On Thu, 20 Aug 2015 11:55:22 -0500, Kirk Wolf wrote: > >The problem, of course, is that DSCBs don't have "last update timestamps". > And in systems that have them, timestamps often can be forged by the user. I once recovered a z/OS HMS backed up data set and was dismayed to see that the timestamp was set to the time of recovery rather than the time of last access.
>My initial whack at this would be to use a two-part hash: > >part 1: a shortened SHA1-hash of the format-1/8 DSCB >part 2: a full SHA-1 hash of all of the data > Your hashing should be sensitive to record boundaries, else an operation as simple as splitting a record in two will not be detected as a change. Perhaps hash the RDWs also. (I argued this on CMS-PIPELINES a while ago. The Bad Guys won.) Would performance be better by replacing hash with diff(1) or cmp(1)? Same amount of I/O; less computation. And cmp could exit early on detecting the first difference; no need for a preliminary short hash. -- gil ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
