Dell EqualLogic PS-series SANs have long led the iSCSI pack with excellent performance, good management tools, 10G Ethernet support and the ability to scale capacity and I/O simultaneously by stacking together appliances. The latest addition to the line offers more reasons to take a close look, with a high performance solid-state drive (SSD) array, dynamic storage tiering and tighter integration with VMware.

The new Dell PS6010XVS model differs from its 16-drive, 4U PS-series brethren only on the inside, where it houses eight 100GB Samsung SSDs and eight 450GB 15K RPM SAS drives, each run in their own RAID6 array. By itself, the SSD-SAS combination isn't terribly interesting, but what the new version of the EqualLogic firmware can do with it is. 

Version 5 of the EqualLogic firmware brings a host of new features to the EqualLogic storage line, but the major benefit to the Dell PS6010XVS is the dynamic storage tiering capability. In traditional tiered storage configurations, volumes are created on disparate storage technologies, SAS, SATA and perhaps SSD, and data is placed on those volumes according to how heavily that info will be accessed. For example, databases might reside on the SAS or SSD arrays, while the cheaper SATA arrays house data that doesn't get much attention. Basically, admins make an educated guess as to the usage patterns of the data in their care and place it accordingly. Most times they're on the right track, but it's impossible to be sure that you have everything tuned correctly.

EqualLogic's new code keeps tabs on how data is being accessed at the block level and shifts heavily accessed portions to the SSD storage on the fly, increasing I/O substantially in some cases. As the workload shifts, it will return some blocks to SAS storage, other blocks to the SSD and so forth. There's no more guesswork involved; it all happens automatically based on actual production workloads.

There's more to the Dell than tiered storage, including significant VMware-based virtualisation performance improvements, but those features are not limited to the EqualLogic PS6010XVS. The release of the version 5 firmware brings those enhancements to the full line of Dell EqualLogic arrays.

In the lab: Dynamic SAS and SSD storage tiers

The Dell EqualLogic PS6010XVS in our test was outfitted with a pair of 10G controllers and the fixed configuration noted above with eight 100GB SSDs and eight 450GB 15K RPM SAS drives. It was connected to a Dell PowerConnect 8024F 10G switch, as were a number of Dell servers running Windows Server 2008 R2 and VMware ESX 4.1.

The workloads placed on the PS6010XVS were driven by five Windows Server 2008 R2 virtual machines, all running continuous IOMeter workloads. I then added a Red Hat Enterprise Linux 5 virtual machine running a dd-based sequential read/write test that varied the number of processes to create a quasi-random workload.

Watching through the EqualLogic SAN HQ monitoring and trending software, it was apparent that the automated tiering was at work, as a graphical representation of the disk I/O shifted from being largely SAS-based to increasingly SSD-based as the workloads continued to run. The response times increased and the latency decreased as the SSDs picked up more of the work, leaving the SAS drives to handle the data that was less utilised.

This shift doesn't happen immediately. It generally takes anywhere from 15 minutes to several hours for the automated tiering to kick in, partly to prevent data from bouncing between the SAS and SSD drives, but also to ensure that the workload is going to last long enough to benefit from the SSD's performance.

One workload that can benefit greatly from storage tiering is VDI (virtual desktop infrastructure). In implementations where many virtual desktops are built from a base desktop image, ie., all the desktop virtual machines are thin clones of the original, the base image gets quite a workout, while the thin clones themselves mostly sit idle. During times of heavy reads, such as the morning when all the users log into their VDI sessions, the Dell PS6010XVS will move the blocks containing the heavily accessed portions of the base image to the SSD storage, which can greatly increase access times and speed up the whole process.