How to access VM disk

I have created a snapshot and its name is in the form of RHEL6x64bit-000009.vmdk. I am able to access the disk when it is powered off. But i am not able to access the snapshot file when the VM is powered on.

What i understand that we should create a redo log for a snapshot. I am trying to use the DoRedo() function to understand in the sample but still it is throwing an error stating that disk is in use when i call this function :

vixError = VixDiskLib_CreateChild(parentDisk.Handle(),

                                     appGlobals.diskPath,

                                     VIXDISKLIB_DISK_MONOLITHIC_SPARSE,

                                     NULL, NULL);

and also QueryDiskChangedArea API its work fine when VM is powered off but not when VM is powered on. Can  some help me to solve this issue. BTW i am using Linux64 bit ans also can someone tell me the command line parameters i have to pass for vixdisklibsample to invoke DoRedo function.

with thanks,

Santosh