InfoWorld review: Dell iSCSI SAN sizzles with SSD, dynamic storage tiering

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 utilized.

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 -- i.e., 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.

Another aspect of the Dell PS6010XVS can come in handy in some workloads: The SSDs can also be used for write caching if they're not loaded with data moved from the SAS tier. In heavy write scenarios, writes to the array are cached on the SSDs and pushed down to disk later, offering a significant write performance boost when necessary. This decision is made by the controller firmware based on an assortment of factors, such as how hard each side of the array is working and how much free space is available.

With all this caching at the controller and disk level, the high performance of the SAS and SSD disks themselves, and the 10G Ethernet interfaces, the Dell PS6010XVS simply flies. It wasn't uncommon for loaded sequential reads run from a VMware virtual machine to hit 150MBps, with writes peaking at around 250MBps. Naturally those numbers fall off dramatically without the caching.