Document how to merge all Grid subjobs for a run by hand (#1599)

* Document how to merge all Grid subjobs for a run by hand

* Update DevelopersTips.md

Author: knopers8

doc/DevelopersTips.md

@@ -394,3 +394,42 @@ query to see for a given run the number of objects per detector:
 ```
 select ccdb.metadata -> '1337188343' as detector, count(distinct path), SUM(COUNT(distinct path)) from ccdb, ccdb_paths where ccdb_paths.pathid = ccdb.pathid AND ccdb.metadata -> '1048595860' = '529439' group by detector;
 ```
+
+### Merge and upload QC results for all subjobs of a grid job
+
+Please keep in mind that the file pattern in Grid could have changed since this was written.
+```
+#!/usr/bin/env bash
+set -e
+set -x
+set -u
+
+# we get the list of all QC.root files for o2_ctf and o2_rawtf directories
+alien_find /alice/data/2022/LHC22m/523821/apass2_cpu 'o2_*/QC.root' > qc.list
+# we add alien:// prefix, so ROOT knows to look for them in alien
+sed -i -e 's/^/alien:\/\//' qc.list
+# we split the big list into smallers ones of -l lines, so we can parallelize the processing
+# one can play with the -l parameter
+split -d -l 25 qc.list qc_list_
+
+# for each split file run the merger executable
+for QC_LIST in qc_list_*
+do
+  o2-qc-file-merger --enable-alien --input-files-list "${QC_LIST}" --output-file "merged_${QC_LIST}.root" &
+done
+
+# wait for the jobs started in the loop
+wait $(jobs -p)
+
+# we merge the files of the first "stage"
+o2-qc-file-merger --input-files merged_* --output-file QC_fullrun.root
+
+# we take the first QC config file we find and use it to perform the remote-batch QC
+CONFIG=$(alien_find /alice/data/2022/LHC22m/523821/apass2_cpu QC_production.json | head -n 1)
+if [ -n "$CONFIG" ]
+then
+  alien_cp "$CONFIG" file://QC_production.json
+  # we override activity values, as QC_production.json might have only placeholders
+  o2-qc --remote-batch QC_fullrun.root --config "json://QC_production.json" -b --override-values "qc.config.Activity.number=523897;qc.config.Activity.passName=apass2;qc.config.Activity.periodName=LHC22m"
+fi
+```