David, Where your Primary and ML1 disk are on the same disk platform, basically costing the same $/GB, the saving realized from ML1 is the fact that it is compressed. If compression reduces the size of migrated datasets by 75% than an aggressive ML1 policy can reduce the cost of idle data by up to 75%.
ML2 is usually on tape, so the savings are in the media, and tape is doing its own compression so in most cases ML2 is not compressed. As you noticed, there is no free lunch because compression uses cycles, and if DFSMShsm ML1 activity is driving your peak CPU the net savings of ML1 may not be worth it. If you are going to turn off ML1 compression then I would suggest that your either (a) change your ML1 policies to migrate less data, because there is no point putting a dataset on ML1 if it uses the same space; or (b) use a cheaper storage platform for Ml1, like SATA drives, so you still realize a saving when you migrate to ML1. You've probably guessed that when you turn of compression you will need to increase the size of ML1 by 300 - 400%. Ron > -----Original Message----- > From: IBM Mainframe Discussion List [mailto:[email protected]] On Behalf Of > O'Brien, David W. (NIH/CIT) [C] > Sent: Wednesday, March 23, 2011 5:46 AM > To: [email protected] > Subject: [IBM-MAIN] HSM Compaction question(s) > > The system I inherited has compaction turned on for Dasdmigrate and > Dasdbackup. We notice spikes in CPU utilization during HSM Migration whether > it is manual or interval. Since we have DASD in abundance while CPU cycles are > sometimes in short supply, I'm considering turning off compaction to save on > overhead. > > My question to the group is whether the HSM users out there have compaction > turned on or not and if yes what is your COMPACTPERCENT setting? > Does anyone know of any downsides to turning off compaction, other than a > larger ML1 pool? > > David O'Brien > NIH Contractor > > ---------------------------------------------------------------------- > 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 ---------------------------------------------------------------------- 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
