BLKSIZE is always multiple of Lrecl in recfm=fb?

[Sharad Nikam]

Hi,

is it true that BLKSIZE is always multiple of Lrecl in recfm=fb?

[Maven JJ]

Hi,

is it true that BLKSIZE is always multiple of Lrecl in recfm=fb?

If you mention the BLKSIZE as greater than 0, then it should be multiple of LRECL. Else 0 also works.

[Abbey Tiwari]

If BLKSIZE(0) is specified, the system will determine the optimal block size based on the maximum record length (LRECL) and the physical characteristics of the disk, or approximately half of a physical track.

[Anuj Dhawan]

Long back 'memory & storage' were very expensive. As a programmer, you were needed to calculate the combination of LRECL and BLKSIZE. "BLK" short of "Block" is multiple of LRECL - which means when system read data it could read data of those many "records (one record is one LRECL, considering RECFM=FB)" which could fit in a given BLKSIZE. However that simply means give the highest BLKSIZE, it degraded the performance (there is more to it) - and there was a upper limit set. Said that, if you mention BLKSIZE it should be multiple of LRECL - usually this is the case of coding done in "old time". After the invention of SMS, one can code BLKSIZE=0 to choose from optimum size. Remember, software developments from IBM are usually backward compatible and coding BLKSIZE=0 didn't invalidated the previous BLKSIZE=??, where ?? represent a number. Have oversimplified it all, hopefully, it gets the concept across.

[Abbey Tiwari]

Why can't mainframe do it all themself?

[Robert Sample]

Why can't mainframe do it all themself?

You have to remember that IBM mainframes go back to 1964 when the s/360 and it's operating system became available. Systems were expensive relative to programmers and hence letting the programmer make the decisions was cheaper than putting things into the hardware or operating system. And things are still the way they are because IBM has been strongly committed to backwards compatibility so things that worked in 1964, for the most part, continue to work today.

[Maven JJ]

Thanks Robert. But it becomes very tough to learn all this.