Now the new part. I have never updated an OSM rendering table, so let's see
[what the docs say about it](https://osm2pgsql.org/doc/manual.html#updating-an-existing-database):

To keep an `osm2pgsql` database up to date you have two options: you either provide the original import data file, or
you provide the replication URL. In my case, the latter is `http://download.geofabrik.de/europe-updates/`, but as you
can see, it can be inferred from the extract; it even picks up the right date[^1]:

    $ osm2pgsql-replication init --verbose --port 5433 --database gis --osm-file europe-latest.osm.pbf
    2023-08-23 15:07:18 [DEBUG]: Replication information found in OSM file header.
    2023-08-23 15:07:18 [DEBUG]: Replication URL: http://download.geofabrik.de/europe-updates
    2023-08-23 15:07:18 [DEBUG]: Replication sequence: 3787
    2023-08-23 15:07:18 [DEBUG]: Replication timestamp: 2023-08-11T20:21:59Z
    2023-08-23 15:07:18 [INFO]: Initialised updates for service 'http://download.geofabrik.de/europe-updates'.
    2023-08-23 15:07:18 [INFO]: Starting at sequence 3787 (2023-08-11 20:21:59+00:00).

Then the update is really simple: you just tell it to do it and it will do it, automatically! The only condition is that
you give the update the same parameters you gave to the original `osm2pgsql` invocation. It will proceed to download
a batch of updates until some value is reached (around 120MiB?), then call `osm2pgsql`, rinse, repeat, until all missing
updates have been applied:

