On Fri, 07 Jan 2011 12:05:44 -0500 (EST), Rob van der Heij wrote: > On Thu, Jan 6, 2011 at 10:04 PM, Stephen Powell wrote: >> ... >> That's eleven combinations. The DIAG driver is only supported in a >> virtual machine under z/VM, of course. The current production >> version of GNU parted supports only the first two combinations. >> With my enhancements, all eleven combinations are supported. > > The complexity caused by the number of combinations is not an obvious > advantage. > It's unfortunate we have not been able to separate transport and > layout. If we had done that, we could select diagnose I/O or straight > SSCH based on performance aspects without any change to the data > layout. > > As you point out, some combinations are WORN devices (write it once, > read never afterwards) when the superblock or partition info > overwrites the CMS FST. Several of the possible combinations are not a > good idea at all.
I quite agree. Just because the kernel supports it does not mean that it is a good idea. Personally, I recommend using CMS RESERVED minidisks for everything. If you do that, then for CKD DASD you can turn DIAG on and off and use the disk either way. (Of course, you must get the disk offline to switch disciplines.) Strangely though, if I recall correctly, you cannot do this with FBA DASD unless your CMS logical block size is 512. The difference is that the FBA driver ignores the CMS logical block size and always handles the disk as if the blocksize were 512; whereas the DIAG driver honors the CMS logical blocksize. The bottom line is that, for CMS minidisks on FBA DASD with a CMS logical blocksize greater than 512, if you change the discipline from FBA to DIAG, or vise versa, you pretty much have to reformat the disk and start over. -- .''`. Stephen Powell : :' : `. `'` `- ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
