![]() Storage vMotion also uses CBT in vSphere 4.0. If a defrag is run during a backup operation, the number of blocks that changes will increase, which means more data will have to be backed up, meaning a longer backup time. This feature tracks changes to a VM’s disk blocks during a backup operation. The CBT feature is used heavily by backup products, including VMware Data Recovery (VDR). Change Block Tracking (VMware Data Recovery).Any VMs running off of a snapshot which ran a defrag would cause the snapshot to inflate considerably, depending on how many blocks were moved during the defrag operation. This is a similar use case to Linked Clones. If your VM was being replicated, and you defragemented the VM on the protected site, it could well cause a lot of data to be sent over the WAN to the replicated site. Replicated VMs (Site Recovery Manager, vSphere Replicator).In the case of a VM running off of a linked clone, the defragmenter bloats up the linked clone redo logs. Linked Clone VMs (vCloud Director, View). ![]() If you defragment a Thin Provisioned VM, as file blocks are moved around, the TP VMDK bloats up, consuming much more disk space. You can then make up you own mind about whether it is a good idea or not. The easiest way to explain the concerns is to give you some scenarios of what might happen to a VM which is defraged, and what impact it has on the various vSphere technologies. However, there are other concerns that you need to keep in mind. Therefore the overall I/O to the underlying LUN is going to be random so defragmenting individual Guest OS’es is not really going to help performance. Typically you are going to have multiple VMs running together on a VMFS or NFS volume. This is very different to running a defrag on a physical host with a local disk. What about defragmentation of a Guest OS in a Virtual Machine? Here’s a view of the Disk Fragementer that is part of the System Tools with Windows 7: ![]() This means that sequential I/O operations should be faster after a defrag. Defragmentation moves blocks around the disk to bring together blocks belonging to the same file in an effort to make the file contiguous on disk. Well, historically, if you ran a defragmentation* operation against an OS disk (typically Windows), you would expect to see a performance improvement. What is it that defragmentation is supposed to give you? In fact, we’ve been having these conversations for a long time now. This has come up time and time again, and I am going to share with you some conversations that have been occurring within VMware on this topic.
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