Best Practices: Viewing virtualization from every angle


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Boot from SAN (with reminders)
The concept of boot from SAN used to be fairly simple. Rather than using local disks for the boot image, make a Fibre Channel or iSCSI LUN the boot disk. If you run a tight and disciplined operation, there's value in booting your systems from SAN. Indeed, a key benefit SANs bring to systems virtualization is the ability to create this huge high-performance repository for storing all of the virtual server boot images. Fair enough. But I have two comments.

The first is that when you aggregate multiple boot images onto a single large LUN, the performance of that LUN comes under scrutiny by the virtual servers running on it. Care should be taken to maintain a healthy boot image number to LUN ratio. Instead of creating a single large LUN, you may be better off creating smaller LUNs and spreading the virtual server images onto them, thus maintaining a lower and well-balanced ratio.

The second reminder is that you still need to decide if the hypervisor itself boots from SAN or not. The answer to this depends on whether you follow boot from SAN as a standard practice for the other nonvirtualized hosts in your environment. If you're moving to a fully virtualized environment, it may make sense to go the route of booting the hypervisor from SAN as well.

Treat your virtual servers as "independent apps"

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servers should be treated as independent entities when storage resources are allocated. You wouldn't want your Oracle database to share the same file system as a Web server on a single host if both are very IO intensive--in the same manner, you should take care that a virtual server running Oracle and a Web server don't share storage resources or, if they do, that there are clear separators between them.

This means keeping your data and boot images separate. It's a bad idea to have everything for all virtual servers on a single resource, whether it's a LUN or a file system.

It also isn't a wise idea to create and share large metaLUNs (each vendor chooses to call them different things) among all of the virtual servers. I see a lot of VMware environments where LUNs in the range of 600GB and higher are presented to a single hypervisor. This doesn't make sense to me. Virtual servers or hypervisors should be treated in a traditional sense from a storage provisioning perspective and all of the standard storage practices should apply to them.

This was first published in December 2007

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