I seem to recall some doc from pre-VSAM SMF (pre-MVS/SP[SE?]) that many blocks
were written 'short'.
For example: the Type-74 (Device Activity) RMF records (a collapsed link list
of multiple devices) were always started on a block boundary, even if there was
'room' in the previous record.
But, it's been over 30 years since I cared about that level of detail; I don't
know if it's still true (or, really, ever was)
-
Ted MacNEIL
eamacn...@yahoo.ca
Twitter: @TedMacNEIL
-----Original Message-----
From: Barry Merrill <ba...@mxg.com>
Sender: IBM Mainframe Discussion List <IBM-MAIN@LISTSERV.UA.EDU>
Date: Mon, 10 Jun 2013 17:18:48
To: <IBM-MAIN@LISTSERV.UA.EDU>
Reply-To: IBM Mainframe Discussion List <IBM-MAIN@LISTSERV.UA.EDU>
Subject: Re: I/O Optimization
VBS Blocksize in dumped SMF data with RECFM=VBS,LRECL=32760,BLKSIZE=0 does NOT
produce interior same-sized blocks, as this small file from my DEV machine
shows:
LENGTH FREQUENCY PERCENT
15712 1 1.45
27995 2 2.90
27998 66 95.65
The 15712 was the last block, but other two blocks were interior blocks that
were not quite filled. I think I recall seeing similar values of up to 4 bytes
less than full blocksize when written to tape with BLKSIZE=32760.
Barry
Herbert W. “Barry” Merrill, PhD
President-Programmer
MXG Software
Merrill Consultants
10717 Cromwell Drive
Dallas, TX 75229
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-----Original Message-----
From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On Behalf
Of Paul Gilmartin
Sent: Monday, June 10, 2013 4:03 PM
To: IBM-MAIN@LISTSERV.UA.EDU
Subject: Re: I/O Optimization
On Mon, 10 Jun 2013 17:43:58 +0000, DASDBILL2 wrote:
>
>Spanned means that a single logical record might span multiple physical
>blocks, but the way it is implemented results in having all block sizes be the
>same, except for possibly the last one, which might be short.
>
Is uniform lengths a requirement? Do any applications rely on being able to
calculate a cylinder and track address within a VBS data set?
Is any error reported if an interior block is short? Does this mean one can't
append (MOD) to a VBS data set? What happens if after writing the last segment
of a logical record the block is not filled up to BLKSIZE but fewer bytes are
available than the length of a SDW?
I suppose using null segments might relax some of these constraints.
By "the way it is implemented" are you referring to the behavior of QSAM?
-- gil
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