Consider for example "flash copy" and similar technologies. The DASD subsystem is able to make a "copy" of an entire volume without using any significant amount of actual honest-to-gosh disk space.
It's a little hard to explain the technology in a quick e-mail paragraph but basically the controller makes a "pretend" copy of the disk by making a duplicate copy of an "index" to all of the volume's tracks. Whenever a track changes, it creates the track image in new disk space and updates the index to point to that track. Lets companies make an internally consistent backup of an entire DB2 volume while only having to "freeze" DB2 for a second or so. Charles -----Original Message----- From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of Gerhard Adam Sent: Saturday, May 20, 2017 11:09 AM To: [email protected] Subject: Re: SDB (system determined Blksize) I don't see how the space would not be wasted. Where would it be assigned or accounted for? If you ignored such waste, you could have more capacity available than the volumes you've defined. Sent from my iPhone On May 20, 2017, at 10:40 AM, Charles Mills <[email protected]> wrote: >> if you write a 32K block to an emulated 3390 track, the balance of >> the > space will be wasted > > Is that true? (Serious question -- everything I know about DASD > management could be written in one paragraph of an e-mail.) Sure, it wastes "virtual" > space on the emulated 3390 track, no doubt, but aren't modern storage > arrays smart enough not to waste the real disk space that you are > paying for on empty 3390 track space? > > Charles > > > -----Original Message----- > From: IBM Mainframe Discussion List [mailto:[email protected]] > On Behalf Of Ron Hawkins > Sent: Friday, May 19, 2017 4:47 PM > To: [email protected] > Subject: Re: SDB (system determined Blksize) > > Lizette (OP), > > > > I would not recommend going to 32K as a blocking factor. Generically > speaking, all three vendor emulate a CKD track, allocating up to 64KiB > of space for every track. > > > > Whether you use a regular formatted volume, or thin provisioning > (DP-VOL in Hitachi speak), if you write a 32K block to an emulated > 3390 track, the balance of the space will be wasted. Ipso fact you > will use 41% more space for the same amount of data at half-track. > > ---------------------------------------------------------------------- > For IBM-MAIN subscribe / signoff / archive access instructions, send > email to [email protected] with the message: INFO IBM-MAIN ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
