If you have database logs in your VMs, they can create what LOOKS like unaligned IO, but the nature of how the DB logs happen, they are false positives.
On Wed, Aug 24, 2011 at 10:16 PM, Fletcher Cocquyt fcocquyt@stanford.eduwrote:
I originally tried posting this back on April 11 - now that the list is "fixed" I want to try again - thanks:
Hi, we’ve aligned all our Vmware vmdk’s according to the Netapp best practices while tracking the pw.over_limit counter see: http://www.vmadmin.info/2010/07/quantifying-vmdk-misalignment.html
Counters that indicate improper alignment ( ref: ftp://service.boulder.ibm.com/storage/isv/NS3593-0.pdf) “There are various ways of determining if you do not have proper alignment. Using perfstat counters, under the wafl_susp section, “wp.partial_writes“, “pw.over_limit“, and “pw.async_read,“ are indicators of improper alignment. The “wp.partial write“ is the block counter of unaligned I/O. If more than a small number of partial writes happen, then IBM® System StorageTM N series with WAFL® (write anywhere file layout) will launch a background read. These are counted in “pw.async_read“; “pw.over_limit“ is the block counter of the writes waiting on disk reads.”
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So the pw.over_limit counter is still recording an 5 minute average of 14 with 7-10 peaks in the 50-100 range at certain times of the day. If I look at the clients talking to the Netapp those times its mostly Oracle RAC servers with storage for data and voting disks on NFS.
This leads me to the question: What if any are the other possible sources for unaligned IO on Netapp? All references I find are vmware vmdk – but are there others like Oracle which may be doing block IO over NFS?
Many thanks
-- Fletcher Cocquyt Principal Engineer Information Resources and Technology (IRT) Stanford University School of Medicine
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