Monday, October 31, 2011

Finding Misaligned VMDK's

Received a useful email from a colleague today:

"Starting with ONTAP 7.3.5, ONTAP has a nice feature to help identify misaligned VMDKs...this feature will be helpful as we roll out 7.3.6P1. At the end of the "nfsstat -d" command, you will see a section named "Files Causing Misaligned IO's".  This will have a list of files that are doing misaligned I/O, along with a counter that indicates the frequency at which this IO is happening.  If you want to start the counters over again, you can use "nfsstat -z" to zero the counters.

Below is a snippet of this output from a filer (the VMDKs with high counter values), which has been having some performance problems lately.  We have 18 VMs here doing a significant amount of misaligned IO since the upgrade was done on Saturday night (there are 48 VMs in total doing misaligned IO).  We need to get these VMDKs aligned in order to help improve the write performance on this system."

Files Causing Misaligned IO's [Counter=48113], Filename=infra_pv_vms_v03_snap14/ds1/c111asz/c111asz_1-flat.vmdk [Counter=18865]

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