It was pointed out in an earlier message that the count area uses a 2-byte field for the data length, so let's consider the capacity using that limitation.
The maximum track capacity is then 65,535 bytes. The maximum number of tracks per cylinder is 65,535. The maximum number of cylinders per logical device is 65,535. For mathematical simplicity let's use 65,536 (2**16). The total capacity of a logical device then becomes 2**48 bytes (262,144 Gb). This is roughly 1,000 times the capacity of the largest physical drives currently available. It has already been noted that IBM has extended support for 2-byte cylinder numbers (i.e., use ICM vs. LH, etc.). IBM can do the same thing for 2-byte head numbers. There will come a time in the future when the ECKD will need be extended or a new architecture introduced. However, for the foreseeable future, ECKD offers ample capacity. I would recommend that IBM go ahead with the changes necessary to fully support 2-byte head numbers as they have previously done for 2-byte cylinder numbers. You are correct that DASD manufacturers do not currently provide emulation for 64K heads. However, PC DASD manufacturers have provided emulation for 256 tracks / cylinder for years. No hardware change is required here. Just some revised microcode. John P Baker Software Engineer -----Original Message----- From: IBM Mainframe Discussion List [mailto:[EMAIL PROTECTED] On Behalf Of R.S. Sent: Wednesday, July 20, 2005 05:55 To: [email protected] Subject: Re: capacity of largest drive I can see small one: nobody uses other geometry than 3390 (please, let's forget 3380 and older stuff, I'm talking about life datacenters, not musea). z/OS does not support devices with 64k heads or 16M B/trk. DASD manufacturers also does not offer such emulation. This is capacity limitation problem. This problem can be solved by IBM. They can do anything they decide: - provide new geometry - more cylinders, despite of word/halfword considerations - more heads, longer tracks - use 'reserved' fields in CCW - make revolution - FBA support - or even something completely new (linear storage) - or drop mainframe business - or just let it die by not changing anything. -- Radoslaw Skorupka Lodz, Poland ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
