Virtualization: Tales from the trenches


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Why virtualize?
Many users virtualize their storage to gain better control and increase their capacity utilization levels. Whether faced with data center consolidation or just normal expansion, storage virtualization technologies promise to lower costs by managing more terabytes with fewer people.

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Virtualization with EMC Invista
While it's not yet in its production storage environment, a large legal technology provider in the Midwest is testing EMC Corp.'s Invista, a switch-based, out-of-band virtualization product. Steven McCaa, storage manager at the firm, says they've been testing the product since December 2005, with a test environment ranging from 1TB to 10TB of storage behind the Invista product. All told, the company currently has 2 petabytes (PB) of online storage, with 70% on Fibre Channel (FC) disk and the remainder on SATA.

The key motivations for the Invista pilot were business process improvement and data lifecycle management. With 1.4PB of FC disk, any substantive amount of data that can be moved to SATA can quickly pay for the Invista. The company installed pre-general availability Invista code and ran into some known bugs, but it's been a relatively straightforward install since then.

"You still have to be smart about what you are doing [with virtualization]," says McCaa, who only intends to virtualize the firm's Clariion-class storage. The company's Symmetrix storage is a highly tuned environment where the right LUNs are mapped to the right physical spindles, and McCaa doesn't want to upset that arrangement.

Virtualization products in general "don't give a good global map of virtualized storage, so that when something is broken you can't see end to end what is impacted," says McCaa. "If an ISL [inter-switch link] goes down and you see performance problems, you can tie the two together." Virtualization adds a layer of abstraction that needs to be understood to determine why performance went bad.

McCaa says the company hasn't seen a performance impact with Invista. He notes that there are four modes to data migration on Invista: trickle, very slow, slow and ASAP. "When you use ASAP," says McCaa, "you can tell right away" if performance degrades. In contrast, he indicated that migrating a 1TB volume at slow speed didn't impact performance, but did take a long time.

Typically, companies tend to run with only a single vendor's storage behind the virtualization engine, although they might mix different classes of disk such as SATA and Fibre Channel (FC). Often, data migration features are used to migrate current storage to a virtualized environment while applications continue to access the data. Some companies are also using storage virtualization to create tiered storage because it makes it easier to move data among storage tiers without disrupting applications using the data.

Using IBM's SVC
Andrew Madsen, a senior storage administrator at Harley-Davidson Inc., says his company runs three IBM SVC clusters. One cluster virtualizes more than 50TB of IBM DS83xx storage with two I/O groups. There are two nodes per I/O group in the SVC providing hardware redundancy and data availability. All of the I/O groups within an SVC cluster are managed as a single entity sharing the I/O workload.

Harley-Davidson's second SVC cluster has 11TB of storage using a single I/O group, while the third is still being tested. The company implemented the SVCs as part of a data center consolidation effort.

Harley-Davidson installed its first SVC in November 2005. At the time, it was an EMC shop and initially used SVC to migrate from its EMC equipment to IBM storage while applications continued to access data. The company had to move 42TB of data, with LUNs ranging from 1GB to 2TB in size. Without virtualization, a significant operational outage would have been required.

Madsen is currently using flash copy (point-in-time copy) as part of his backup scheme, and data migration to move data from FC to SATA disk. He has plans to use IBM's Metro Mirror, which provides synchronous remote replication over short distances, to provide for disaster recovery.

One misconception about in-band storage virtualization is that it can degrade performance. The reality is more complex. While in-band products add some latency to every I/O operation, they also add cache to compensate. Often, the additional overhead is so negligible that it's invisible to normal applications; the additional caching can also improve performance considerably, depending on the back-end storage. "Once it [SVC] was working correctly," says Harley-Davidson's Madsen, "it was equal to or better than direct-attached [storage]."

But there are some pitfalls to virtualized storage. Early on, Madsen had some configuration issues with SVC and EMC storage as his company's Clariion-to-SVC configuration wasn't done correctly and had to be modified. Also, the Symmetrix-to-SVC configuration was generating many I/O error retries, which caused hosts to take EMC volumes offline. At that time, SVC didn't see any problem with the volumes. Ultimately, it was determined that a fix to the host driver software resolved the problem.

This was first published in August 2006

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