In a message dated 3/11/2006 1:51:16 P.M. Central Standard Time, [EMAIL PROTECTED] writes:
>"copy is immediately available before the physical tracks are copied". I can tell you how the IBM ESS (aka 2105) FlashCopy does it. At 03:00:00:00 you execute a TSO command (or use some other means to send the I/O request to the 2105) to establish a FlashCopy relationship between volume X and volume Y with the NOCOPY option specified. At 03:00:00:10 (or thereabouts) the 2105 has set enough information inside its control storage so that the net effect is that volume Y now looks exactly like volume X from the point of view of software that tries to access volume Y. The reason you specified the NOCOPY option is so that there will never be any copy of volume x moved onto volume Y. The details are in the devil: Whenever some software (any software - could be a backup job or something else) attempts to read track Z on volume Y, the 2105 checks to see if track Z on volume X was altered at any time after 03:00:00:10. If it has not been so altered, then the 2105 redirects the I/O aimed at volume Y onto volume X instead but just for track Z. Next part of the microcode: whenever any software writes onto track W on volume X after Y was declared to be a mirror of X, then the 2105 first copies track W from volume X onto its corresponding (same CCHH) location on volume Y before allowing the write operation to proceed onto track W on volume X. So if software attempts to read track W from volume Y, then the 2105 allows the I/O to go to volume Y, as Y now contains a valid copy of track W from volume X as that track was at 03:00:00:10. The only tracks on volume Y that will ever contain valid copies of their corresponding tracks on X are those tracks on X that are altered with some kind of write operation after the FlashCopy relationship was established. The intent is for backup software to read all the tracks that it wants to back up from what it thinks is volume Y. Presumably most of the read requests will end up going to volume X if those tracks were never altered since the FlashCopy relationship was established hours earlier. Some small fraction of the read requests will end up going to volume Y for the tracks on X that were altered after the FlashCopy relationship was established. The NOCOPY option allows you to make valid point-in-time backup copies of data from volume X with the minimum amount of controller overhead (i.e., having to copy tracks from X to Y). Only the tracks that are about to be changed are first copied to Y and then the write is allowed to go through to X. Once track W on volume X has been copied, it is never copied again. The 2105 is smart enough to remember which tracks it has copied. Is the copy instantaneous? Yes and no. First define "copy" and then we can answer. The backup copy on tape which you remove from the data center and ship to the vault is not finished until all 10,000 cylinders on volume X have been read by the backup process. The data still has to be read by software and transferred to the output file (either tape or another DASD). You can't copy 10,000 cylinders of data in 1/10 of one second. Unless the data center is hit by a space alien's laser bomb before the backup job finishes, then you have a good backup copy available for whenever you feel like finally getting around to running the backup job (hours or days later if you like). At some time after your backup copy process is complete, you execute another FlashCopy command, this time to break the relationship. And after that you can use volume Y again for any purpose. But please be aware that almost never will volume Y be an exact, full copy of volume X. Do not use it as a copy of X. The intent of FlashCopy is very different. IBM has several different hardware copy options from which to choose now: XRC, PPRC, FlashCopy, Dual Copy, Snapshot, and Concurrent Copy. They work differently from each other. Bill Fairchild ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
