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.

One more thing is that vixdisklib options works fine when the source vm is powered of but it will not work when is powered on.

For ex:-

./sanv_rep -dump -host 172.168.1.114 -user root -password Sanovi123$ "[datastore1] ACTVM/ACTVM.vmdk"

[NFC ERROR] NfcFssrvrProcessErrorMsg: received diskLib error 16392 from server: NfcFssrvrOpen: Failed to open '[datastore1] ACTVM/ACTVM.vmdk': Failed to lock the file
Error: [vixDiskLibSample.cpp:606]  40080000000f The file is already in use

with thanks,

Santosh