Tuesday, May 02, 2006

Dynamic Queue Management

When we (Netapp) rolled out Fibre Channel support almost 4 years ago, one of our goals was to simplify the installation, configuration, data and protocol management as well as provide deep application integration. In short, we wanted to make sure the burden does not fall squarely on the shoulder of the Administrator to accomplish routine day to day tasks.

One of the things we paid particularly attention to, was Host side and Target side Queue Depth management. Setting host Queue depths is a much more complicated process than the various disk subsystem vendors documentation make it to be and requires specific knowledge around application throughput and response times in order to decide what the appropriate Host Queue Depth should be set to.

All SAN devices suffer from Queue Depth related issues. The issue is that everybody parcels out finite resources (Queues) from a common source (Array Target Port) to a set of Initiators (HBAs) that consider these resources to be independent. As a result, on occasion, initiators can easily monopolize I/O to a Target Port thus starving other initiators in the Fabric.

Every vendor documentation I've seen, explicitly specifies what the host setting of the Host Queue Depth setting should be. How is that possible when in order to do this you need to have knowledge of the application's specific I/O requirements and response rime? Isn't that what Little's Law is all about (N=X * R)?

It's simply a "shot in the dark" approach hoping that the assigned queue depth will provide adequate application performance. But what if it doesn't? Well, then, a lot of vendors will give it another go...Another "shot in the dark". In the process of setting the appropriate Host Queue Depth, and depending on the OS, they will edit the appropriate configuration file, make the change, and ask the application admin to take an outage and reboot the host.

The above procedure is related to two things: a) Poor Planning without knowing what the Application requirements are b) Inadequate protocol management features

To address this challenge we decided to implement Dynamic Queue Management and move Queue Depth management from the Host to the Array's Target Port.

So what is Dynamic Queue Management?

Simply put, Dynamic Queue Management manages queue depths from the Array side. By monitoring Application response times on a per LUN basis, and QFULL conditions it dynamically adjusts the Queue Depth based on the application requirements. In addition, it can be configured to:

  1. Limit the number of I/O requests a certain Initiator sends to a Target Port
  2. Prevent initiators from flooding Target ports while starving other initiators from LUN access
  3. Ensures that initiators have guaranteed access to Queue resources

With Dynamic Queue Management, Data ONTAP calculates the total amount of command blocks available and allocates the appropriate number to reserve for an initiator or a group of initiators, based on the percentage you specify (0-99%). You can also specify a reserve Queue Pool where an initiator can borrow Queues when these are needed by the application. On the host side, we set the Queue Depth to its maximum value.

The benefit of this practice is, that it take the guessing game out of the picture and guarantees that the application will perform at its maximum level without unnecessary host side reconfigurations, application shutdowns or host reboots. Look Ma', No Hands!!!

Several of our competitors claim that we're new to the FC SAN market. While I will not disagree, I will augment that statement by saying that we're also...wiser and we've addressed challenges in a 4 year span that others haven't since 1997. After all, there's nothing mystical or cryptic about implementing a protocol that's been around for several years.

No comments: