Heads Up! VOMA – ERROR: Trying to do IO beyond device Size

This is a short note on trying to use VOMA, the vSphere On-disk Metadata Analyzer, on a dump taken from a VMFS-5 volume which was upgraded from VMFS-3. This is not an issue if VOMA is run directly on the volume; it is only an issue if a dump is taken from the volume and then you try to run VOMA on the dump. It may error during phase 1- ‘Checking VMFS header and resource files‘ with an error ‘ERROR: Trying to do IO beyond device Size‘.

When a VMFS-3 is upgraded to VMFS-5, a new system file, pb2.sf, is created for additional pointer blocks. One of the major differences between VMFS-3 & VMFS-5 is the introduction of double-indirect pointer blocks which means that we can created 2TB VMDKs with a unified 1MB file block. With a very full VMFS-3, this new system file could be placed quite a ways down the volume. With offline dumps for VOMA, our support folks will typically request the first 1.5GB of a volume to do the analysis. If the pb2.sf file is outside of this first 1.5GB of disk, then you will encounter this error. In this case, VOMA should be run against the actual volume, ensuring of course that it is completely quiesced. I recently did an article here which details what might be heartbeating to a datastore, even when there are no running VMs.

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