Virtualization: Tales from the trenches


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One thing Madsen would like to improve is the way SVC handles a node outage. Each SVC node in an I/O group has an affinity to some of the LUNs assigned to the I/O group. If a node fails, its LUNs are moved to the other node in the I/O group; however, when this happens, they're put in "write-through mode" (no write caching). This hampers write performance and can cause volumes to be taken offline by hosts detecting bad performance, which results in application outages. Madsen would rather see the volumes moved to another I/O group (he can do this manually, but it's time consuming and is usually too late to avoid application outages).

The worst incident Madsen encountered was an outage that lasted more than eight hours for some SVC storage and 36 hours for the rest, which occurred while Harley-Davidson was still migrating data to SVC. Some of the SVC storage was in "image" or pass-through mode when the SVC went down, which meant it wasn't fully virtualized yet. Madsen was therefore able to bring that storage up outside the SVC within eight hours. The rest of the SVC storage had already been completely virtualized with no way to back out, so Madsen had to wait until IBM fixed the SVC.

Despite a few bumps in the road to virtualization, Madsen likes SVC and thinks IBM has a real winner. "SVC will be here for a long time," he says, adding that it allows him to "better manage where storage sits" within his environment. In pre-SVC days, moving data

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between Clariion and Symmetrix storage required a long application outage, so it was never done. With SVC in place, Madsen can make the moves without any application outages.

FirstMerit Corp., Akron, OH, was one of IBM's first 100 SVC customers. Dave Samic, senior network analyst at FirstMerit, says the bank installed SVC in the spring of 2004 and currently has 16TB operating through one SVC cluster with a single I/O group.

Samic had a couple of IBM TotalStorage FAStT storage subsystems that were causing performance delays when the bank booted multiple virtual servers. In addition, FirstMerit had three disk LUNs with 146GB disks supporting a usable 280GB of space, but it could only use 100GB before the LUN began performing poorly.

IBM suggested implementing SVC as an alternative to buying faster storage. With SVC and its additional cache, FirstMerit can use much more disk space and boot 30 to 40 virtual servers without delay.

FirstMerit virtualizes its blade servers with VMware. Samic says the improved performance and manageability realized with SVC allows his firm to support 22 virtual servers per physical blade--considerably more than the 12 to 15 that VMware recommends. Samic also reports that FirstMerit is getting 60% to 75% blade utilization.

Both Samic and Harley-Davidson's Madsen say SVC's GUI is a bit slow. "[The GUI] only displays 10 disks per screen," says Samic, "and with 200 disks, that takes 20 screens."

Samic noted some internal resistance when he tried to include SATA disk under SVC. At the time, SVC licensing was based on the amount of storage under SVC control, and paying premium for cheaper SATA storage made little economic sense. However, Samic demonstrated that the bank could actually save money by taking better advantage of the SATA storage once it was under SVC. Since then, IBM has made some changes to SVC licensing, which would have made Samic's justification easier.

This was first published in August 2006

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