    $ osm2pgsql-replication update --verbose --port 5433 --database gis -- --cache 0 --number-processes 4 --slim --flat-nodes $(pwd)/nodes.cache --hstore --multi-geometry --style $osm_carto/openstreetmap-carto.style --tag-transform-script $osm_carto/openstreetmap-carto.lua
    2023-08-23 15:13:16 [INFO]: Using replication service 'http://download.geofabrik.de/europe-updates'. Current sequence 3787 (2023-08-11 22:21:59+02:00).
    2023-08-23 15:13:16 [DEBUG]: Starting new HTTP connection (1): download.geofabrik.de:80
    2023-08-23 15:13:16 [DEBUG]: http://download.geofabrik.de:80 "GET /europe-updates/state.txt HTTP/1.1" 200 123
    2023-08-23 15:13:16 [DEBUG]: Calling osm2pgsql with: /usr/bin/osm2pgsql --append --slim --prefix planet_osm --cache 0 --number-processes 4 --slim --flat-nodes /home/mdione/src/projects/osm/data/osm/nodes.cache --hstore --multi-geometry --style /home/mdione/src/projects/osm/osm-carto/openstreetmap-carto.style --tag-transform-script /home/mdione/src/projects/osm/osm-carto/openstreetmap-carto.lua -d gis -P 5433 /tmp/tmpk7ml1gi9/osm2pgsql_diff.osc.gz
    2023-08-23 15:13:16 [DEBUG]: Importing from sequence 3787
    2023-08-23 15:13:16 [DEBUG]: Starting new HTTP connection (1): download.geofabrik.de:80
    2023-08-23 15:13:16 [DEBUG]: http://download.geofabrik.de:80 "GET /europe-updates/state.txt HTTP/1.1" 200 123
    2023-08-23 15:13:16 [DEBUG]: Starting new HTTP connection (1): download.geofabrik.de:80
    2023-08-23 15:13:16 [DEBUG]: http://download.geofabrik.de:80 "GET /europe-updates/000/003/788.osc.gz HTTP/1.1" 200 30348254
    2023-08-23 15:13:25 [DEBUG]: Downloaded change 3788. (389531 kB available in download buffer)
    2023-08-23 15:13:25 [DEBUG]: Starting new HTTP connection (1): download.geofabrik.de:80
    2023-08-23 15:13:25 [DEBUG]: http://download.geofabrik.de:80 "GET /europe-updates/000/003/789.osc.gz HTTP/1.1" 200 35284953
    2023-08-23 15:13:36 [DEBUG]: Downloaded change 3789. (245491 kB available in download buffer)
    2023-08-23 15:13:36 [DEBUG]: Starting new HTTP connection (1): download.geofabrik.de:80
    2023-08-23 15:13:36 [DEBUG]: http://download.geofabrik.de:80 "GET /europe-updates/000/003/790.osc.gz HTTP/1.1" 200 32891529
    2023-08-23 15:13:46 [DEBUG]: Downloaded change 3790. (114339 kB available in download buffer)
    2023-08-23 15:13:46 [DEBUG]: Starting new HTTP connection (1): download.geofabrik.de:80
    2023-08-23 15:13:46 [DEBUG]: http://download.geofabrik.de:80 "GET /europe-updates/000/003/791.osc.gz HTTP/1.1" 200 35347966
    2023-08-23 15:13:57 [DEBUG]: Downloaded change 3791. (-26371 kB available in download buffer)
    2023-08-23 15:14:16  osm2pgsql version 1.8.0
    2023-08-23 15:14:16  Database version: 15.3 (Debian 15.3-0+deb12u1)
    2023-08-23 15:14:16  PostGIS version: 3.3
    2023-08-23 15:14:16  Setting up table 'planet_osm_point'
    2023-08-23 15:14:16  Setting up table 'planet_osm_line'
    2023-08-23 15:14:16  Setting up table 'planet_osm_polygon'
    2023-08-23 15:14:16  Setting up table 'planet_osm_roads'
    2023-08-23 16:26:17  Reading input files done in 4321s (1h 12m 1s).
    2023-08-23 16:26:17    Processed 3014131 nodes in 1506s (25m 6s) - 2k/s
    2023-08-23 16:26:17    Processed 687625 ways in 977s (16m 17s) - 704/s
    2023-08-23 16:26:17    Processed 28176 relations in 1838s (30m 38s) - 15/s
    2023-08-23 16:27:11  Going over 217062 pending ways (using 4 threads)
    Left to process: 0........
    2023-08-23 16:30:04  Processing 217062 pending ways took 173s (2m 53s) at a rate of 1254.69/s
    2023-08-23 16:30:04  Going over 89496 pending relations (using 4 threads)
    Left to process: 0.......
    2023-08-23 17:24:42  Processing 89496 pending relations took 3277s (54m 37s) at a rate of 27.31/s
    2023-08-23 17:24:43  Done postprocessing on table 'planet_osm_nodes' in 0s
    2023-08-23 17:24:43  Done postprocessing on table 'planet_osm_ways' in 0s
    2023-08-23 17:24:43  Done postprocessing on table 'planet_osm_rels' in 0s
    2023-08-23 17:24:43  All postprocessing on table 'planet_osm_point' done in 0s.
    2023-08-23 17:24:43  All postprocessing on table 'planet_osm_line' done in 0s.
    2023-08-23 17:24:43  All postprocessing on table 'planet_osm_polygon' done in 0s.
    2023-08-23 17:24:43  All postprocessing on table 'planet_osm_roads' done in 0s.
    2023-08-23 17:24:43  osm2pgsql took 7827s (2h 10m 27s) overall.
    [...]


I'm not going to put all 4 or 5 iterations, this should be enough. 2h10m to process around 120MiB od data, meaning around
1m/MiB. I have 12 days of updates, each around 30MiB, except for the last three, that are 90, 160 and 130 MiB each, due
to [an ongoing edit battle](https://www.openstreetmap.org/user/snoozingnewt/diary/402206). A total of around 620MiB of
updates, it's going to take 10h at this pace.


\[The next day\]

    [...]
    2023-08-23 23:31:36 [INFO]: Data imported until 2023-08-22 20:21:53+00:00. Backlog remaining: 1 day, 1:09:43.865536

After ~8h10m it finished. Disk space grew 10GiB, which is more than the 10x of the import (more like 15x!). At
this pace I should get space issues in around 220 days. On the other hand, these diffs are quite unusual, so probably
the stats are skewed. Also, I guess at some point I should redo some of the clustering and analyzing done in the import
process, but I don't see anything on the docs, so I'll ask around. There are not big peaks to talk about. The most space
I see being freed is around 200MiB.


The `osm2pgsql` people have automated it to the point where they even provide in their documentation an example
`systemd` unit file to do the updates for you. Check the docs linked above.

[^1]: Yes, I started writing these posts 13 days ago.