> My system's very small - approx 20 users and very little content at present. 
> It's for a small collaborative research group which may grew to ~100 members. 
> I expect quite a few largish attachments and have already configured 
> filesystem attachments.

Sounds quite reasonable.
Limited memory?

> I'm using PostgreSQL for various reasons, so can't add all the suggested 
> indexes (names the string prefix ones that are specific to MySQL), but I'd be 
> surprised to be hitting limits so soon.

The database difference might be something to explore.
I've seen rather often that things are more battle-tested for MySQL.
But I fully agree you have reasons to prefer PostgreSQL.

> The slowdown occurred soon after I opened the wiki to the other members, who 
> probably did some exploring and perhaps triggered something painful.

You definitely need to explore when it happens.
- something such as show full processlist (that's a MySQL thing) should be 
available to show if there's an SQL query taking long and if others are slow 
(typically, big LIKE queries tend to block all writes)
- invoke a thread-dump (kill -HUP or use JMX or 
 the threads generally carry the path of the http requests
- check memory
- make sure you enable -verbose:gc as a JVM option to indicate when memory 
limits are reached


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