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Comment by kccqzy | Hacker News Reader

The read-only system volume really seems like a neat idea. It reminds me of the Transactional Server role in openSUSE where they make system upgrades as atomic as possible: https://news.opensuse.org/2018/05/15/transactional-updates-i...

That said, I don't understand the use case for firm links. Seems like the problem it's trying to solve can just be solved with simple mount points: mounting read-write /Users and /usr/local. Am I missing something?

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From the video:

"Traditionally, it was done by mounting file systems on top of directories in the root file system. With the number of crossing points which we need to introduce and the number of volumes which it would require in the file system, that approach becomes rather expensive."

In the diagram, there are just two directories, and mount points could easily handle that case. In the video he describes how the scale of things makes using mount points here unwieldy.

A few more details that I've managed to suss out. The Data volume is mounted on /System/Volumes/Data. Currently, there are 21 firm links into it. They are enumerated in /usr/share/firmlinks (included below).

    /AppleInternal Device/AppleInternal
    /Applications Device/Applications
    /Library Device/Library
    /System/Library/Caches Device/System/Library/Caches
    /System/Library/Assets Device/System/Library/Assets
    /System/Library/PreinstalledAssets Device/System/Library/PreinstalledAssets
    /System/Library/AssetsV2 Device/System/Library/AssetsV2
    /System/Library/PreinstalledAssetsV2 Device/System/Library/PreinstalledAssetsV2
    /System/Library/CoreServices/CoreTypes.bundle/Contents/Library Device/System/Library/CoreServices/CoreTypes.bundle/Contents/Library
    /System/Library/Speech Device/System/Library/Speech
    /Users Users
    /Volumes Device/Volumes
    /cores Device/cores
    /home Device/home
    /mnt Device/mnt
    /opt Device/opt
    /private Device/private
    /sw Device/sw
    /usr/local Device/usr/local
    /usr/libexec/cups Device/usr/libexec/cups
    /usr/share/snmp Device/usr/share/snmp

It's interesting that /usr/X11 isn't in that list!

Well, X11 hasn't been provided by Apple since 10.8, about 7 years ago. So it wouldn't be touched by OS upgrades anyway (it's a separate product).

/sw isn't provided by Apple either, but they provide it so that Fink will work.

On my older High Sierra box, /usr/X11 is a symlink to /opt/X11. Catalina ships with /usr/X11 symlinked to ../private/var/select/X11. So it should still be possible to get X11 to work.

> I don't understand the use case for firm links

Haven't watched the video yet, so I'm speculating. We know APFS doesn't support directory hard links, which means Time Machine can't back up to APFS. I'm guessing firm links are designed to fix this problem (perhaps among others).

I suspect they will eventually migrate time machine backups in one go to APFS+snapshots, then use the new ASR tool also mentioned in this talk to stream local time machine snapshots to the volume.

I also suspect that will launch 'soon' (after September but possibly in March) , but I have to imagine the complexity and testing requirements for such a in-place migration are staggering.

With APFS supporting proper snapshots, I don't think you need directory hard links any more.

Why is Linux file system never really improving when there are so many potential developers? In the last 15 years I've only switched from ext3 to ext4 which isn't much of a change.

And the only alternative I hear recommended is xfs which is also around forever but not advanced, zfs is license restricted and btrfs isn't widely used enough to consider using it daily.

It's sad Linux is still stuck with basic file systems without any of the modern features to this day even though there must be huge demand for a more modern file system that can go with kernel GPL2.

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