-----Original Message----- From: Yong Huang [mailto:yong321@xxxxxxxxx] Sent: Saturday, May 22, 2010 1:28 PM To: redhat-list@xxxxxxxxxx Cc: Allen, Jack Subject: RE: Multipath I/O stats While we're on this topic, here's some not very technical thought on load balancing of multipath. When we talk about "load balance", we always tend to associate it with overall performance improvement (overall means scalability or throughput of multiple "clients", not latency of a single "client"). For example, an Oracle cluster database (called RAC by Oracle) allows more clients to connect to the database without degraded response time. But here we're dealing with multipath I/O. It's different in that the work done underneath is on one single piece of storage hardware, a hard disk (or a virtual one provided by some storage technology). Because read speed on the storage itself is always much slower than any of the multi- paths which is usually fiber channel, whether you have a single or multiple paths to access the single slow disk will not provide performance improvement. Am I missing anything obvious? No doubt multipath provides failover capability or failure resilience. Even with that one advantage, it's worth it. Yong Huang ========= This was sort of my reason for the question I ask in the first place. I agree that multipath provides multiple paths to a single resource (Physical or Virtual disk). On some SAN implementations there can only be an Active and standby path, others can have active/active paths. This helps performance some if both paths can be accessed at the same time as you point out and maybe the Virtual disk is made up of multiple Physical disk. If both request are for different areas of the Virtual disk, that is different Physical disk, the SAN can process both request at the same time, basically as if they were total unrelated storage. Most SANs that have multiple controllers generally have LUNs assigned to them as the primary controller. The other controller can access the same area if needed, but some have poorer performance when this is done. Then there can be an imbalance on the amount of request a given controller has to process as compared to the other controller so it can become saturated causing poor performance for those systems accessing the storage through that controller. This is generally due to heavy I/O systems all having the same primary controller for the LUNs assigned to them. Some vendors like EMC have their on implementation called Powerpath. they claim does true load balancing. Basically when a request is made it decides which path to which controller is less busy based on current queue length and response times and will route the I/O request to that controller. With Multipath round-robin and I have found out that rr_min_io controls the number of I/O that are sent down one path before switching to the next path. It does not analyze anything else related to the paths other than are they available. We have a database application that some of our IT staff want Powerpath to be used when we install it at a customer location. I want to use Multipath because each time the kernel is updated it does not break or have to be reinstalled like Powerpath does. Then there has been times when Red Hat has had a point release update, and Powerpath could not be reinstalled because it checked /etc/redhat-release and stated it was not supported. So my question about how to tell if multiple paths were really being used and by how much was to determine if the load balancing of Powerpath really made that big of a difference and that all paths were truly being utilized. I know there is more to it than are all the paths being used. The underlying FC interface has to be able to send multiple I/O request to the SAN and the SAN has to be able to accept them and process them in the best order. If multiple I/O request can not be sent at once, that is having several outstanding I/Os in progress at the same time, then it does no good because the OS will have to wait for each I/O to complete before it can send another. I am glad to see my question has caused others to think about what Multipath ready does and how. ----- Jack Allen -- redhat-list mailing list unsubscribe mailto:redhat-list-request@xxxxxxxxxx?subject=unsubscribe https://www.redhat.com/mailman/listinfo/redhat-list
