confusions about how to implement a tool to backup/export old prometheus data to a remote write storage

[Jennings Liu]

Hi,

we have several prometheus instanaces running for years, and we have several TB data served as local storage, now we plan to  upgrade our monitoring stack to use a remote read/write storage(m3db). 

so we wish to backup/export old data to m3db. so I started to implement a tool to read the persistence block files and send them all to remote read endpoint.  but as an user I don't have enough insight on the core of 

prometheus that how the data is stored and how is sent.

Now I can read the data from the directory, my step is :

tsdb.OpenDBReadOnly------> blockReaders--- > blockMeta ----->MinTime

                                                                        |                        |--------->MaxTime

                                                                    |--->storageQuerier--------> storageQuerier.Select(labelSelectParams, labelMatcher) -----> storage.SeriesSet ------->storage.series  -------> labels(labels.Label)  ---transform to ---->prompb.Label 

                                                                                                                                                                                                                                                                                        |-------> iterator (cotains the sample of timestamp and value)---transform to ---->prompb.Sample    

then I don't figure out how to send these metrics to the remote storage, in repo https://github.com/prometheus/prometheus/storage/remote, since prometheus itself has implemented the remote write function, so I don't want to implement it again, but I didn't get the logic here.

and anther thing is that I don't find a way  how to extract the metric metadata from local storage, I can't see any libs/functions that exists inside the repo .