N-LEVEL FILE SHADOWING AND RECOVERY IN A SHARED FILE SYSTEM

A shared file environment permits multiple users to read a file that is being updated concurrently. The process maintains N level shadows for a file to allow multiple users to read a file even though that file may be updated by one or more updaters in succession. A reader of a file does not need to wait on an updater of the file nor does the reader see any updates as they are being made. Each reader that opens the file sees the latest committed level of the file; that is, if reader A opens the file for read before updater B commits his changes, then there will exist one level shadow for the file after B commits. The process maintains N level shadows for a file, although only one updater of a file is allowed at a time. A new construct, referred to as the Not Available Yet (NAY) structure, contains entries for all files that are currently open for read. The NAY structure has an entry for each level of a file that is open for read. A counter associated with the entry indicates the number of readers of that level of the file. These entries are anchors for a list of block numbers of shadow blocks that cannot be unallocated because there is at least one reader of that level. A logging procedure allocates and unallocates blocks to track the N-level shadow blocks.