W dniu 2012-02-07 17:01, Dana Mitchell pisze:
> That leads to a question that I've been thinking about for some
> time. Since the 3390 geometry is emulated by modern storage
> control units, why then are the inefficiencies of small blocks
> emulated also? There are not SLEDs actually storing the data, why
> are IBG's, sectors, and all the other CKD nastiness emulated that
> makes 80 byte blocks such a bad idea? IOW, why can't the control
> unit simply store 708 * 80 byte blocks on a 56,664 byte 3390
> track? Does zos's calculations take these inefficiencies into
> account and only write 78 of these blocks per track?
>
> Inquiring minds want to know
Because of COMPATIBILITY. When you write the physical block, there are
still gaps between them. Still the number of blocks per tracks is
limited, so writing small blocks is wasting space.
Performance - I think (my guess only) that reading many blocks vs one
big block means you have to perform many channel programs which causes
overhead. While I'm not sure about the reason I'm pretty sure about
results. It's enough to use in ISPF PS F 80 and PS FB 80 (with optimal
blksize) to feel the difference.
BTW: There were CU's which were able to eliminate all the wasted space
and use real storage much more efficiently. I mean IBM RVA family which
includes STK ICEBERG, SVA, etc.
--
Radoslaw Skorupka
Lodz, Poland
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