Virtualization and Storage: Overview, Vendor Solutions: Page 2

Posted December 7, 2010

Chris Evans

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Virtualizaton and Backup

The move to virtualized environments meant a new approach to backup. Although possible, it is impractical to backup each virtual guest individually. Instead, functionality within the virtual server enables backup images of each virtual guest to be taken and accessed by a separate backup server. To improve the performance of this feature, the storage array can perform the snapshot process, offloading CPU and I/O resources from the virtual server.


Performance is clearly an issue in backup, however, performance in general is a key storage array feature for virtualization.

Storage arrays have been developed to process large volumes of I/O, which can be either sequential or random. In virtual environments the I/O is typically random and this doesn’t work well with DAS storage, which would require more expensive, high-speed drives.

Storage arrays can benefit from large numbers of disks (as it is a shared environment), dedicated cache and multiple I/O connectivity, all of which both improves performance and delivers a more consistent I/O response time.

Stateless Boot

As more workload is virtualized, the hypervisor itself becomes a significant part of the support effort, because it is the platform that is tied to the hardware itself.

Boot from SAN enables the hypervisor to be disconnected from the hardware and allows a single hypervisor instance to be booted on any server; it also allows the hypervisor to be replaced with another instance that could be (for example) an upgraded version.

By removing the boot device from the server and placing it on the SAN, the server holds no state information and so becomes a commodity. This is most easily demonstrated with the use of blade servers, where multiple physical servers exist in a single chassis; they can be added or removed from the blade infrastructure at any time. Ultimately, blade flexibility is served best with shared SAN storage.

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Both hypervisor and boot disk stored on SAN; hypervisor can be booted from any physical server.

Why VAAI Is So Important

Although storage arrays already offer many important features to virtual environments, there are additional requirements not met by today’s hardware. That is why for VMware vSphere, VAAI (vStorage API for Array Integration) was developed.

VAAI defines a set of API calls that are implemented within the storage array through amendments to the SCSI protocol. Most notably of these are the following:

Block Zero – implemented as Write Same in SCSI, this pushes the task of zeroing out large blocks of data down to the array. In fact, Write Same could be used to write any values over a large range of data, however it’s most useful to vSphere to write zeroed out data when creating new virtual disks (VMDKs).

Full Copy – implemented within the array as SCSI EXTENDED COPY, this feature allows bulk movement of data both within and between storage arrays, taking the load off the vSphere hypervisor when performing storage vMotion or guest cloning functions.

Hardware Assisted Locking (HAL) – this moves the SCSI hardware lock from the LUN to the block level, improving performance on certain vSphere operations that require locking for data integrity. However, HAL will potentially resolve the issue of LUN replication, allowing I/O on both sides of a replicated LUN pair.

Virtualization and Storage Vendor Solutions

Storage vendors are starting to offer new features and products that specifically meet the needs of virtual environments.

Example 1: EMC VPLEX

EMC’s VPLEX product virtualizes the storage LUN and permits I/O to either side of a replicated LUN pair. This enables virtual guests to be moved between storage arrays (typically in geographically distant locations) with no outage and without waiting for data to be replicated.

Example 2: Compellent Live Volume

Compellent’s newly announced Live Volume feature enables a single logical LUN to be spread across multiple storage arrays. The LUN can be associated with one array and dynamically moved to another in order to meet workload balancing or DR requirements.

Virtualization and Storage: Summary

It’s clear that as virtualization continues to have a greater importance in the enterprise, storage will form a critical part in delivering that infrastructure. The features of the storage array will continue to evolve and deliver better performance, availability and resilience than could be achieved using directly attached storage (DAS) alone. Storage and virtualization are and will continue to remain, closely linked.

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