inexplicable difference between `fileSystemSize` and `fileSystemMaxSpace`

[calestyo]

Hey.

I was trying to understand the meaning of the various pool spaces in the REST output (for adding them to my Prometheus exporter).
E.g. there's:

        "space": {
          "total": 17583515092665,
          "free": 13804589006848,
          "precious": 0,
          "removable": 0,
          "lru": 1783180729,
          "gap": 53687091200,
          "breakEven": 0.7

whose values seem to be always(?) the same than those:

    "repositoryData": {
      "label": "Repository",
      "state": "OPEN",
      "files": 13544,
      "totalDiskSpace": "17583515092665",
      "freeDiskSpace": 13804589006848,
      "usedDiskSpace": 3778926085817,
      "preciousDiskSpace": 0,
      "removableDiskSpace": 0,
      "usedDiskSpaceRatio": 0.21491300606858676,
      "preciousDiskSpaceRatio": 0.0,
      "removableDiskSpaceRatio": 0.0,
      "fileSystemSize": 17592186044416,
      "fileSystemFree": 13804589006848,
      "fileSystemRatioFreeToTotal": 0.7847000351175666,
      "fileSystemMaxSpace": 17583515092665,
      "gap": 53687091200
    },

Now my understanding would be as follows:

• totalDiskSpace is always whatever fileSystemMaxSpace is

• usedDiskSpace is the sum of all data files on the pool, taking the apparent size not the used space

• freeDiskSpace is simply the former - the later (or can it be anything else?)

• fileSystemSize and fileSystemFree is whatever the filesystem reports
  Do you by chance know what exactly that is? I mean df would use statfs(2) or fstatfs(2)… I tried to quickly skim through dCache's code and I guess it uses Java FileStore’s getTotalSpace and getUsableSpace? And what these use probably depend on the platform?

• fileSystemMaxSpace my undrestanding is that this is always <= fileSystemSize and the value/percentage set by the set max diskspace command?

Right so far?

But here the issue:
I have e.g.

> info
…
--- rep (Repository manager) ---
State : OPEN
Files : 13544
Disk space
    Total    : 17583515092665
    Used     : 3778926085817    [0.21491300606858676]
    Free     : 13804589006848    Gap : 53687091200
    Precious : 0    [0.0]
    Removable: 0    [0.0]
File system
    Size : 17592186044416
    Free : 13804589006848    [0.7847000351175666]
Limits for maximum disk space
    File system (limit)  : 17583515092665 (100%)
    Statically configured: -
    Runtime configured   : -
Sweeper Policy
    lru   : 0
    margin: 0.0

The pool in question has no set max diskspace , and indeed it shows 100%, yet: 17592186044416-17583515092665 = 8670951751

Where does that discrepancy come from? It's not the gap.

Does it have to do with @kofemann’s commit 208bfbe? And if not... is that (even further?) reduction somewhere exported?

Thanks,
Chris.

[calestyo]

Ah seems I found the "offender":

dcache/modules/dcache/src/main/java/org/dcache/pool/repository/v5/ReplicaRepository.java

Line 1022 in 6288b6c

info.setFileSystemMaxSpace(fsFree + used);

Anyway, would still be nice if the other points above could be confirmed/clarified.