Ron,
Thank you for the good response. It is true that the DFSMSdss
COMPRESS keyword and HWCOMPRESS keyword do not perform the same types of
compression. Like Ron said, the COMPRESS keyword is using a Huffman
encoding technique, and works amazing for repeated bytes (just the types of
things you see on system volumes). The HWCOMPRESS keyword utilizes a
dictionary based method, and works well, supposedly, on customer type data.
The CPU utilization of the HWCOMPRESS (dictionary based) is indeed larger
due to what it is doing. So you should choose the type of compression that
suits your CPU utilization needs and data type.
It was mentioned elsewhere in this thread about using the Tape
Hardware compaction. If you have it available, that's what I would go for.
The main intent of the HWCOMPRESS keyword was to provide the dictionary
based compression for the cases where you were using the software
encryption, and thus couldn't utilize the compaction of the tape device.
Thanks,
Andrew Wilt
IBM DFSMSdss Architecture/Development
Tucson, Arizona
IBM Mainframe Discussion List <[email protected]> wrote on 12/02/2010
04:20:15 PM:
> From:
>
> Ron Hawkins <[email protected]>
>
> To:
>
> [email protected]
>
> Date:
>
> 12/02/2010 04:21 PM
>
> Subject:
>
> Re: Hardware-assisted compression: not CPU-efficient?
>
> 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:[email protected]] On
> Behalf Of
> > Tony Harminc
> > Sent: Thursday, December 02, 2010 9:09 AM
> > To: [email protected]
> > Subject: Re: [IBM-MAIN] Hardware-assisted compression: not
CPU-efficient?
> >
> > On 2 December 2010 05:53, Ron Hawkins <[email protected]>
> 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.
> >
> > ----------------------------------------------------------------------
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>
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