On Tue, Jun 16, 2009 at 9:21 AM, Agblad Tore<[email protected]> wrote: > Thank's I will try that out. > I have read in the help pages for RESERVE and I see what it does, but what is > the > major difference from just run FORMAT, what does FORMAT not do ?
RESERVE allocates a single file on a CMS formatted disk that occupies all space. It's the technique that some CMS applications (like SFS and DB2) use to do "raw I/O" (treat the disk as a series of blocks without a file system). To avoid confusion, RESERVE puts a dummy FST on the disk so users are not tempted to write on the disk. It's mostly history why the Linux diag driver requires the disk to be prepared with RESERVE, though it probably helps to avoid mistakes. But then that should have been done for the other drivers as well. When we started with what is now called the LDL, I've seen at least one Proof of Concept fail when the MVS storage admin "recovered" a number of volumes that "were not even labeled" and wiped out the effort of a week or two. After that, some arms were twisted and the Linux driver was made to do OS-compatible disk layout so that everyone could tell whose data was there. IMHO the confusion comes from mixing the device layout and the access method. I think Linux should use DIAG250 when running in a virtual machine, and SSCH when in LPAR. There is no reason to associate that with different layout of the device (well, it would need a DOC APAR against z/VM to bless this). I really don't know whether right now there is an advantage to use the diag driver for ECKD (or rather which workloads would benefit from it). Performance of disk I/O with Linux on z/VM is rather complex. I do know that there is a lot of potential in that interface, and if we would all be using it there might be a case for z/VM to polish the interface to exploit that. Rob ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
