Not all snapshots are created equal. We have many customers who contact us about VMware Backup and get this question very often, “Why is your way of backing up VMware is more efficient than others?”. This triggered us to write a blog post detailing how Vembu Backup & Disaster Recovery product performs VMware Backup . For starters, most companies that offer VMware Backup uses APIs provided by VMware for performing snapshots. There are two ways snapshots can be created COW (Copy on Write) and ROW (Redirect on Write). Some of the leading vendors in the VMware backup domain perform COW based snapshots for backing up the data. Now the problem with this approach is that it works good only for smaller VMs. Problems starts to sprout typically when the VM size is larger – more than 5 TB or if some applications are running inside the VM like Exchange or SQL. Many customers have faced hard time in backing up Applications inside VMs. Most of the problems are related to backups not being ‘Application Aware’. When you need to perform VM snapshot, you should also make sure that all VSS writers are invoked and applications should be requested to commit their transactions so that a ‘Application Aware’ snapshot can be performed. If not you will end with corrupted applications or database. Now both COW and ROW snapshots helps you perform application aware backups, but COW snapshots have to be kept open till the applications complete committing their transactions. During COW snapshots, a dummy snapshot is created and then data from snapshot as and when applications commit transactions is continuously written to this dummy snapshot. The more time this COW snapshot remains open, the more IO and system resources it’s going to use. So imagine an Exchange Server of size greater than 5 TB running thousands of Mailboxes. In this situation, the COW based backups is either going to be terribly slow or fail big time.
Vembu BDR’s ROW Snapshot
With Vembu BDR we utilize ROW (Redirect-on-Write) snapshots. According to Net-app, “By design a RoW snapshot is optimized for maximum write performance so that any changes/updates are redirected to new blocks. Instead of writing one copy of the original data to a snapshot reserved space plus a copy of the changed data that is required with CoW, RoW writes only the changed data to new blocks”. Hence we make sure our backups are highly efficient even in larger environments. Hence our VMware backups complete faster than other products in the market and is more reliable for restores. Since the backups are application aware, we allow granular recovery of Applications like Mailbox Level Recovery, & Mail Level Recovery for Exchange and even Table Level Recovery for SQL.
Other Vendors Backup Methodologies using COW Snapshot for VMware Backup
Method 1 – Application aware snapshot using VMware’s Guest tools
In this method, the vendor will require you to download VMware’s Guest tools on the host machine where the backup needs to be performed. This will take care of everything (performing VSS snapshot on Guest machine and transporting it to Host machine etc…) VMware Guest tool will perform “COPY-ONLY” snapshot. However backup application tasks like log purging of Exchange, SQL cannot be performed by using this method. Basically this method will not render any ‘Application Aware’ backups
Method 2: Application aware snapshot using vendor’s own Agent.
In this method the Vendor will use VSS (Volume Shadow Copy) to freeze the applications inside VMs and use VMware snapshot methodologies to create the snapshot in the host machine. Vendor’s backup Agent will also inject some agents on the particular guest VM to perform the application aware snapshots at time of backup and revoke these agents once when the backup is complete. These agents are as follows VSS requestor: This agent is responsible for creating the snapshot in the guest machine VSS Writer: This agent will notify the backup agent when receive the ‘freeze call’ for snapshots. The freeze call means that all applications in the guest machine have to freeze their IOs and are ready to take a snapshot. When vendor’s own VSS writer receives this freeze call, it will communicate with the backup agent though RPC and initiate VMware snapshot. VMware snapshots will then perform the snapshot . The advantage this way of snapshot is that the vendor is able to perform the application consistent snapshot as well as log purging at the end of the backup using COW (Copy on Write snapshot). But the big problem as we mentioned earlier is that if the VMware snapshot takes more than 60 seconds (in some cases 20 seconds if particular VM has high transactional application like Exchange), VSS snapshot will be aborted. Hence your backups will fail. If you are interested in trying Vembu BDR’s VMware Backup free for 30 days, you can download the same here – https://www.bdrsuite.com/try/
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