[ZODB-Dev] REPLACE INTO queries in history free schema

[Daniel Garcia]

Hello,

Recently I was troubleshooting a system using Relstorage/MySQL with a
history free schema. The zope application became unresponsive and a
show processlist on the server showed a REPLACE INTO query taking a
long time. The mysql process had saturated the cores on the box. The
server had 64GB RAM, 300 GB SAN storage, 3.6 GHz Xeon x 8 cores.

https://gist.github.com/015fb86fbd96d7871460

As soon as I killed the REPLACE INTO query the web application became
responsive. I patched relstorage with this patch:

https://gist.github.com/2790549