On 2018-04-07, at 5:04 PM, Rainer Müller <rai...@macports.org> wrote:

> For distfiles, it would be possibile to apply a trick with a union mount
> to create a local overlay that allows writing new files:
> mount -t nfs -o ro,union mirror:.../distfiles \
>    /opt/local/var/macports/distfiles
> hdiutil create -o /tmp/distfiles-overlay.dmg -size 20g -fs HFS+J
> hdiutil attach -nomount /tmp/distfiles-overlay.dmg  # diskX in output
> mount -t hfs -o rw,union /dev/diskXs1 /opt/local/var/macports/distfiles

I discovered the hard way that union mounts are not supported in Mac OS. When I 
tried something with union mounts that did not work, and reported it to Apple 
tech support, they told me explicitly that they do not support union mounts. As 
I understand it, the issue is that there are two different sets of file access 
routines in the operating system. While the POSIX calls will support union 
mounts, the calls used by the internal Apple code bypass the POSIX calls, and 
use a different, lower access level. The result is that any type of AppKit, or 
other Apple-aware program (such as Finder, etc.) will fail to work properly.

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