5 ms·

Maybe this is a really dumb question but how does one use a read-only filesystem? Can you mount it as writable temporarily or something? Or is it that you crea

Maybe this is a really dumb question but how does one use a read-only filesystem? Can you mount it as writable temporarily or something? Or is it that you create a compressed 'file' that you can mount as a file system? Like a zip file kinda I guess?

koolba 4y ago
You mount it as a read only file system to a mount point. It’s like putting in a CD (remember those?) into a drive. The source data can be a raw block device or more likely a local file. It doesn’t matter as either way the kernel is just reading blocks of bytes.
- staticassertion 4y ago
  > remember those? Not really tbh. I haven't used a CD in my adult life. I remember "burning a CD" was a thing and that's about it.
  - o_1 4y ago
    we won't ask about 3.5 drives, funny thing is they have their special uses too.
    - chongli 4y ago
      Or 5 1/4” for that matter! I remember my cousin’s old PC used those. Very flimsy but seemed to work remarkably well!
      - AndyJames 4y ago
        IMHO they wasn't flimsy just a bit floppy
  - bee_rider 4y ago
    How could you make us all feel so old. You monster.
    - unsafecast 4y ago
      I was born in the early 2000s and I've used CDs a lot.
      - staticassertion 4y ago
        As an adult? For what? I was born in 1991 and I haven't used one since I was in 9th grade and made a girl a 'mix tape'.
hansvm 4y ago
Just like a read-only file or service. There's some kind of a construction step, and thereafter it's read-only. One might do that to make it explicit that updates are expensive, to grant read-only privileges to a less trusted process, or whatever. Somebody else can chime in with the exact mechanism by which this one is written, but common solutions include being writable sometimes or having a program to build the filesystem from known data. That might be filesystem-as-a-file, filesystem on a separate partition, or what have you.
- staticassertion 4y ago
  Right I want to know what the construction step is in this case, but that makes sense that there's multiple approaches.
  - Nekit1234007 4y ago
    Just like creating any sort of archive. You (or a program of sorts) create the fs structure in some directory, then invoke something like `mkdwarfs -i /path/to/that/directory -o /path/to/output/file.dfs`
    - Arnavion 4y ago
      And squashfs works the same way - mksquashfs takes a directory as input and writes a file as output. That file can then be loopback-mounted to present the readonly filesystem.
    - staticassertion 4y ago
      Got it, yeah so that tracks then, thanks.
syntheticnature 4y ago
Another option is to use a read-only filesystem with overlayfs to provide a writeable filesystem (like JFFS2) that also has a solid static FS underneath in case of error. OpenWrt does, or at least did, this.
hinkley 4y ago
You got a few good “how” answers but as to “why”, it’s common for containers to use an overlay file system to handle writes. So in a container situation, you’ve already paid the overlay tax. Adding compression is a smaller incremental cost.

koolba 4y ago

You mount it as a read only file system to a mount point. It’s like putting in a CD (remember those?) into a drive. The source data can be a raw block device or more likely a local file. It doesn’t matter as either way the kernel is just reading blocks of bytes.

staticassertion 4y ago
> remember those? Not really tbh. I haven't used a CD in my adult life. I remember "burning a CD" was a thing and that's about it.
- o_1 4y ago
  we won't ask about 3.5 drives, funny thing is they have their special uses too.
  - chongli 4y ago
    Or 5 1/4” for that matter! I remember my cousin’s old PC used those. Very flimsy but seemed to work remarkably well!
    - AndyJames 4y ago
      IMHO they wasn't flimsy just a bit floppy
- bee_rider 4y ago
  How could you make us all feel so old. You monster.
  - unsafecast 4y ago
    I was born in the early 2000s and I've used CDs a lot.
    - staticassertion 4y ago
      As an adult? For what? I was born in 1991 and I haven't used one since I was in 9th grade and made a girl a 'mix tape'.

hansvm 4y ago

Just like a read-only file or service. There's some kind of a construction step, and thereafter it's read-only. One might do that to make it explicit that updates are expensive, to grant read-only privileges to a less trusted process, or whatever. Somebody else can chime in with the exact mechanism by which this one is written, but common solutions include being writable sometimes or having a program to build the filesystem from known data. That might be filesystem-as-a-file, filesystem on a separate partition, or what have you.

staticassertion 4y ago
Right I want to know what the construction step is in this case, but that makes sense that there's multiple approaches.
- Nekit1234007 4y ago
  Just like creating any sort of archive. You (or a program of sorts) create the fs structure in some directory, then invoke something like `mkdwarfs -i /path/to/that/directory -o /path/to/output/file.dfs`
  - Arnavion 4y ago
    And squashfs works the same way - mksquashfs takes a directory as input and writes a file as output. That file can then be loopback-mounted to present the readonly filesystem.
  - staticassertion 4y ago
    Got it, yeah so that tracks then, thanks.

syntheticnature 4y ago

Another option is to use a read-only filesystem with overlayfs to provide a writeable filesystem (like JFFS2) that also has a solid static FS underneath in case of error. OpenWrt does, or at least did, this.

hinkley 4y ago

You got a few good “how” answers but as to “why”, it’s common for containers to use an overlay file system to handle writes. So in a container situation, you’ve already paid the overlay tax. Adding compression is a smaller incremental cost.