Virtual, Real and System Files

In the SeaDAS framework and within the SeaDAS documentation, regarding memory storage location, we can discuss files as being one of 3 types: System, Real or Virtual.

A "system" file is a file whose data is stored directly on the file system of your computer. Other names for this file might be "stored" file, or perhaps "physical" file.

A "real" file, is a file existing within SeaDAS. It occupies the temporary memory being used while SeaDAS is running. When you load a "system" file into SeaDAS, this essentially copies that file's data into a new memory (or RAM) location of your computer creating what we are calling a "real" file. Any changes subsequently made within SeaDAS to the "real" file will not alter the stored "system" file unless you specifically save the file. Other names for a "real" file might be "loaded" file or "session" file.

A "virtual" file is a file which has been generated within SeaDAS, but does not contain a complete set of its own data. Instead it contains a mix of metadata, and associated operations set to be performed when they are needed. The actual raster data needed to perform these operations exists in other file(s). You can think of a virtual file as a small file (not actually stored on your computer) which is mathematically linked to other files. Tools which have an option to create either a virtual file or a saved file will run very fast when creating a virtual file, as compared with the time needed to create the saved file. This is because all the operations must be performed when creating the saved file, but not when creating the virtual file. An example of a virtual file would be if you run the Reproject Tool with the option "save the file" turned off. The output would be a virtual file which contains information about the reprojection operation which has not actually been performed yet. If you were to then open a band within the virtual file, at that point the reprojection operation would need to occur in order to display that band. If you "save" a virtual file, it's associated operations are run and it is transformed into a real file. A system file copy of this real file is made. Other names for a "virtual" file might be a "linked" file or perhaps a "logical" file.

For instance, if the file on which you are doing statistics was created in the current SeaDAS session using the Reproject Tool but the resultant processed file was not saved, then the band(s) on which you are doing statistics on have not actually been reprojected yet. Consequently, the statistics may run extremely slowly or computer memory issues may arise.

Note: If you use a tool to create a virtual file from another virtual file, you are creating a chain of virtual files which gets increasing more difficult for SeaDAS and the computer memory to handle. A chain of virtual files, depending on how long it is and the type of operations being performed, can bog the performance down to the point where the computer memory or some other issue just won't let the operation proceed. In general it is advised to avoid generate a file using a virtual file as input, especially if that chains involves more than one intermediary virtual file along the chain.