Behind the scenes, some controllers only store the active amount of data for a track, and monitor the VTOC for datasets being deleted. This is called Thin Provisioning and allows a small amount of overcommitting.
On Tue, Feb 7, 2012 at 1:24 PM, Hunkeler Peter (KIUP 4) <[email protected]> wrote: >>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? > > The "C" in CKD stands for "count" and this is some information about the > amount of data following in the next data block (the amount may vary > from block to block). Likewise "K" in CKD stands for "key" and some > access methods are using key data that is stored before the actual data. > So, both are bits of information that are used and thus cannot be > dropped. > > -- > Peter Hunkeler > > ---------------------------------------------------------------------- > For IBM-MAIN subscribe / signoff / archive access instructions, > send email to [email protected] with the message: INFO IBM-MAIN -- Mike A Schwab, Springfield IL USA Where do Forest Rangers go to get away from it all? ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
