Storage Savings from space-optimized snapshots

With the large number of virtual machines housed per physical server, the number of boot images used on a single server is also significant. A single bare-metal Linux boot image needs around 3 GB of space at a minimum. Installing software stacks and application binaries on top of that requires additional space typically resulting in using around 6 GB of space for each virtual machine that houses a database application.

When a user provisions a new virtual machine, the boot image can be a full copy or a space-optimized snapshot. Using a full copy results in highly inefficient use of storage. Not only is storage consumed to house identical boot images, storage is also consumed in making the boot images highly available (mirror across enclosures) as well in their backup.This large amount of highly available, high performance storage is very expensive, and likely to eliminate the cost advantages that server virtualization would otherwise provide. To add to it, backup and recovery of such capacity is also an expensive task.

In order to address the above issue, Veritas recommends the use of space-optimized snapshots of the gold image as boot images of the various VM guests. Space-optimized snapshots do not make a full copy of the data in the gold image, rather they work on the copy-on-write principle where only the changed blocks are stored locally. This set of changed blocks is called a Cache Object and it is stored in a repository for all such space-optimized snapshots, called the Cache Object Store, which is backed by physical storage. The Cache Object offers a significant storage space reduction, typically occupying a 5-20% storage footprint, relative to the parent volume (the gold image volume in this case). The same Cache Object Store can be used to store changed blocks for multiple snapshot volumes.

Each Snapshot held in the Cache Object Store contains only changes made to the gold image to support that installation's boot environment. Hence, to achieve the best possible storage reduction, install software on data disks rather than root file systems and limit as many changes as possible to the gold image operating files (i.e., system, hosts, passwd, etc.).