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I am trying to understand about the file system of bootable system on CD. For CDs which can only be written once, how exactly does system boot?

Aren't there any configuration parameters which need to be changed/added upon every boot? Or are there not folders like tmp which contains the temporary data (although it vanishes away after every boot).

In simpler terms, how does CD-Read-only is able to boot ?

Am I missing something here ?

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This is achieved using union mounts. The two main methods are OverlayFS and aufs. It works like any other file system in the Linux VFS but as part of its configuration will point to two other file systems. As part of the union fs mount configuration will determine where data written to it will go. This is usually to an in memory file system.

There are varied implementations for semi-persistence live cds. But generically these systems will write what ever is in the temporary part of the union fs onto persistence storage such as a usb drive.

  • So does this mean, when we boot from 'read-only CD' , it does require a persistent storage (to mount temporary folder eg '/tmp') ? Because when we talk about file systems like OverlayFS and aufs, these file system does contain a read-write portion (on top of read-only portion). And if that is the case, how can I know which files/folders are never modified ? – gabbar Jan 29 '18 at 15:24

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