Avoidance of write collisions

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A FCTC connection can potentially be used for communication in both directions.

However, if both sides try to send through a particular FCTC connection at the same time, a so called write collision occurs. While this is a recoverable situation, recovery consumes considerable processing and I/O resources and hence should be avoided.

In order to avoid write collisions, z/VM® implements a specific usage convention for the FCTC connections configured for an ISFC logical link. By means of that usage convention z/VM internally designates certain FCTC connections for unidirectional use.

The usage convention depends on the FCTC device numbers and the system names of the connected z/VM members; for details, see ISFC Improvements.

In summary, if two z/VM members are connected through an ISFC logical link, then

  • The z/VM member with the lexicographically lesser system name1 uses the FCTC devices starting from the lowest device number, up to higher device numbers
  • The z/VM member with the lexicographically greater system name uses the FCTC devices starting from the highest device number, and down to lower device numbers
  • If the ISFC logical link is composed of two up to eight FCTC devices, then each side uses at most one FCTC device less than configured, leaving the unused FCTC device for exclusive use of the adjacent side
  • If the ISFC logical link is composed of nine up to sixteen FCTC devices, then each side uses at most two FCTC devices less than configured, leaving the unused FCTC devices for exclusive use of the adjacent side

For example, if z/VM members in LPARs LPAR1 and LPAR2 were connected through an ISFC logical link with the FCTC connections with device numbers D000 through D00F, then LPAR1 (the system with the lexicographically lesser name) would perform its write operations on FCTC devices D000 up to D00D, leaving D00E and D00F exclusively for write operations of LPAR2. On the other hand, LPAR2 would perform its write operations on D00F down to D002, leaving FCTC devices D000 and D001 exclusively for write operations of LPAR1. This is illustrated in Figure 1.

Figure 1. Devices designated by z/VM for unidirectional use, based on their device number
Diagram showing selected devices.

With live guest relocation, asymmetric load on an ISFC logical link is typical, such that at a particular point in time only one side is sending a large amount of data, while the adjacent side at that time typically only sends small amounts of control information back to the sender.

As long as more than one FCTC connection is configured for an ISFC logical link, the two latter of the above rules ensure that there always remains at least one (or two) FCTC device(s) designated for the recipient for sending back control information, thereby – for the typical asymmetric load conditions – minimizing the risk that write collisions occur.

However, these two rules also imply that not all connections of an ISFC logical link are used for transferring relocation data; this is particularly significant

  • If the ISFC logical link is composed of only few FCTC connections, and / or
  • If the unidirectional FCTC devices are the only ones configured from a particular FICON® channel.
1 Lexicographical comparison is based on the lexicographic order of characters. Lexicographical order is a generalization of alphabetical order to non-alphabetic characters. For example, “AA” is lexicographically less than “AB”, or “A1” is less than “A2”.