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Also please call it >>> round-robin, or 'rr' for short. >> >> I'm fine with round-robin. The [ ] part is not mandatory; if the >> min_contiguous_read value is not specified, it will default to a >> predefined value. >> >>> The default of sector size is IMHO a wrong value, switching devices so >>> often will drop the performance just because of the io request overhead. >> >>> From what I rememer values around 200k were reasonable, so either 192k >>> or 256k should be the default. We may also drop the configurable value >>> at all and provide a few hard coded sizes like rr-256k, rr-512k, rr-1m, >>> if not only to drop parsing of user strings. >> >> I'm okay with a default value of 256k. For the experimental feature, >> we can keep it configurable, allowing the opportunity to experiment >> with other values as well > > Yeah, for experimenting it makes sense to make it flexible, no need to > patch and reboot the kernel. For final we should settle on some > reasonable values. > >>>> 5. Tested FIO random read/write and defrag compression workloads with >>>> min_contiguous_read set to sector size, 192k, and 256k. >>>> >>>> RAID1 balancing method rotation is better for multi-process workloads >>>> such as fio and also single-process workload such as defragmentation. >>>> >>>> $ fio --filename=/btrfs/foo --size=5Gi --direct=1 --rw=randrw --bs=4k \ >>>> --ioengine=libaio --iodepth=256 --runtime=120 --numjobs=4 \ >>>> --time_based --group_reporting --name=iops-test-job --eta-newline=1 >>>> >>>> >>>> | | | | Read I/O count | >>>> | | Read | Write | devid1 | devid2 | >>>> |---------|------------|------------|--------|--------| >>>> | pid | 20.3MiB/s | 20.5MiB/s | 313895 | 313895 | >>>> | rotation| | | | | >>>> | 4096| 20.4MiB/s | 20.5MiB/s | 313895 | 313895 | >>>> | 196608| 20.2MiB/s | 20.2MiB/s | 310152 | 310175 | >>>> | 262144| 20.3MiB/s | 20.4MiB/s | 312180 | 312191 | >>>> | latency| 18.4MiB/s | 18.4MiB/s | 272980 | 291683 | >>>> | devid:1 | 14.8MiB/s | 14.9MiB/s | 456376 | 0 | >>>> >>>> rotation RAID1 balancing technique performs more than 2x better for >>>> single-process defrag. >>>> >>>> $ time -p btrfs filesystem defrag -r -f -c /btrfs >>>> >>>> >>>> | | Time | Read I/O Count | >>>> | | Real | devid1 | devid2 | >>>> |---------|-------|--------|--------| >>>> | pid | 18.00s| 3800 | 0 | >>>> | rotation| | | | >>>> | 4096| 8.95s| 1900 | 1901 | >>>> | 196608| 8.50s| 1881 | 1919 | >>>> | 262144| 8.80s| 1881 | 1919 | >>>> | latency | 17.18s| 3800 | 0 | >>>> | devid:2 | 17.48s| 0 | 3800 | >>>> >>>> Rotation keeps all devices active, and for now, the Rotation RAID1 >>>> balancing method is preferable as default. More workload testing is >>>> needed while the code is EXPERIMENTAL. >>> >>> Yeah round-robin will be a good defalt, we only need to verify the chunk >>> size and then do the switch in the next release. >>> >> >> Yes.. >> >>>> While Latency is better during the failing/unstable block layer transport. >>>> As of now these two techniques, are needed to be further independently >>>> tested with different worloads, and in the long term we should be merge >>>> these technique to a unified heuristic. >>> >>> This sounds like he latency is good for a specific case and maybe a >>> fallback if the device becomes faulty, but once the layer below becomes >>> unstable we may need to skip reading from the device. This is also a >>> different mode of operation than balancing reads. >>> >> >> If the latency on the faulty path is so high that it shouldn't pick that >> path at all, so it works. However, the round-robin balancing is unaware >> of dynamic faults on the device path. IMO, a round-robin method that is >> latency aware (with ~20% variance) would be better. > > We should not mix the faulty device handling mode to the read balancing, > at least for now. A back off algorithm that checks number of failed io > requests should precede the balancing. > >>>> Rotation keeps all devices active, and for now, the Rotation RAID1 >>>> balancing method should be the default. More workload testing is needed >>>> while the code is EXPERIMENTAL. >>>> >>>> Latency is smarter with unstable block layer transport. >>>> >>>> Both techniques need independent testing across workloads, with the goal of >>>> eventually merging them into a unified approach? for the long term. >>>> >>>> Devid is a hands-on approach, provides manual or user-space script control. >>>> >>>> These RAID1 balancing methods are tunable via the sysfs knob. >>>> The mount -o option and btrfs properties are under consideration. >>> >>> To move forward with the feature I think the round robin and preferred >>> device id can be merged. I'm not sure about the latency but if it's >>> under experimental we can take it as is and tune later. >> >> I hope the experimental feature also means we can change the name of the >> balancing method at any time. Once we have tested a fair combination of >> block device types, we'll definitely need a method that can >> automatically tune based on the device type, which will require adding >> or dropping balancing methods accordingly. > > Yes we can change the names. The automatic tuning would need some > feedback that measures the load and tries to improve the throughput, > this is where we got stuck last time. So for now let's do some > starightforward policy that on average works better than the current pid > policy. I hope that tha round-robin-256k can be a good default, but of > course we need more data for that. Sending v3 with rotation renamed to round-robin. Code review appreciated; I'll wait a day. Thanks, Anand