This appendix details the on-disk organization of logging databases. Though a logging database can be accessed both via user interfaces as well as programmatically, knowing its organization makes it possible to manipulate the database via the file system. This should never be done while the "databases" runlevel is active because parts of the database will then be cached in memory.
Tags are contained in subdirectories of the database directory. These directories have the same name as the tags themselves. Meta tags are contained in subdirectories of the directory of their parent tag. Their directories have the same name as their name extension. It is possible to copy tags, including their logged data, from one database to another simply by copying their directories. Also, it is possible to rename or delete tags by renaming or deleting their directories.
In addition to data, the tag's directory contains its configuration inside a script named "config.lua". The script defines a table of configuration parameters that includes both the tag's creation parameters as well as the run-time modifiable tag configuration parameters. Similarly, the root of the database directory will hold a "subsets.lua" script when subsets have been defined via the "Tag Subset Editor". It is possible to load and modify these scripts using a text editor. Care should be taken to retain the proper table format, otherwise the database will fail to be initialised by the "databases" runlevel.
Since logging databases do not have to worry about referential integrity or mutation of already stored data, the storage format is simple. It is therefore not hard to write a custom program to accesses the logged data. There should be no need for doing so since a comprehensive set of tag access functions and VIs is provided. Still, for completeness, the remainder of this section details the data storage format
The data of each tag is spread out over one or more numbered "points<nnnn>.dat" files. By using multiple point files, number tags can contain more data than the LabVIEW file size limit allows. Point files simply contain successive timestamp and value pairs, both represented as double precision floating point numbers. For string tags, the values indirectly specify a string through a byte offset into an additional "strings.dat" file. The "strings.dat" file stores strings as successive flattened LabVIEW strings: a big-endian 32-bit string length followed by as many bytes of string data. Since there is only a single "strings.dat" file, the amount of string data is constrained by the file-size limitations of LabVIEW. Note that the data logger performs run-length compression such that when a particular string is logged multiple consecutive times, points with the same string offset are written so that the "strings.dat" gets appended to only once. Also, the "UNDEFINED" string is never stored but instead represented via a NaN offset value.
The "index.dat" file starts with a double that specifies the number of points in the log (the log size). This may be less than the size of the last point file indicates because the point files are grown in size by padding them with zeros in fairly large power-of-two increments (so-called chunks) once the log grows beyond a certain size. This prevents disk fragmentation by favouring the allocation of a continuous range of disk blocks when a point file is grown. After the log size, the "index.dat" file contains the double-precision timestamps of the first point in each chunk. These timestamps allow the quick localisation of logged data, given a timestamp, to within a chunk. This speeds up queries.
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