High Availability Modular Storage: USP VM
by Hu Yoshida on Jul 18, 2008
Last night the executives of our Hitachi development laboratories hosted a dinner for the Asia Pacific Media tour at a traditional Japanese Restaurant in Odawara. It was a fabulous meal which lasted almost three hours. I had forgotten what it feels like to sit on a tatami floor for three hours. As I struggled to stand up while maintaining my balance, I was reminded about modular storage.
The advantage of modular storage is its lower cost and modular architecture which makes it well suited for the midrange market. Modular storage can be expanded by adding drawers of disks into a standard 19 inch rack. It can be configured in the same rack with server, switch, and other peripherals that are also packaged in 19 inch drawers. Modular storage does not need a raised floor data center since each drawer has fans which draw the cooling air in from the front and exhaust out the back.
Up to now the limitation of modular storage is that it only has two controllers. Each controller has its own ports to the servers, manages its own cache, and provides RAID parity generation and access to the disks that are assigned to it. The controllers are designed to fail over to the other in the event of a controller failure. In order to maintain cache coherency, the controllers are assigned separate volumes or LUNs for processing. Writes to one controller’s cache is mirrored into the redundant controller’s cache to avoid data loss in the event of a controller failure. While this avoids the loss of cached data for the failed controller, this does not provide availability since the remaining controller now must take over ownership of the other’s volumes and do the work of both controllers. With one controller we no longer have write protection for cached data and we must either switch off the cache to write directly to disk or take the risk of data loss if the remaining controller should fail. Sooner or later the system would have to be shut down to fix the failed controller. This also applies when the controllers have to be taken down for maintenance. While modular storage is more affordable than monolithic storage, the two controller, modular storage is not high availability. However, it is very cost effective if you have maintenance windows.
That is why I was reminded of modular storage after sitting on my legs for three hours. A two controller modular storage system is like a pair of legs. My legs are redundant but if I injure one leg, I usually stop and wait for it to heal it before I go back to work.
For high availability modular storage, Hitachi provides the USP VM. It is packaged in a 19 inch rack and priced for the midrange market. But instead of two controllers it has 32 controllers which share a common cache with write protection. With a common cache for all the controllers, volumes do not have to be partitioned between the controllers for cache coherency. With this system you can lose one or more controllers with no data loss, and if you have alternate controller paths to the same cached data, there is no need to transfer LUN ownership and no disruption to the application. Since the controllers and cache are connected through a switch that can switch around hotspots and failures, you may not even see a performance degradation. The availability of more than two controllers and the ability to share the same cache provides high availability with no disruption to the application.
The USP VM also has all the functions provided in the USP V. This includes the ability to virtualize existing storage assets behind the USP VM, and enhance their capability with services such as dynamic (thin) provisioning, replication, and tiered storage. If you attach a modular storage system behind the USP VM and need to do maintenance on it, the USP V or VM can move the data off during the maintenance and then move it back afterwards, all without disruption to the application, making your existing modular storage a high availability storage system.
Eventually I was able to recover the use of my legs and hobble out of the restaurant. My legs are definitely not built for high availability or for sitting on them.