Use VMKFSTOOLS instead of cp

Posted by virtualization.info Staff   |   Sunday, August 28th, 2005   |  

Quoting from VMTS:

The VMFS-2 metadata manager serializes operations performed on different hosts that require metadata updates. This is a standard file system practice that protects a shared resource (the metadata) from being modified simultaneously by multiple hosts. Here, normal data I/O is not affected—only operations that originate from different hosts and that require a metadata update are serialized. Typically, these operations occur infrequently and performance impact is not significant.
cp , however, can change this dynamic grow (in blocks of 10Kb) as data is appended to them. Growing a copied file is an operation that requires a lock on the VMFS-2 metadata.

When you have a cp in action you may experience a degradation in I/O performance due to increased VMFS-2 metadata contention. Other symptoms may include the inability to change power states or modify a virtual machine.

The extent of any performance degradation depends on several factors, including the number of ESX Server hosts with virtual machines that use the VMFS-2 volume, and the intensity and pattern of I/O activity in the virtual machines with REDO log files and the number of concurrent cp operation.

The only way to do a file copy inside a VMFS volume is to use the undocumented vmkfstools option:

vmkfstools -e /vmfs/vmfsname/target.vmdk -d vmfs /vmfs/vmfsname/source.vmdk



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