(some snippage)


I had to look into 'unique' recently, and the limitations of recorded clock time. Tiemstamps are not granular enough, as has been pointed out, "unique" of course can only be believed for a single clock, as if you have several boxes, their clocks can all return identical values, in fact you'd expect them to.
But look here..


http://publib.boulder.ibm.com/infocenter/iseries/v7r1m0/index.jsp?topic=%2Frzatk%2FMININ.htm

Where we find that a 'uniqueness value' is assigned which ensures the value returned is unique, though the time represented may not be.


Extract from the reference above
Standard Time Format
The Standard Time Format is defined as a 64-bit (8-byte) unsigned binary value as follows:
Offset
Dec Hex Field Name Data Type and Length
0 0 Standard Time Format UBin(8)
0 0 Time Bits 0-51
0 0 Uniqueness bits Bits 52-63
8 8 --- End ---
The time field is a binary number which can be interpreted as a time value in units of 1 microsecond. A binary 1 in bit 51 is equal to 1 microsecond.
The uniqueness bits field may contain any combination of binary 1s and 0s. These bits do not provide additional granularity for a time value; they merely allow unique 64-bit values to be returned, such as when the value of the time-of-day (TOD) clock is materialized. When the uniqueness bits all contain binary 0s, then the 64-bit value returned is not unique. Unless explicitly stated otherwise, MI instructions which materialize the TOD clock return a unique 64-bit value.
A number of MI instructions define fields to contain a binary value which may represent a time stamp or time interval, or may specify a wait time-out period. Unless explicitly stated otherwise, the format of the field is the Standard Time Format.
Note: The following examples are shown with the uniqueness bits containing binary 0s. However, the uniqueness bits may contain any combination of binary 1s and 0s.
Examples of binary values as time intervals:
* A hex value of... Represents...
*
* 0000000000001000 1 microsecond
*
* 0000000000008000 8 microseconds
* 00000000F4240000 1 second
* 00000D693A400000 1 hour
* 0008CD0E3A000000 1 week
Examples of binary values as Coordinated Universal Time (UTC) time stamps:
* A hex value of... Represents...
* 0000000000000000 08/23/1928 12:03:06.314752
* 4A2FEC4C82000000 01/01/1970 00:00:00.000000
* 8000000000000000 01/01/2000 00:00:00.000000
* DFFFFFFFFFFF8000 07/07/2053 20:57:40.263928
*
* EFFFFFFFFFFFF000 06/08/2062 16:27:16.974591
*
Time-of-Day (TOD) Clock
The time-of-day (TOD) clock is a machine facility which provides a consistent measure of elapsed time. The value of the TOD clock can be materialized in "Standard Time Format". The time field of the TOD clock is incremented by adding a binary 1 in bit 51 every microsecond. This gives the TOD clock a granularity of 1 microsecond. However, the observed granularity cannot be accurately predicted because retrieval latency depends on the current implementation of the machine facility and the workload on the machine when the request is made.
Depending upon the MI instruction used, the value of the TOD clock may be materialized either as the Coordinated Universal Time (UTC) for the system, with or without a time zone offset and year offset, or as the local time for the system.


As for "The UUID is unique as an identifier across all time and space"
In a site with more than 1 box I'd trust that about as far as I could throw a politician


Richard Wilson


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