VMware vSphere VM Snapshot Size and Age issues
Virtual machine snapshots are one of the most useful features of VMware vSphere as they preserve the state of a virtual machine’s virtual disk and, optionally, virtual memory before a critical event such as an application upgrade or configuration change. VM Snapshots are also taken by most virtualization backup applications at the start of the backup and then removed at the end of the backup. In fact, besides vMotion, snapshots were one of the original top reasons that organizations adopted virtualization technologies to begin with. As snapshots are a point in time freeze, all new data needs to be written in additional files that can grow into a large VM snapshot size or become old if forgotten.
What many vSphere administrators don’t know is that the quantity and age of snapshots on a virtual machine are directly related to their performance impact on a virtual machine. This was especially true with older vSphere versions (prior to ESXi 4.1) because they locked the entire LUN where the virtual machine was stored using a SCSI-2 reservation while taking a snapshot or when booting virtual machines. With ESXi 5 and above, you have the option to configure VAAI (vSphere API for Array Integration) which only locks the individual virtual machine, and does so with hardware assisted locking, as long as your storage array supports VAAI. With VAAI in place, there is much less performance impact when snapshots are in use but that impact isn’t completely eliminated.
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