On Tue, Sep 13, 2022 at 11:54 AM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> OK. IIUC, you have upper fs files only in the root dir?
Sorry, no, the upper fs files need to be in subdir.
> And the lower root dir has only subdirs?
There could be files.
> Can you give a small example of an upper a lower and their
> union trees just for the sake of discussion?
>
For example, assume lower has the following layout:
$ tree lower
.
└── A
├── B
│ └── lower
└── lower
I can't create files in the fs in the lower.
$ touch A/B/file
touch: cannot touch 'A/B/file': Permission denied
The upper is initially empty.
I would like to overlay a writable fs on top of lower, so the union
tree looks like
$ tree union
.
└── A
├── B
│ └── lower
└── lower
$ touch A/B/file
$ tree union
.
└── A
├── B
│ ├── file
│ └── lower2
└── lower1
Here, 'file' exists in the upper.
Further, directory B could disappear from lower. When that happens, I
think there are two possible behaviors:
- make 'file' disappear from union as well;
- make 'file' and its directory accessible as well.
In behavior 1, it will look like
$ tree union
.
└── A
└── lower1
In behavior 2, it will look like
$ tree union
.
└── A
├── B
│ └── file
└── lower1
IMHO, behavior 1 works better in my use case. But if the FS experts
think behavior 2 makes more sense, I can work around.
>
> If that is all then it sounds pretty simple.
> It could be described something like this:
> 1. merged directories cannot appear/disappear
> 2. lower pure directories can appear/disappear
> 3. upper files/dirs can be created inside merge dirs and pure upper dirs
>
> I think I have some patches that could help with #2.
>
These three semantics looks good to me.
> Thanks,
> Amir.
