Multi-Queue Block Layer Is Merged For Linux 3.13

The multi-queue block layer (blk-mq) has indeed made it for Linux 3.13 as one (of many other) exciting features. The multi-queue block layer can lead to better SSD performance through reducing latency by balancing I/O workload across multiple CPU cores and allowing for multiple hardware queues.

Since earlier this month we were expecting Linux 3.13 To Receive Multi-Queue Block Layer. The block layer pull request was submitted with this support and it didn't receive any objections from Linus Torvalds or any other kernel developers, so it's now in.

While this is a big win, not all drivers have been converted yet to the new interface but more work will happen for Linux 3.14. More details can be found via the Git pull message for this blk-mq support.

This is a new and more scalable queueing model that marries the best part of the request based interface we currently have (which is fully featured, but scales poorly) and the bio based "interface" which the new drivers for high IOPS devices end up using because it's much faster than the request based one.

The bio interface has no block layer support, since it taps into the stack much earlier. This means that drivers end up having to implement a lot of functionality on their own, like tagging, timeout handling, requeue, etc. The blk-mq interface provides all these. Some drivers even provide a switch to select bio or rq and has code to handle both, since things like merging only works in the rq model and hence is faster for some workloads. This is a huge mess. Conversion of these drivers nets us a substantial code reduction. Initial results on converting SCSI to this model even shows an 8x improvement on single queue devices. So while the model was intended to work on the newer multiqueue devices, it has substantial improvements for "classic" hardware as well. This code has gone through extensive testing and development, it's now ready to go. A pull request is coming to convert virtio-blk to this model will be will be coming as well, with more drivers scheduled for 3.14 conversion.