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Radiology firm uses Compellent Data Progression to cut disk costs

Compellent Data Progression automated tiering software expands data storage through cheaper tier 3 SATA disks; offers tiering and sub-tiering level options.

The most remarkable benefit Princeton Radiology Associates P.A. has noted after using Compellent Data Progression (Compellent has been acquired by Dell Inc.) automated tiering software for close to five years is that it hasn't seen a need to add any tier 1 storage.

"It's just pretty amazing. Every time we expand, we're adding tier 3 disk," said Alan Howard, director of information technology at the imaging company, which has five locations in New Jersey.

More than 90% of Princeton Radiology's 37.5 TB of data resides on its less expensive, high-capacity tier 3 drives, which are a mix of 500 GB Fibre Channel ATA (FATA) disks and 750 GB, 1 TB and 2 TB SATA disks, all at RAID 5-9 (eight data drives and one parity drive).

The company's primary application profile includes a radiology information system (RIS) and two picture archiving and communication systems (PACS), each running off Microsoft Corp. SQL Server databases.

Tier 1 consists of a 16-disk enclosure filled with 15,000 rpm Fibre Channel (FC) disks, all at 300 GB, RAID 10, with one hot spare. The partially filled tier 2 is 10,000 rpm FC, 300 GB for all 10 disks, some at RAID 10 and some at RAID 5-9.

Compellent Data Progression automated tiered storage software also gives Princeton Radiology the option to sub-tier based on RAID level and a separately licensed option called Fast Track, which directs the system to place the most frequently accessed data on the outer edge of a drive.

"Mainly, it provides slightly better I/O performance on the top tier," Howard said about Fast Track. "Compellent tries to automate a lot of that, and for our purposes, that's exactly what we want because we don't want to have to learn about all those details."

Howard is also impressed with Data Progression's ability to distinguish between regular user data and snapshots, which Compellent refers to as replays. He wanted to take frequent snapshots, with varying expiration points, so he could recover data without having to go to backups. The Compellent system helps keep costs in check by storing snapshots on a lower, less expensive tier or sub-tier of storage than the user data.

"You kind of have this epiphany where you say, 'Wow, if that really works the way they advertise, then that means we could probably buy a few terabytes of high-performance disk and not really ever need any more,'" Howard said, recalling his initial discovery of Compellent's technology. "And that's actually, for the most part, turned out to be true."

Managing the levels of tiering and sub-tiering became a lot easier once Compellent introduced the concept of storage profiles, in which the user defines a tiering structure object, or template, to which it can add or remove volumes. Every volume within a template adheres to the characteristics the user has set for it.

Profiles make it simpler for Howard to change default settings to better suit his needs. He noted that the recommended tier structure for writable data on tiers 1 and 2 is RAID 10; for read-only replays on tiers 1, 2 and 3, it's RAID 5-9. But Howard found some volumes don't ever need tier 1, and some need it only for a limited time frame.

For instance, when Princeton Radiology's PACS processes new patient studies, a designated storage volume gradually fills with image data on tier 1. But once the volume is full, only one or two users typically need to access any of the images on an occasional basis.

"Given full rein to use any tier it wants, that data might move up to tier 1, but it never really needs to be on tier 1 once it's a full volume," Howard said. "We'll have records that are very active for a few days, and then they just go dormant for months, if not years, if not forever."

So, whenever the radiology firm adds a new empty volume to its PACS, it instructs the system to shift the just-filled volume from the recommended storage profile of all tiers to only tier 3, RAID 5-9.

Doing more fine-grained tweaking of algorithms, however, requires extra care. Howard said he once attempted to improve the system's performance by adjusting the defaults to keep data a bit longer in tier 1 and not pull data out of tier 3 too quickly. But his changes had the unintended effect of turning off tier 2 and tier 3, which caused the system to run out of tier 1 space. Customer support had to come to the rescue to fix the problem.

Compellent has since removed some of the fine-tuning capabilities, including Fast Track, from the end-user interface, but Howard said he's still able to accomplish more granular tuning by working with customer support. He said he asked customer support to disable RAID 10 on tier 3 to save disk space, for instance.

Built-in monitoring software and an optional Enterprise Manager can help users to check if the automated tiered storage software is allocating space and performing as expected. That's how Princeton Radiology spotted a problem in the summer of 2008 with an earlier version of Data Progression that Howard thinks was related to combining the FATA and SATA disks into one tier. Monitoring showed that a volume pegged to use mostly tier 3 started to consume a considerable amount of tier 1 storage.

Although Howard wasn't able to pinpoint the cause of the problem, he speculated it may have been related to a software upgrade or disk expansion. He said Compellent support seemed to agree with him and fixed the issue, and he hasn't encountered the problem again.

Bob Fine, director of product marketing at Compellent, said he was unfamiliar with the Princeton Radiology problem, but it's possible the system had a bug. "The fact that they've been running since 2008 without any significant need to have a tweak is a huge testament for the software and how well it works on an automated basis," he added.

Moving forward, Princeton Radiology could follow the general industry trend for a three-tier setup, with solid-state drives (SSDs) as tier 1; 15,000 rpm and/or 10,000 rpm FC or SAS as tier 2; and low-cost, high-density 7,200 rpm SATA as tier 3. Princeton Radiology's Howard said he might consider adding SSDs if the price drops and if the company's growing virtual server environment runs into performance bottlenecks.

In the meantime, the Compellent system provides ample ROI to a company that doesn't have the resources to assign a full-time employee or group of IT staffers to manage storage, according to Howard.

"We're not storage experts," he said. "It all comes down to price and the fact that we don't have to have specialized software that understands storage hierarchy. We can put about 90% of our data on relatively cheap disk without really having to have any management overhead."

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