DSSU - Staging & Watermarks
After much investigation, reading and research I still have a confusion over the operation of a basic disk DSSU and it's staging to tape. Let me give an example:
Storage Unit created on local disk, specified as BASIC disk, on demand only, low watermark 20%, high watermark 80%, staging schedule set to run 24x7 - hourly to tape.
In this instance my understanding is as follows:
1. data is written to the DSSU during the backup window
2. every hour the staging shcedule runs and will duplicate ALL images not already duplicated to the tape storage unit
3. during the backup window should the storage unit not have sufficient space for a backup image then the oldest (assuming no data classification) image which has already been copied to tape will be deleted from disk and the new backup image will be written to the DSSU.
4. should there be no images when the DSSU staging schedule runs every hour then the staging job simply starts and finishes with no action
Where do the watermarks come in ? I have read various explanations - and in the case of BASIC disk some say the watermark values are not considered, some say that the low watermark is ignored, some say the high watermark is ignored....... My confusion lies in the area of when the images on disk are deleted, and how many are deleted. i.e. if the watermarks are valid, then when the DSSU reaches the high watermark (assuming all images have been copied to tape already) do sufficient images get deleted to bring the capacity down to the low watermark ? Obviously I wamt to keep as many images on disk as possible, so should I have the high watermark at the default 98%. If the watermarks are ignored then I believe that all images will remain on disk until a status 129 (disk full) occurs, then a backup will pause, images will be deleted (but how many ??) and then the backup will re-commence.