IBM Mainframe Forum

Hello and Merry Christmas to everyone!

As a beginner in MVS and Systems Programmer, one thing I have always struggled with, is finding the information I want in the manuals, there are just so many of them!

For example, I have decided over the holidays to refine my zOS knowledge and fill in the pieces where I am still not so clear about, in this case I want to know all about dataset allocation parameters.

I want to understand things like Directory Blocks, LRECL and know all possible Space Units etc...

So, going through all the manuals here http://www-03.ibm.com/systems/z/os/zos/library/bkserv/r13pdf/#IEA I just do not know which manual has the information I am looking for. I did scan a few DFSMS manuals, as I felt this type of information would be categorised under this, but I did not find what I was looking for.

So I was hoping that someone can advise me on how to use the manuals or other IBM documentation, this would be a great help, as I feel using these manuals is going to be the key to me learning zOS faster and more accurately. I tend to avoid them and just Google for information as I find them time consuming to navigate.

Thank you for any input at all.

I got news for you. I started with IBM OS/360 and System/360 in 1968. Forty six years ago there were TOO MANY [expletive deleted] manuals. Finding the manual with the correct information was too [expletive deleted] hard then! Now it seems there are an order of magnitude more manuals! A big part of learning to be effective in this trade is knowing how to find the information you need.

PDS directory blocks. That one is hard because there is no single source of information.

• All directory blocks have 256 data bytes.
• A used directory block has 2 overhead bytes.
• All directory entries require 12 bytes: 8 bytes for the member or alias name, 3 bytes for the address of the first data block, and 1 byte for flags and the size of user data, if any, for the directory entry.
• All load module directory entries require 24 bytes of user data. Some require more.
• A directory entry for a member with ISPF member status requires 30 bytes of user data. Members with "extended" ISPF member information require more user data.
• The last used directory block requires a dummy 12 byte directory entry.

LRECL depends on the data. Sorry, but by definition it cannot be in a manual.

Space units is hard, too. In 1968 there were a large number of possible disk types, though very few installations had more 2 installed. In 2014 installations with DASD types other than 3390 are few and far between, which makes your job easier. As for all the space types check the JCL Reference manual.

Google can be useful, but you should not be too dependent on it.

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deucalion0 (Sat Dec 27, 2014 1:10 am)

Thank you for your reply Steve!

And thank you for the information you have provided!

I know I am kind of spoiled with having all of the manuals all online in one place, I can scan them easily using Control-F, it must be a lot easier now than it was back in the day when there was only one or two hard copy's to in one shop.

And my exact reason for posting this in the first place is because I realize how important it is to know how and where to find information. As a beginner in MVS I pretty much always need information

I feel like I am putting together the biggest jigsaw puzzle ever made, I just need to get all the pieces together so I have an understanding of zOS.

Can I just ask you a little more regarding directory blocks?

In relation to bytes per Tracks and directory blocks: there are 56,664 bytes per track, are all of these bytes used for data, or are some lost? According to your info about each block being 256 bytes, this would mean a track can fit 221.3 directory blocks, so I assume 3/10's of a directory block - 76.8 bytes would be lost?

I am trying hard to learn as much as possible, but also making sure anything I learn can be validated as correct knowledge. I was trying to figure out the most efficient space for a PDS, using LRECL of 80.

I really appreciate you taking the time to help me out.

Thanks!!!

there are 56,664 bytes per track, are all of these bytes used for data, or are some lost?

You need to Google gx26-4577 and download a copy of this IBM manual. It gives information about 3390 disk drives and is still relevant despite coming out 25 years ago. The number of unusable bytes per track depends upon the block size of the data set and GX26-4577 gives you the number of blocks per track for various block sizes. One example: a block size of 30,000 means 26,664 bytes of each track cannot be used.

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deucalion0 (Sat Dec 27, 2014 6:20 pm)

deucalion0 wrote:... In relation to bytes per Tracks and directory blocks: there are 56,664 bytes per track, are all of these bytes used for data, or are some lost? According to your info about each block being 256 bytes, this would mean a track can fit 221.3 directory blocks, so I assume 3/10's of a directory block - 76.8 bytes would be lost?

It's much, much worse than you think. If you go to the manual Mr. Sample suggests, find the information for data records with an 8 byte key and 256 byte data area. In other words, PDS directory records. You will find 45 records per track. That's 11520 data bytes per track!

Sometime in the 1970s or 1980s, I converted the size of the record gaps to their travel time under the read/write heads. I found this time was roughly the same for all the DASD types I investigated. My conclusion at the time was the gap size had more to do with the controller than with the disk drive.

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deucalion0 (Sat Dec 27, 2014 6:20 pm)

deucalion0 wrote:... I know I am kind of spoiled with having all of the manuals all online in one place, I can scan them easily using Control-F, it must be a lot easier now than it was back in the day when there was only one or two hard copy's to in one shop.

It was both easier and harder. IBM was much more generous with freebies in those days, and the manuals themselves were cheap to buy, though finding out how to buy them or where to buy them was not so easy.

The hard part was the updates. IBM would send out update pages quite often. Inserting the update pages into the manual was often moderately difficult. Most of the manuals were bound using staples and were single sheets, not "signatures" like regular books. To insert the the updates, first you removed the staples, then you inserted the updates. After inserting the updates you then had the problem of keeping the manual together.

The JCL reference manual (SA22-7597) will exlain the meaning of each parameter relating to dataset allocation and their possible values, although not perhaps all the necessary detail to understand what those values mean.

DFSMS Using Datasets (SC26-7410) is a useful reference for understanding space allocations and such like.

To help find the right manual, you can...
Google search for the actual parm, then chase down the IBM book that way
Go to the LookAt page http://www-03.ibm.com/systems/z/os/zos/ ... index.html and enter a message ID from the system you are trying to figure out
Go to IBM site and enter that same message.

You will find links to libraries, or specific books using any of these methods. The books seem to have three flavors: A User Guide, a Reference Manual, and a Programming guide. Some have a separate Message Guide.
The User Guide will have a general overview of what the product does.
The Reference Manual will have detailed discussion of each of the parms/verbs/statements
The Programming Guide will explain how to do certain things with the products