this post was submitted on 07 Apr 2024
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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Because if you're not offline, something is writing to the filesystem and changing blocks while you're copying. If you're lucky what you copied would be outdated. If you're less lucky it would cause fs inconsistency which would be cleaned up by fsck. If you are even less lucky you'd end up with silently corrupted files, e.g. a text file with old parts mixed with new. If you're even less lucky, you'd hit a vital metadata part of the fs and it would not be mountable anymore.
To clarify, the filesystem being block-copied has to be offline or mounted RO, not the whole OS. However if that's the root/home filesystem, then you can't unmount it while the OS is online.
If you don't want to deal with that you need a filesystem or volume manager that supports snapshots, then you can copy the snapshot. E.g. snapshot your root LVM vol, then block-copy the snapshot.