[email protected] (John McKown) writes: > This is not about the z, per se, but is interesting. I don't think > that any of the IBM systems have this type of "filesystem". Hum, > perhaps the i?
original CMS filesystem from mid-60s ... was somewhat brought over from CTSS ... would simulate fixed-block on CKD dasd (somewhat inverse of the current situation where there hasn't been any CKD DASD manufactured for decades and simulated on industry standard fixed-block). The default was to not replace/update existing record ... but write to newly allocated location ... then periodically update alloction map, file directory (aka VTOC) ... also to new location and then rewrite the MFD record (in-place, single write that would flip between the old set of records and the new set of records). however, ibm CKD dasd had a peculiar power failure mode ... that might occur in the middle of a write operation ... there would be sufficient power to complete a write in progress ... but not sufficient power to continue transmitting the data from processor memory over the channel ... so the controller completed the write operation with all zeros (and no indication of a read/write failure). As far as i know, none of the other mainframe systems made any software provisions to handle this particular failure mode of ibm ckd dasd. As a result, in the mid-70s, the CMS extended file system had fix ... which change to a pair of MFD records and would alternatively write to the pair of records. On initial startup ... it would check both records to see if both records had been written correctly (no zeros propogated at the end of the record) and choose the most recent valid record. UNIX filesystem has been notorious for writting records in arbitrary order ... especially the filesystem control information (metadata) and after a shutdown/failure w/o "clean" shutdown (all records cleanly written to disk) ... a start up after non-clean shutdown would have to reread all records looking for inconsistencies ... which might take large tens of minutes. Circa 1990, aixv3 for rs/6000 enhanced the unix filesystem with logging changes to the file directory information (metadata) ... a side-effect was aix could almost immediately record/startup ... by rerunning logged information (it doesn't do anything for consistency of file data ... but does fix the unix filesystem integrity problem). AIX JFS filesystem http://en.wikipedia.org/wiki/JFS_%28file_system%29 http://www.linuxjournal.com/article/6268 the original implementation relied on special hardware in 801/risc where the unix filesystem control information (metadata) was placed in memory area that was specially identified to catch all changes. then all changes to filesystem was captured and journaled ... w/o having to change all the unix code to explicitly call the journaling/logging facility. The original claim was that the hardware implementation was also faster than putting in explicit logging/journaling calls. However, when the ibm paloalto group was porting JFS to generic hardware (w/o the 801/risc features), they had to put in explicit logging/journaling calls for changes. When they back ported that implementation to rs/6000, it turns out the explicit calls ran faster than the original implementation. as an aside, we relied on JFS for faster restart when we did ibm's ha/cmp (high availability, cluster multiprocessor) ... some past posts http://www.garlic.com/~lynn/subtopic.html#hacmp past posts mentioning 801/risc http://www.garlic.com/~lynn/subtopic.html#801 recent references to Jim Gray credited with formalizing transaction semantics and ACID properties http://www.garlic.com/~lynn/2014f.html#69 Is end of mainframe near ? http://www.garlic.com/~lynn/2014g.html#2 Is end of mainframe near ? http://www.garlic.com/~lynn/2014g.html#14 Is end of mainframe near ? http://www.garlic.com/~lynn/2014g.html#15 Is it time for a revolution to replace TLS? http://www.garlic.com/~lynn/2014g.html#38 Fifty Years of BASIC, the Programming Language That Made Computers Personal http://www.garlic.com/~lynn/2014k.html#2 Flat (VSAM or other) files still in use? -- virtualization experience starting Jan1968, online at home since Mar1970 ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
