The Set Watermarks provides the administrator with the ability to determine how directory and file creation is handled for a given volume set.
· Directory and File Creation Watermark- To minimize disc thrashing, InveStore clusters related files and subdirectories on the same volume (as opposed to having the files scattered randomly over many volumes within a volume set). InveStore also avoids having files span volumes, since any access to such a file would very likely cause thrashing.
· However, InveStore will not require clustering, since in some scenarios (backups or chronological archiving of data with no updates, for example) simply filling up the volumes sequentially is the best strategy. To facilitate these goals, we do the following:
1. A file is always written to the same volume that contains its parent sub-directory, if possible.
2. A sub-directory is always written to the same volume that contains its parent sub-directory, if possible.
3. Volume sets have a user-defined File Creation Watermark value. This value determines how full a volume can get before we quit writing new files to it. For example, if the File Creation Watermark value is 80%, and the volume is over 80% full, writing a new file to that volume set will cause its parent subdirectory to be extended to a new member volume and the new file written there. File modifications, on the other hand, will be written to the same volume as the original file even if the File Creation Watermark has been exceeded.
4. Volume sets have a user-defined Directory Creation Watermark value. This value determines how full a volume can get before we quit creating new directories on it. This leaves room to add files to existing directories without having to extend them.
Note: Though the watermark values are defined for each volume set, they are only enforced on individual member volumes—the extent to which a volume set is full has nothing to do with watermarks.