[EtherpadDB] Try to optimize performance of index cache

pica-etherpad-db/CHANGELOG.md

+## mysql8.picapatch2
+
+Try to improve performance of MySQL
+
+* Increase [key_buffer_size](https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_key_buffer_size) given that the index file is way too big for the default value
+* Also increase `read_buffer_size` to a [recommended value](https://dba.stackexchange.com/a/136409) of 256K
+* More precise base image tag
 
 ## mysql8.picapatch1
 

pica-etherpad-db/Dockerfile

 FROM mysql:8
 
 COPY config/binary_log.cnf /etc/mysql/conf.d/binary_log.cnf
+COPY config/myisam_perf.cnf /etc/mysql/conf.d/myisam_perf.cnf
 
 # By default, COPY uses permissions `-rw-rw-rw-` for the file inside
 # the container. But mysql refuses to use config files that are world-

pica-etherpad-db/config/myisam_perf.cnf

+[mysqld]
+# The default value is 8Mo.
+# Given this documentation : https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_key_buffer_size,
+# we see that the  Key_reads/Key_read_requests ratio should normally be less than 0.01.
+# At the moment (04/2020), the ratio is more than 0.05, which indicates that the index is far larger than
+# the cache. The index is 300M. Right now with this setting, we will "waste" 700M of RAM,
+# which we can afford, and foresee further increase.
+key_buffer_size=1G
+read_buffer_size=256K

pica-etherpad-db/docker-compose.yml

@@ -4,4 +4,4 @@ version : "2.4"
 
 services:
   etherpad-db:
-    image: registry.picasoft.net/pica-etherpad-db:mysql8.picapatch1
+    image: registry.picasoft.net/pica-etherpad-db:mysql8.picapatch2