Hello, I've noticed read I/O starvation problems of LIO iSCSI target when used on top of writeback-enabled HW RAID controller (PERC H700 with 1GB cache). For intensive mixed read-write workload in virtualized environments, writes are able to consume over 95% of the IOPS throughput and cause starvation of reads. After a number of tests it seems to me it's a general issue of block layer I/O scheduling when running on top of a writeback device. If there is a write-intensive task, all writes go to the writeback cache with near-zero latency. This allows writer to quickly saturate the device with thousands of writes while using only a minimal fraction of queue depth. However, non-cached reads depend on spinning drive latencies which are orders of magnitude higher than writeback cache latencies, and so readers cannot submit so many requests per second as writers. Consequently, I guess the controller has totally wrong view of the incoming workload pattern, tries to satisfy the write flood first and the net result is inacceptable starvation of reads, with latencies up to hundreds of milliseconds. A simple fio test with 1TiB block device where one thread does 4k random sync writes with iodepth=32 and one thread does 4k random reads with iodepth=32 shows that instead of the theoretical 50:50 IOPS ratio, the block device runs with 95:5 ratio in favor of writes. In fact, the imbalance is so high that even write iodepth=2 is enaugh to achieve the same numbers. Real workloads that tend to exhibit this problem are: initial zeroing of a virtual machine disk, virtual machine migration, virtual machine cloning, intensive swapping of one virtual machine etc. I tried to set WCE=1 on target iblock device, played with queue depths, tested all three I/O schedulers and their parameters, controller's parameters, but with no luck. To achieve reasonably good fairness, the only solution is to set nr_requests to 1 or disable controller's writeback cache at all -- at the expense of degraded overall performance :-( Regarding nr_requests, there's obvious relation between iodepths and read starvation: if (nr_requests >= workload iodepth) then starvation surely occurs. Lowering nr_requests below this threshold slowly starts improving fairness and for every rd+wr iodepths pair, there exists sufficiently low nr_requests value at which IOPS ratio is finally balanced according to rd:wr iodepth ratio. Unfortunately it means there is no minimal nr_requests value suitable for all workloads. For iodepths around 2 to 8, only nr_requests=1 provides fair load balancing. Is this a known problem? Does anybody find block layer parameters that elliminate this problem for iscsi-target storage in mixed random read-write environments like virtualization? Or should I start writing my own I/O scheduler? ;-) Update: I've just found https://lkml.org/lkml/2012/12/10/550 (Read starvation by sync writes), where Jan Kara describes identical symptoms. But setting nr_requests=10000 doesn't help in my case. CC'ing LKML too (I'm not LKML subscriber). Thanks, Martin -- To unsubscribe from this list: send the line "unsubscribe target-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
