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diff --git a/doc/cephfs/mantle.rst b/doc/cephfs/mantle.rst new file mode 100644 index 000000000..064408f71 --- /dev/null +++ b/doc/cephfs/mantle.rst @@ -0,0 +1,263 @@ +Mantle +====== + +.. warning:: + + Mantle is for research and development of metadata balancer algorithms, + not for use on production CephFS clusters. + +Multiple, active MDSs can migrate directories to balance metadata load. The +policies for when, where, and how much to migrate are hard-coded into the +metadata balancing module. Mantle is a programmable metadata balancer built +into the MDS. The idea is to protect the mechanisms for balancing load +(migration, replication, fragmentation) but stub out the balancing policies +using Lua. Mantle is based on [1] but the current implementation does *NOT* +have the following features from that paper: + +1. Balancing API: in the paper, the user fills in when, where, how much, and + load calculation policies; currently, Mantle only requires that Lua policies + return a table of target loads (e.g., how much load to send to each MDS) +2. "How much" hook: in the paper, there was a hook that let the user control + the fragment selector policy; currently, Mantle does not have this hook +3. Instantaneous CPU utilization as a metric + +[1] Supercomputing '15 Paper: +http://sc15.supercomputing.org/schedule/event_detail-evid=pap168.html + +Quickstart with vstart +---------------------- + +.. warning:: + + Developing balancers with vstart is difficult because running all daemons + and clients on one node can overload the system. Let it run for a while, even + though you will likely see a bunch of lost heartbeat and laggy MDS warnings. + Most of the time this guide will work but sometimes all MDSs lock up and you + cannot actually see them spill. It is much better to run this on a cluster. + +As a prerequisite, we assume you have installed `mdtest +<https://sourceforge.net/projects/mdtest/>`_ or pulled the `Docker image +<https://hub.docker.com/r/michaelsevilla/mdtest/>`_. We use mdtest because we +need to generate enough load to get over the MIN_OFFLOAD threshold that is +arbitrarily set in the balancer. For example, this does not create enough +metadata load: + +:: + + while true; do + touch "/cephfs/blah-`date`" + done + + +Mantle with `vstart.sh` +~~~~~~~~~~~~~~~~~~~~~~~ + +1. Start Ceph and tune the logging so we can see migrations happen: + +:: + + cd build + ../src/vstart.sh -n -l + for i in a b c; do + bin/ceph --admin-daemon out/mds.$i.asok config set debug_ms 0 + bin/ceph --admin-daemon out/mds.$i.asok config set debug_mds 2 + bin/ceph --admin-daemon out/mds.$i.asok config set mds_beacon_grace 1500 + done + + +2. Put the balancer into RADOS: + +:: + + bin/rados put --pool=cephfs_metadata_a greedyspill.lua ../src/mds/balancers/greedyspill.lua + + +3. Activate Mantle: + +:: + + bin/ceph fs set cephfs max_mds 5 + bin/ceph fs set cephfs_a balancer greedyspill.lua + + +4. Mount CephFS in another window: + +:: + + bin/ceph-fuse /cephfs -o allow_other & + tail -f out/mds.a.log + + + Note that if you look at the last MDS (which could be a, b, or c -- it's + random), you will see an attempt to index a nil value. This is because the + last MDS tries to check the load of its neighbor, which does not exist. + +5. Run a simple benchmark. In our case, we use the Docker mdtest image to + create load: + +:: + + for i in 0 1 2; do + docker run -d \ + --name=client$i \ + -v /cephfs:/cephfs \ + michaelsevilla/mdtest \ + -F -C -n 100000 -d "/cephfs/client-test$i" + done + + +6. When you are done, you can kill all the clients with: + +:: + + for i in 0 1 2 3; do docker rm -f client$i; done + + +Output +~~~~~~ + +Looking at the log for the first MDS (could be a, b, or c), we see that +everyone has no load: + +:: + + 2016-08-21 06:44:01.763930 7fd03aaf7700 0 lua.balancer MDS0: < auth.meta_load=0.0 all.meta_load=0.0 req_rate=1.0 queue_len=0.0 cpu_load_avg=1.35 > load=0.0 + 2016-08-21 06:44:01.763966 7fd03aaf7700 0 lua.balancer MDS1: < auth.meta_load=0.0 all.meta_load=0.0 req_rate=0.0 queue_len=0.0 cpu_load_avg=1.35 > load=0.0 + 2016-08-21 06:44:01.763982 7fd03aaf7700 0 lua.balancer MDS2: < auth.meta_load=0.0 all.meta_load=0.0 req_rate=0.0 queue_len=0.0 cpu_load_avg=1.35 > load=0.0 + 2016-08-21 06:44:01.764010 7fd03aaf7700 2 lua.balancer when: not migrating! my_load=0.0 hisload=0.0 + 2016-08-21 06:44:01.764033 7fd03aaf7700 2 mds.0.bal mantle decided that new targets={} + + +After the jobs starts, MDS0 gets about 1953 units of load. The greedy spill +balancer dictates that half the load goes to your neighbor MDS, so we see that +Mantle tries to send 1953 load units to MDS1. + +:: + + 2016-08-21 06:45:21.869994 7fd03aaf7700 0 lua.balancer MDS0: < auth.meta_load=5834.188908912 all.meta_load=1953.3492228857 req_rate=12591.0 queue_len=1075.0 cpu_load_avg=3.05 > load=1953.3492228857 + 2016-08-21 06:45:21.870017 7fd03aaf7700 0 lua.balancer MDS1: < auth.meta_load=0.0 all.meta_load=0.0 req_rate=0.0 queue_len=0.0 cpu_load_avg=3.05 > load=0.0 + 2016-08-21 06:45:21.870027 7fd03aaf7700 0 lua.balancer MDS2: < auth.meta_load=0.0 all.meta_load=0.0 req_rate=0.0 queue_len=0.0 cpu_load_avg=3.05 > load=0.0 + 2016-08-21 06:45:21.870034 7fd03aaf7700 2 lua.balancer when: migrating! my_load=1953.3492228857 hisload=0.0 + 2016-08-21 06:45:21.870050 7fd03aaf7700 2 mds.0.bal mantle decided that new targets={0=0,1=976.675,2=0} + 2016-08-21 06:45:21.870094 7fd03aaf7700 0 mds.0.bal - exporting [0,0.52287 1.04574] 1030.88 to mds.1 [dir 100000006ab /client-test2/ [2,head] auth pv=33 v=32 cv=32/0 ap=2+3+4 state=1610612802|complete f(v0 m2016-08-21 06:44:20.366935 1=0+1) n(v2 rc2016-08-21 06:44:30.946816 3790=3788+2) hs=1+0,ss=0+0 dirty=1 | child=1 dirty=1 authpin=1 0x55d2762fd690] + 2016-08-21 06:45:21.870151 7fd03aaf7700 0 mds.0.migrator nicely exporting to mds.1 [dir 100000006ab /client-test2/ [2,head] auth pv=33 v=32 cv=32/0 ap=2+3+4 state=1610612802|complete f(v0 m2016-08-21 06:44:20.366935 1=0+1) n(v2 rc2016-08-21 06:44:30.946816 3790=3788+2) hs=1+0,ss=0+0 dirty=1 | child=1 dirty=1 authpin=1 0x55d2762fd690] + + +Eventually load moves around: + +:: + + 2016-08-21 06:47:10.210253 7fd03aaf7700 0 lua.balancer MDS0: < auth.meta_load=415.77414300449 all.meta_load=415.79000078186 req_rate=82813.0 queue_len=0.0 cpu_load_avg=11.97 > load=415.79000078186 + 2016-08-21 06:47:10.210277 7fd03aaf7700 0 lua.balancer MDS1: < auth.meta_load=228.72023977691 all.meta_load=186.5606496623 req_rate=28580.0 queue_len=0.0 cpu_load_avg=11.97 > load=186.5606496623 + 2016-08-21 06:47:10.210290 7fd03aaf7700 0 lua.balancer MDS2: < auth.meta_load=0.0 all.meta_load=0.0 req_rate=1.0 queue_len=0.0 cpu_load_avg=11.97 > load=0.0 + 2016-08-21 06:47:10.210298 7fd03aaf7700 2 lua.balancer when: not migrating! my_load=415.79000078186 hisload=186.5606496623 + 2016-08-21 06:47:10.210311 7fd03aaf7700 2 mds.0.bal mantle decided that new targets={} + + +Implementation Details +---------------------- + +Most of the implementation is in MDBalancer. Metrics are passed to the balancer +policies via the Lua stack and a list of loads is returned back to MDBalancer. +It sits alongside the current balancer implementation and it's enabled with a +Ceph CLI command ("ceph fs set cephfs balancer mybalancer.lua"). If the Lua policy +fails (for whatever reason), we fall back to the original metadata load +balancer. The balancer is stored in the RADOS metadata pool and a string in the +MDSMap tells the MDSs which balancer to use. + +Exposing Metrics to Lua +~~~~~~~~~~~~~~~~~~~~~~~ + +Metrics are exposed directly to the Lua code as global variables instead of +using a well-defined function signature. There is a global "mds" table, where +each index is an MDS number (e.g., 0) and each value is a dictionary of metrics +and values. The Lua code can grab metrics using something like this: + +:: + + mds[0]["queue_len"] + + +This is in contrast to cls-lua in the OSDs, which