System News
The Storage Architect Reviews Sun's Open Storage
Critic Finds Much to Praise in Amber Road
June 1, 2009,
Volume 136, Issue 1

Scalable storage systems built on commodity hardware and open software components: Amber Road
 

The Sun Storage 7000 Unified Storage System (Amber Road) came under the scrutiny of Chris Evans, writing a two-part review in The Storage Architect. Specifically, he writes about his time with the 7210 array, Sun's mid-range offering, which found favor with him.

When it comes to Sun's Unified Storage System (USS), Evans likes the mix: "highly scalable storage solutions built on commodity hardware and open software components." What is more, Evans applauds Sun's commitment to provide all current and future software features as part of the cost of the standard hardware, which means users can upgrade the USS and take advantage of new software features as they become available.

The model 7210 Evans evaluated came equipped with the standard 48x 1TB drives and two 18GB SSDs. While Evans notes that a server as a storage array is not a brand new idea, what he finds praiseworthy is the use of solid state to drive performance out of the commodity SATA drives in the array.

In Part II of his review Evans examines the disk components of the hardware, along with the flash (SSD) drives and ZFS that make up Sun's commodity storage device.

Evans makes note of how ZFS improves USS performance: holding disk reads in cache (Adaptive Replacement Cache or ARC) and journalling or logging disk writes into the ZFS Intent Log (ZFS ZIL). In the event of a system crash, he writes, the ZIL provides resilience as well as the opportunity for increased filesystem write performance. The USS model implements ARC for cached reads (which is stored in RAM), L2ARC, a level 2 ARC which extends ARC and is stored on read-biased SSDs and the ZIL, which is stored on write-biased SSDs, Evans explains.

As Evans sees it, "L2ARC allows cache reads to be improved by creating an intermediate tier of read cache between disk and main memory. ZIL improves writes by logging them to SSD and periodically flushing them to physical disk. In the event of a system crash, integrity is still maintained as the ZIL is non-volatile."

The author did find it somewhat inconvenient that the current implementation of USS allows only a single disk pool, meaning that all data must be protected with the same RAID level, though he anticipates future releases will enable creation of the separate pools that ZFS allows.

Using tiered LUN models does solve problems encountered in the typical general storage array, where some LUNs are very active while others are idle. Still, determining the amount of storage required in each tier is a problem with the tiered arrangement.

Evans does praise the fact that "the USS provides one option to the Long Tail model [the profile of non-tiered LUNs]. By accepting all writes into SSD and destaging later to SATA, it ensures that high performance non-volatile storage is available at the time of the write and for multiple successive reads. Fronting disk access with SSD ensures that high performance is dynamically provided to LUNs as it is needed," he writes. For all of that, Evans says he did not find it possible to compromise the SSD write cache by flooding a USS array writes.

In summary, Evans makes the following observations on the value proposition delivered by the Unified Storage System:

  • All data is stored on cheap, high capacity SATA drives, reducing the overall cost of the solution.

  • I/O performance demands are managed by a small incremental cost in SSD.

  • Variations in I/O workload performance is dynamically managed, removing the need to implement multiple storage tiers, significantly reducing management overhead.

  • Array expansion is simplified – there’s no need to spend time planning how additional storage should be assigned to an array by tier.

More Information

Sun Storage 7000 Unified Storage Systems

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Other articles in the OpenStorage section of Volume 136, Issue 1:

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