Veeam Backup and Replciation for CBT and Thin Provisioning

Changed Block Tracking and vSphere backup

The vStorage APIs for Data Protection are most beneficial to backup and replication applications and vendors seem to be most excited about the new Changed Block Tracking feature that is included in it. This feature allows third-party applications to query the VMkernel to find out which disk blocks have changed in a virtual machines disk file since the last backup operation. Without this feature, applications would have to figure this out on their own which can be time-consuming. Now with CBT they can instantly find this out so they know exactly which disk blocks need to be backed up. This enables much faster incremental backups and also allows for near continuous data protection (CDP) when replicating virtual disk files to other locations. In addition, point-in-time restore operations are much quicker as CBT can tell exactly which disk blocks need to be restored to the virtual machine.

Changed Block Tracking is supported on any storage device and datastore in vSphere except for physical mode Raw Device Mappings, this includes iSCSI, VMFS, NFS and local disks. It also works with both thin and thick disk types. CBT is a new feature to vSphere, so it does require that the virtual machine hardware be version 7, which is the default in vSphere. By default, CBT is disabled as there is a very slight performance penalty that occurs when using it. It can be enabled on select VMs by adding parameters (ctkEnabled=true and scsi#:#.ctkEnabled=true) to the configuration file of the virtual machine, backups applications can also enable it using the SDKs. Once enabled, a VM must go through something called a stun/unstun cycle for it to take effect; this cycle happens during certain VM operations including power on/off, suspend/resume and create/delete snapshot. During this cycle, a VM's disk is re-opened, which allows a change tracking filter to be inserted into the storage stack for that VM.

The Changed Block Tracking feature stores information about changed blocks in a special "-ctk.vmdk" file that is created in each VM's home directory. This file is fixed length and does not grow and the size will vary based on the size of a virtual disk (approximately .5 MB per 10 GB of virtual disk size). Inside this file the state of each block is stored for tracking purposes using sequence numbers that can tell applications if a block has changed or not. One of these files will exist for each virtual disk that CBT is enabled on.

The vStorage APIs for Data Protection and the CBT feature make backups quicker and easier in vSphere and are a big improvement over VCB. VMware has provided third-party vendors with a much improved backup interface in vSphere, now it's up to them to adapt their products to take advantage of them.

Thin provisioning and backups

Thin provisioned disks are virtual disks that start small and grow as data is written to them. Unlike thick disks where are all space is allocated at the time of disk creation, when a thin disk is created its initial size is 1 MB, (or up to 8 MB depending on the default block size) and it then grows up to the maximum size that was defined when it was created as data is written to it by the guest OS. The benefit of thin provisioned disks is that they allow for the over-allocation of storage on a VMFS volume to make use of the often wasted unused space inside of a VM's disk. Thin provisioned disks are not new to vSphere and also existed in VI3, however, there were numerous changes to make them more usable in vSphere.

Why are thin disks important to backups? Many backup applications for virtualization do not operate inside the guest operating system and operate outside of it at the virtualization layer. Instead of backing up individual files inside the guest OS, they back up the single large virtual disk files (vmdk) that contain the encapsulated VM. Because of this, backup applications must search for empty disk blocks contained inside the virtual disk file so they do not back them up. This process of identifying empty blocks takes additional time and resources to complete. With thick disks all space is allocated at once, so a 40 GB virtual disk will actually take up 40 GB of disk space on a datastore regardless of how much space is used by the guest OS running on it. So, if only 10 GB of disk space is in actual use by the guest OS you will want to avoid backing up the extra 30 GB of empty space inside the virtual disk file.

Thin disks only take up as much space on a datastore as what is actually used by the guest OS, so if only 10 GB of a 40 GB virtual disk is in use the virtual disk file will only be 10 GB in size. Because of this, backup applications no longer have to worry about searching for those empty disk blocks because there are none in a thin disk. Not having to do this results in faster and more efficient backups which is just one of the advantages of using thin disks.

Veeam Backup provides the best solution for CBT and Thin provisioning in the Virtualization environment.

Thanks,

Veera

Software Engineer,

Climb Inc,

http://www.climb.co.jp

Veeam Backup and Replication

http://www.veeam.com