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 in the past (e.g., week-old, month-old, three-month-old, etc.). Thus if any information on the disk should be lost or garbled, any or all user files could be restored to the status of an earlier day.

For example, were the entire contents of the disk to be erased, the last set of dump tapes would be used for reloading, and the loss confined to less than twenty-four hours worth of new information. (This is not expected to happen since erasure does not occur easily and the disk normally retains all information in case of power failure.)

If both sets of duplicate dump tapes were to prove unreadable, the loss would be no greater than twenty-four hours, since the preceding day's tapes would be available. If the dump tapes were being used to restore information after disk loss, and both sets of tapes were unusable (an unlikely concatenation of misfortunes), the loss would still be confined to forty-eight hours since an earlier day's tapes would be available.

If an individual user's file is lost or garbled due to undetected failure of one or more disk tracks or to undetected tape error, the single file could be retrieved from earlier dump tapes either in earlier form or precisely, using the document number of the file for confirmation. If the user does not reference the file very frequently, however, it is possible that he might not ascertain the error until, say, a week after the loss occurred, and it might not exist on any older backup tape. Consequently, as with every type of information storage, it behooves the user to take his own precautions concerning valuable files.

Disk Editing Procedures
Despite the rather large capacity of the IBM 1301 disk modules for secondary storage they are still capable of being filled by various means, from runaway programs to careless user file "housecleaning." To minimize the latter effect, each user is assigned an initial but usually adequate track quota, which can only be extended by a formal request to the Center. Furthermore a technique is used of automatically displacing, from the disk secondary storage to magnetic tape tertiary storage, those user files which have been unused and inactive for 27