027017 by sreyan32 on Friday 7th of December 2018 06:17:02 AM
Old 12-07-2018
Quote:
Originally Posted by bakunin
You already got some excellent answers to your questions at hand but you might profit from a little "theory" behind all that, so here it goes:

I hope that connects a few loose ends.

bakunin
It does connect a few loose ends. But I really didn't think docker was a para-virtualization product. I thought they were just implemented as jailed processes.

AFAIK Docker mainly uses Linux Namespaces and Cgroups to create a virualized environment for execution.

One more thing that is bugging me is : When you install a software within a docker container does it get installed into the host OS also? Intution tells me that is how it should be since a Docker mainly shares the host kernel. If so, then shouldn't it share the host package management system.

Where this logic falls apart is when on an Ubuntu system we can run a Debian or Alpine docker. How is that possible? How can debian binaries even run on an Ubuntu system?

Lastly, why does docker need the root filesystem of the OS that it is trying to emulate on disk? What does it mean to have a rootfs of an OS and again how can utilities within in run an another OS. This refers mostly to the video that I have linked in my original question. If it is at all possible please watch it ( I have marked the actual place where it got generated)- you will know why I am getting confused.

--- Post updated at 04:47 PM ---

Quote:
Originally Posted by MadeInGermany
Hard to explain. An example is a disk mount (filesystem like ext3,ext4,reiserfs,xfs,...), that is only allowed once, because writes to the two mount points would cause a corruption in the filesystem on the disk. But: a bind mount of the primary disk mount to another mount point is allowed; all writes occur at the primary mount point.
Didn't get what you explained. Let me tell you of what idea I have of the Linux mount process then may be you will get a better idea of why I am failing to grasp the concept.

Any block device that the kernel identifies can be mounted to a location in the VFS. The location, which is just a directory is called a mount point.

Now one question is that can an already mounted device be mounted twice? A partition that is mounted twice? If so I don't see why it would cause corruption as you say it would. You writing to the same block device, the kernel just identifies it to the user by 2 mount points.

Next, is what is a primary mount point? Is the first mount of the block device? Can a mount point be mounted again? Why would you want to do that?

Where does bind mount fit into all of this?
 

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xfs_freeze(8)						      System Manager's Manual						     xfs_freeze(8)

NAME
xfs_freeze - suspend access to an XFS filesystem SYNOPSIS
xfs_freeze -f | -u mount-point DESCRIPTION
xfs_freeze suspends and resumes access to an XFS filesystem (see xfs(5)). xfs_freeze halts new access to the filesystem and creates a stable image on disk. xfs_freeze is intended to be used with volume managers and hardware RAID devices that support the creation of snapshots. The mount-point argument is the pathname of the directory where the filesystem is mounted. The filesystem must be mounted to be frozen (see mount(8)). The -f flag requests the specified XFS filesystem to be frozen from new modifications. When this is selected, all ongoing transactions in the filesystem are allowed to complete, new write system calls are halted, other calls which modify the filesystem are halted, and all dirty data, metadata, and log information are written to disk. Any process attempting to write to the frozen filesystem will block waiting for the filesystem to be unfrozen. Note that even after freezing, the on-disk filesystem can contain information on files that are still in the process of unlinking. These files will not be unlinked until the filesystem is unfrozen or a clean mount of the snapshot is complete. The -u flag is used to un-freeze the filesystem and allow operations to continue. Any filesystem modifications that were blocked by the freeze are unblocked and allowed to complete. One of -f or -u must be supplied to xfs_freeze. NOTES
A copy of a frozen XFS filesystem will usually have the same universally unique identifier (UUID) as the original, and thus may be pre- vented from being mounted. The XFS nouuid mount option can be used to circumvent this issue. In Linux kernel version 2.6.29, the interface which XFS uses to freeze and unfreeze was elevated to the VFS, so that this tool can now be used on many other Linux filesystems. SEE ALSO
xfs(5), lvm(8), mount(8). xfs_freeze(8)
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