NEP.FOUTS%ames-vmsb@sri-unix.UUCP (03/11/84)
In an environment where users are not allowed physical access to the machines, for whatever reason (in addition to security / paranoia on the part of those who operate the machines, there is also the case of the remote user,) there must be an ability to instruct the operator using simple command level procedures, with predefined protocals. After all, the operator cannot always be expected to know how your latest software wants him to respond to a tape hanging request. The 'hang' command discussed on the net is an excelent starting point towards bringing UNIX towards production environment use of tape drives. An additional form of tape hanging in an environment where "real" operators cover all three shifts of operations is to make tape hanging as invisible as possible to the user. There are two cases where this can be applied relatively easily. First, it is possible to design a hierarchial file migration system which causes files to migrate from fast, expensive disk to slow, cheap tape, to archive. In this case, the file entry still exists in user directory, with a pointer to the actual storage medium. When the user wants the file again, the operating system invokes routines which cause the operator to hang the correct tape, copies the file down to the disk and then turns it over to the user. Since the time to retrieve a file is nontrivial, there has to be a mechanism to request a file before it is needed, along with a way to gracefully recover if the user doesn't want to wait for the file. Second, it is possible to define a directory hierachy such that the medium of storage is not readily apparent. This is typically done in a similar fashion, with directory entries on line pointing to files on tape. To use this mechanism, the file creator is given the option of specifying the medium, and the medium is transparent to all other users. Again, the retrieval time is nontrivial, and an abort option must be available. The next obvious extension to this is to make not only the medium transparent, but the actual machine location. This would be useful in the case of a large research installation where a Cray with a small number of local disks was supported by a mass storage system, possibly blue. It would make life much easier for both the system user and the system designers if the software didn't have to know where in the hardware topology a file resided, but rather only where in a logical file system it could be found. ------