has well-defined arguments +(e.g., input/output bufferlists). Exposing the metrics directly makes it easier +to add new metrics without having to change the API on the Lua side; we want +the API to grow and shrink as we explore which metrics matter. The downside of +this approach is that the person programming Lua balancer policies has to look +at the Ceph source code to see which metrics are exposed. We figure that the +Mantle developer will be in touch with MDS internals anyways. + +The metrics exposed to the Lua policy are the same ones that are already stored +in mds_load_t: auth.meta_load(), all.meta_load(), req_rate, queue_length, +cpu_load_avg. + +Compile/Execute the Balancer +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Here we use `lua_pcall` instead of `lua_call` because we want to handle errors +in the MDBalancer. We do not want the error propagating up the call chain. The +cls_lua class wants to handle the error itself because it must fail gracefully. +For Mantle, we don't care if a Lua error crashes our balancer -- in that case, +we will fall back to the original balancer. + +The performance improvement of using `lua_call` over `lua_pcall` would not be +leveraged here because the balancer is invoked every 10 seconds by default. + +Returning Policy Decision to C++ +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +We force the Lua policy engine to return a table of values, corresponding to +the amount of load to send to each MDS. These loads are inserted directly into +the MDBalancer "my_targets" vector. We do not allow the MDS to return a table +of MDSs and metrics because we want the decision to be completely made on the +Lua side. + +Iterating through tables returned by Lua is done through the stack. In Lua +jargon: a dummy value is pushed onto the stack and the next iterator replaces +the top of the stack with a (k, v) pair. After reading each value, pop that +value but keep the key for the next call to `lua_next`. + +Reading from RADOS +~~~~~~~~~~~~~~~~~~ + +All MDSs will read balancing code from RADOS when the balancer version changes +in the MDS Map. The balancer pulls the Lua code from RADOS synchronously. We do +this with a timeout: if the asynchronous read does not come back within half +the balancing tick interval the operation is cancelled and a Connection Timeout +error is returned. By default, the balancing tick interval is 10 seconds, so +Mantle will use a 5 second second timeout. This design allows Mantle to +immediately return an error if anything RADOS-related goes wrong. + +We use this implementation because we do not want to do a blocking OSD read +from inside the global MDS lock. Doing so would bring down the MDS cluster if +any of the OSDs are not responsive -- this is tested in the ceph-qa-suite by +setting all OSDs to down/out and making sure the MDS cluster stays active. + +One approach would be to asynchronously fire the read when handling the MDS Map +and fill in the Lua code in the background. We cannot do this because the MDS +does not support daemon-local fallbacks and the balancer assumes that all MDSs +come to the same decision at the same time (e.g., importers, exporters, etc.). + +Debugging +~~~~~~~~~ + +Logging in a Lua policy will appear in the MDS log. The syntax is the same as +the cls logging interface: + +:: + + BAL_LOG(0, "this is a log message") + + +It is implemented by passing a function that wraps the `dout` logging framework +(`dout_wrapper`) to Lua with the `lua_register()` primitive. The Lua code is +actually calling the `dout` function in C++. + +Warning and Info messages are centralized using the clog/Beacon. Successful +messages are only sent on version changes by the first MDS to avoid spamming +the `ceph -w` utility. These messages are used for the integration tests. + +Testing +~~~~~~~ + +Testing is done with the ceph-qa-suite (tasks.cephfs.test_mantle). We do not +test invalid balancer logging and loading the actual Lua VM. |