Is volume compressed?

[WojTec]

Zitat von DeddyH:

Do you really mean a compressed volume or a volume which supports compression of indivual files?

So, compressed volume, in properties when checked specified checkbox and then volume showed in blue (usually) - I want to detect this situation

[WojTec]

None of above flags (flags and flag <> 0) are working, some always returns false, another always true. Any help?

[Der schöne Günther]

I might be wrong, but I believe there is no clear indication if a volume is either compressed or not compressed at all. We're talking about NTFS, right? I think the FILE_VOLUME_IS_COMPRESSED flag still hails from the time of the FAT file system. Back then, a volume was either entirely compressed ("double space volume") or no compression at all was taking place.

Take a look: Set a volume of yours to "compressed". Create some folders, they're all blue (compressed). Now set a folder to "uncompressed". Your volume will still state "compressed". I believe "volume compression" only means "Default setting for compressing content or not".

Keeping this in mind, this is what I baked together:

Delphi-Quellcode:

procedure TForm18.checkCompression();
const
   // According to http://www.swissdelphicenter.ch/en/showcode.php?id=1053
   COMPRESSION_FORMAT_NONE = 0;
   COMPRESSION_FORMAT_LZNT1 = 2;
   COMPRESSION_FORMAT_DEFAULT = 1;
var
   fileHandle: THandle;
   outBuffer: ^USHORT;
   bufferLength: DWORD;
begin


   // Fetch Handle
   fileHandle := CreateFile(
      'F:\.',
      GENERIC_READ,
      FILE_SHARE_READ,
      nil,
      OPEN_EXISTING,
      FILE_ATTRIBUTE_NORMAL or FILE_FLAG_BACKUP_SEMANTICS,
      0
   );

   if (fileHandle = INVALID_HANDLE_VALUE) then raise Exception.Create('CreateFile Failed');

   // Init Buffer
   New(outBuffer);

   if not DeviceIoControl(
      fileHandle,
      FSCTL_GET_COMPRESSION,
      nil,
      0,
      outBuffer,
      SizeOf(outBuffer),
      bufferLength,
      nil
   ) then raise Exception.Create('DeviceIoControl failed');

   if (outBuffer^ and COMPRESSION_FORMAT_LZNT1) <> 0 then
      ShowMessage('it is compressed');


end;

[himitsu]

Zitat:

lpFileSystemFlags [out, optional]

A pointer to a variable that receives flags associated with the specified file system.

This parameter can be one or more of the following flags. However, FS_FILE_COMPRESSION and FS_VOL_IS_COMPRESSED are mutually exclusive.

Remarks

The FS_VOL_IS_COMPRESSED flag is the only indicator of volume-based compression. The file system name is not altered to indicate compression, for example, this flag is returned set on a DoubleSpace volume. When compression is volume-based, an entire volume is compressed or not compressed.

...

Es kann nur Einen geben.



Entweder ist alles immer komprimiert,

oder das Volume unterstützt eine eine Komprimierung.
Und wenn das Volume nun manuell komprimiert wurde, dann ist das andere Flag dennoch nicht aktiv, da nicht das Volume, sondern nur das Root-Verzeichnis komprimiert wurde.

[Der schöne Günther]

Ja, aber er hat ja schon gesagt, dass das Flag trotzdem nicht gesetzt wird, auch wenn man im Explorer für das gesamte Laufwerk das "komprimiert"-Häkchen gesetzt hat. Und das kann ich auch bestätigen. Das war vielleicht unter FAT noch so, aber unter NTFS zwingt kein übergeordnetes Verzeichnis/Laufwerk einen, jetzt gefälligst komprimiert zu sein.

Wie das unter exFat, ReFS und was weiß ich aussieht- Keine Ahnung.

[WojTec]

Thanks, your explanation is helpful and now I understand more