Mbuttive io 828

so the os-360 genre operating systems have been vulnerable to this CKD record write problem with propagated zeros in case of power failure ... because some number of the records were static-fixed location.

the original cms filesystem (circa '66, cp67-cms then morphing to vm370-cms) had sequence were it wrote all the changed filesystem structure data to new disk record location ... and then carefully replaced the MFD (master file directory) record. This worked for all cases except the situation where the power failure occured while the MFD record was actually being written and a power failure occured ... resulting in zeros being propagated thru the end of the record (and their would be no error indication). The nominal logic (modulo the partial zero filled MFD record) was that either the MFD pointed to the old copies of all the file structure or the changed-updated records written to new disk locations (rewritting the MFD was effectively a commit like operation ... but was vulnerable to the power failure zero fill problem).

the "EDF" cms filesystem, introduced in the mid-70s, creaed a pair of MFD records ... and recovery-startup would read both MFD records, determine which was the most current valid MFD. on filesystem updates ... it would write updated-changed records to new disk location (just like the original cms filesystem) but alternate writting the MFD record. This caught various kinds of failure modes occuring during the updating of the MFD record.