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.
In a thread on TSO-REXX a couple days ago, I hinted at how this might be crafted with a file granularity in a UNIX filesystem by using "pax -lrw" to create the "index" to the "pretend" copy. This might be a use for PDSE generations. -- gil ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
