As I said, I am no expert. My point was simply to give an example to illustrate the answer to
> 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. and illustrate that defined apparent 3390 space could be greater than actual occupied hardware space. Good discussion of CoW here: http://stackoverflow.com/questions/628938/what-is-copy-on-write Charles -----Original Message----- From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of Paul Gilmartin Sent: Saturday, May 20, 2017 7:46 PM To: [email protected] Subject: Re: SDB (system determined Blksize) On Sat, 20 May 2017 13:33:09 -0700, Charles Mills wrote: >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. > The technique is known as "Copy on Write". CoW is also used by quality implementations of fork(), by ZFS (not zFS; the real one; GIYF), by btrfs, and by old StorageTek products, Iceberg and EchoView. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
