Tony, You are surprised, and then you explain your surprise by agreeing with me. I'm confused.
I'm not sure if you realized that the Huffman encoding technique used by DFMSdss COMPRESS keyword is not a dictionary based method, and has a symmetrical CPU cost for compression and decompression. Finally, as I mentioned in another email, there may be intrinsic Business Continuance value in taking advantage of the asymmetric CPU cost to speed up local recovery of an application, or Disaster Recovery that is based on DFSMSdss restores. An improvement in Recovery time may be worth the increased cost of the backup. Ron > -----Original Message----- > From: IBM Mainframe Discussion List [mailto:ibm-m...@bama.ua.edu] On Behalf Of > Tony Harminc > Sent: Thursday, December 02, 2010 9:09 AM > To: IBM-MAIN@bama.ua.edu > Subject: Re: [IBM-MAIN] Hardware-assisted compression: not CPU-efficient? > > On 2 December 2010 05:53, Ron Hawkins <ron.hawkins1...@sbcglobal.net> wrote: > > Johnny, > > > > The saving in hardware assisted compression is in decompression - when you > read it. Look at what should be a much lower CPU cost to decompress the files > during restore and decide if the speed of restoring the data concurrently is > worth the increase in CPU required to back it up in the first place. > > I am a little surprised at this. Certainly for most of the current > dynamic dictionary based algorithms (and many more as well), > decompression will always, except in pathological cases, be a good > deal faster than compression. This is intuitively obvious, since the > compression code must not only go through the mechanics of > transforming input data into the output codestream, but must do it > with some eye to actually compressing as best it can with the > knowledge available to it, rather than making things worse. The > decompression simply takes what it is given, and algorithmically > transforms it back with no choice. > > Whether a hardware assisted - which in this case means one using the > tree manipulation instructions - decompression is disproportionately > faster than a similar compression, I don't know, but I'd be surprised > if it's much different. > > But regardless, surely it is a strange claim that an installation > would use hardware assisted compression in order to make their > restores faster, particularly at the expense of their dumps. What > would be the business case for such a thing? How many installations do > restores on any kind of regular basis? How many have a need to have > them run even faster than they do naturally when compared to the > dumps? > > Tony H. > > ---------------------------------------------------------------------- > For IBM-MAIN subscribe / signoff / archive access instructions, > send email to lists...@bama.ua.edu with the message: GET IBM-MAIN INFO > Search the archives at http://bama.ua.edu/archives/ibm-main.html ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@bama.ua.edu with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
