As I understand it, the filesystem knows the last byte written. With an FBA device, the access method reads that block and starts writing after the last used byte within the block. It then rewrites the entire block. I don't know why BSAM could not do something similar with FBS files. Of course, this was designed long ago and perhaps it just was not considered due to the poor performance or maybe the number of bytes required in the access method which would cause a region size problem. An FBA device is, in effect, a totally preformatted set of physical blocks and so doing random I/O to a random block is simple. CKD devices are much more complicated because you can read just any old CCHHR. Because the R may not exist yet.
I do understand that supporting FBA on z/OS would be a large undertaking. Especially for things like IPL and NIP. But I wish that z/OS would support FBA for some access methods. I don't see why the Media Manager could not easily do FBA for things like VSAM and PDSe data sets. They are fixed physical blocks at the hardware level as it is. Of course, there is the old "cash cow" mentality of locking z/OS users into the more costly ECKD emulated devices instead of being able to use the less expensive FBA. Heavens, might even put the z/OS disk on a SAN. On Wed, Jun 5, 2013 at 9:12 AM, Paul Gilmartin <paulgboul...@aim.com> wrote: > On Wed, 5 Jun 2013 09:32:18 -0400, Blaicher, Christopher Y. wrote: > > > >Do not MOD data to a FBS file. A short block in an FBS file is a EOF > condition. This means that if you luck out and fill the last buffer of a > file, you can MOD to it, but 99 44/100 percent of the time it doesn't work > and a program that goes to read the file doesn't see the added records in > the file. > > > <RHETORIC> How, then, do OSes with native FBA filesystems manage to > support appending to files?</RHETORIC> Couldn't QSAM have been > designed to to likewise? > > -- gil > > ---------------------------------------------------------------------- > For IBM-MAIN subscribe / signoff / archive access instructions, > send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN > -- This is a test of the Emergency Broadcast System. If this had been an actual emergency, do you really think we'd stick around to tell you? Maranatha! <>< John McKown ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
