How to cCheck Size of a share/folder?

Hephaestus wrote:

Hypothetically, if a file system consisted of one file only, the file being 1GB, this file consumes 1GB of physical disk space. If a file system consists of more files (usually the case), the same file does not consume 1GB of physical disk space (shared blocks). Thus "size of a single file" really means: the size when the file is copied outside of ReadyNAS OS environment. Adding all files sizes within a folder, and showing the result as folder/share size would simply follow already adopted logic: this is the size of the folder when it is copied outside of ReadyNAS OS environment. Showing size of files and not showing the size of folders is logically inconsistent.

Thought experiment (a): Let's say you have a share with a single 1 GB MP4 file in it. You take 1 snapshot.

The folder+snapshot will be reported as 2 GB total space by btrfs, but the actual disk space used is only 1 GB.

Now let's change the thought experiment.

Thought experiment (b): Now you add a tag to that MP4 file which rewrites all the data in the file (shifting it a little bit). BTRFS will again report the folder+snapshot as taking 2 GB. And now it will actually take 2 GB of space.

So btrfs (without the quota option enabled) can't tell you how much actual disk space is used by the folder and its associated snapshots.


Now there is some use for simply knowing the total file space in the main share - ignoring the snapshots and CoW and simply summing up the sizes of the files as you say. That number is readily available in ssh (the normal du command will give it to you). And it could be easily added to the GUI.

But I am interested in knowing the actual disk space used by the folder+snapshots too - because I can use that to tell me where I need to delete snapshots as the disk fills.

Yes, agree. I guess my angle was: data on NAS is often moved out to different environments; to copy folders to external computer, to backup selected folders from NAS to external drive, etc. I have extensive knowledge of other file systems architecture, but only very basic knowledge of BTRFS. With my limited BTRFS understanding it appears to me that changing what is understood here by "folder size" is necessary. My initial impression is that some conceptual problems could be solved by saying: "[in BTRFS] we can only say that a folder size is 10GB when it is copied outside of BTRFS environment. Within the BTRFS environment concept of "folder size" is different, and saying that it is 10GB is meaningless". Perhaps even changing naming convention within BTRFS could also help, to something like: "Ext Size" for "exported" or "external" size. With that paradigm change in place knowing "Ext Size" would be very helpful for users. I need to think about it more in my spare time.

Now there is some use for simply knowing the total file space in the main share - ignoring the snapshots and CoW and simply summing up the sizes of the files as you say

I think that should be added to start with; just about every second post in the forum talks about backing up your data but it would be nice to know how much space is required on the external storage to actually do that backup, and that number would not include snapshots/CoW.

So just add that with a note saying "file space (excluding snapshots & CoW data)".

For solution see: viewtopic.php?f=7&t=81129