A couple of releases ago, I measured an increase in CPU time for some benchmark-type tasks that used ZFS vs. HFS when both were caching equally. That pattern was later confirmed with one or two real workloads. According to IBM this is not really unexpected because ZFS does more (journaling, more sophisticated caching), but I still was surprised--it was something on the order of 10% and for workloads that were doing a *lot* of HFS/ZFS cache friendly reads it did result in measurably longer run times.
Nonetheless, we're slowly moving towards ZFS because it does a better job caching than HFS. In fact, HFS caching is flat broken for us at times and IBM is not particularly interested in fixing it because they want everybody to go to ZFS. (E.G. I still have an ETR open with them that's been open for something like 18+ months, waiting for L3 to have time to work on finding the problem.) So despite the increased CPU time for similarly cached workloads, workloads that are not being cached well in HFS may be cached better in ZFS and may have a significant performance improvement. The cache reporting and controls in ZFS are also much better. So it's not all bad, I just wish there wasn't such a significant CPU hit for things that are already well-cached. And we did move the root to ZFS. ---------------------------------------------------------------------- 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
