I upgraded to the latest version (from only the more recent previous version), and it seemed to upgrade without error.
Now, however, when I visit my Piwik homepage, I get
SQLSTATE[HY000]: General error: 1 Can’t create/write to file ‘/tmp/#sql_6b2_0.MYI’ (Errcode: 28)
It isn’t clear if that’s the root filesystem /tmp, or the /tmp folder in the Piwik directory, but BOTH have permissions that should allow Piwik to write to the directory.
drwxrwxrwx 8 www-data www-data 4096 2013-06-07 10:32 .
and /tmp is
drwxrwxrwt 6 root root 471040 2015-04-01 11:45 .
(and contains files written by another process as “www-data” so it seems clearly accessible to www-data.