| |
|
RSS Feed - WebSphere MQ Support
|
RSS Feed - Message Broker Support
|
  |
|
| 0S/390 Logical Record Length |
« View previous topic :: View next topic » |
| Author |
Message
|
| solomon_13000 |
 Posted: Fri Jul 04, 2008 12:12 am Post subject: 0S/390 Logical Record Length |
 |
|
Master
Joined: 13 Jun 2008
Posts: 284
|
| 256 is the OS/390 Logical Record Length. What does it mean?. |
|
| Back to top |
|
 |
| bruce2359 |
| Posted: Sat Jul 05, 2008 7:02 pm Post subject: |
|
|
Poobah
Joined: 05 Jan 2008
Posts: 9469
Location: US: west coast, almost. Otherwise, enroute.
|
A quick search of Mr. Google for logical record yielded several good definitions. Please do a little research before you post. Here's a good definition:
Record (Logical Record)
A collection of data items (fields) that describe an entity. All the fields of a record are stored or retrieved together. A collection of records is called a file or a data set.
------------------------------------------------------------
This is as brief an explanation as I can manage in a few minutes:
Think of a logical record as a row in a database table. The row represents an instance of a transaction - like a payroll row for each employee, or a inventory row for an inventory item.
Midrange systems and applications move data in byte-streams. The logical record concept is unique to the mainframe. Data is transfered from disk (or tape) in physical blocks. Physical blocks contain one or more logical records. If the physical block contains only one logical record, it is said to be unblocked. If the physical record contains more than one logical record, it is said to be blocked.
The real difference is seen by the application program. A midrange application reads a byte-stream. It is up to the application to determine where one piece of data ends and another begins. This often requires a lot of code and virtual storage to accept the entire byte-stream.
Mainframe applications are written to request (read, in this example) only one logical record. The application only need allocate space for a single logical record, not all of them.
At dataset (file) open, one or more buffers to contain many physical blocks are allocated in the address space where the application executes.
At first read, control is passed from the application to system routines (access methods) that understand the architecture of the data There are many file types on the mainframe (sequential, PDS, VSAM, etc.), and there are access methods for each.
If there are no (more) logical records in the buffers, then the access method passes control to another system routine that requests that a physical I/O take place. A physical block is the unit of data transfer.
When the buffer is filled, control is passed back to the access method which takes the first (next) logical record, passes it to the space in the application program. Subsequent reads are satisfied by the access method from logical records in the buffers.
System programmers (system admins) and automation can change the physical block size to improve I/O throughput. All this dramatically reduces the application program size, reducing the demand for virutal and real storage, the amount of I/O required, thus improving throughput and application concurrency.
As has been suggested in your prior posts, please consider taking an introductory z/OS class. IBMs ES155 z/OS Facilities class will give you a substantial introduction to what's different about mainframes.
A logical record length of 256 bytes means that all of the fields (name, address, city, state, zip, as examples) total 256.
I hope this helps.
_________________
I like deadlines. I like to wave as they pass by.
ב''ה
Lex Orandi, Lex Credendi, Lex Vivendi. As we Worship, So we Believe, So we Live. |
|
| Back to top |
|
|
| rtsujimoto |
| Posted: Tue Jul 08, 2008 9:44 am Post subject: |
|
|
Centurion
Joined: 16 Jun 2004
Posts: 119
Location: Lake Success, NY
|
| Given that the platform is OS/390, I think the "traditional" usage of logical record for that platform is used in conjunction with the term physical record. A physical record (on disk) may contain one or more logical records, depending on the blocking factor used. |
|
| Back to top |
|
|
| bruce2359 |
| Posted: Tue Jul 08, 2008 12:15 pm Post subject: |
|
|
Poobah
Joined: 05 Jan 2008
Posts: 9469
Location: US: west coast, almost. Otherwise, enroute.
|
| Quote: |
| Given that the platform is OS/390 |
This is basic MVS concept. A logical record is what a mainframe application reads or writes. Logical records may be blocked to save media space and improve i/o throughput.
_________________
I like deadlines. I like to wave as they pass by.
ב''ה
Lex Orandi, Lex Credendi, Lex Vivendi. As we Worship, So we Believe, So we Live. |
|
| Back to top |
|
|
| zpat |
| Posted: Tue Jul 08, 2008 12:46 pm Post subject: |
|
|
Jedi Council
Joined: 19 May 2001
Posts: 5866
Location: UK
|
The LRECL is used because the IBM mainframe is record oriented not byte stream oriented.
There is no mainframe use of line termination characters (in general) so the length of a logical record is pre-defined as the LRECL.
Records can be fixed LRECL (FB) where each record is the same record length (usually padded with blanks), or variable (VB) where the individual record length is set in the RDW (Record Descriptor Word) which is a four byte field at the start of each record.
In the case of VB, the LRECL is the maximum record length allowed.
LRECLs can be no more than 32767. FB datasets are typically LRECL 80 bytes (card images in the old days). |
|
| Back to top |
|
|
| bruce2359 |
| Posted: Tue Jul 08, 2008 2:40 pm Post subject: |
|
|
Poobah
Joined: 05 Jan 2008
Posts: 9469
Location: US: west coast, almost. Otherwise, enroute.
|
For all the details, refer to a manual entitled MVS Using Datasets. It describes the various dataset types and attributes.
_________________
I like deadlines. I like to wave as they pass by.
ב''ה
Lex Orandi, Lex Credendi, Lex Vivendi. As we Worship, So we Believe, So we Live. |
|
| Back to top |
|
|
|
|
|
| |
|
Page 1 of 1 |
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
|
|
|
|
