Attempting to reduce RAM, SQL I/O write errors

Greetings to all the UNA family! With TMD Hosting and a dedicated server, I get frequent and frustrating sql I/O write errors to tmp files. I am allowed 4 gigs of RAM, they say.

The issue persists when even a few users are logged in to the site. Once logged in, every page access generates a new sql, notifications-based, i/o error.

Is there a setting or a strategy whereby I can reduce RAM loads? On the Timeline, the number of entries can be reduced, I am aware. I also reduced the number of notification categories. Is there perhaps a rewrite or cache setting I'm overlooking?

When I turned off the Notifications module - the errors stop. Also, if not logged in - again, no sql errors are generated by the Notification Module. Like this one , for example:

(SELECT DISTINCT `bx_notifications_events`.* FROM `bx_notifications_events` LEFT JOIN `bx_notifications_handlers` ON `bx_notifications_events`.`type`=`bx_notifications_handlers`.`alert_unit` AND ((ISNULL(`bx_notifications_settings2users`.`active`) AND .........` bx_notifications_settings2users`.`active`='1')) ORDER BY `bx_notifications_events`.`date` DESC, `bx_notifications_events`.`id` DESC LIMIT 0, 6) ORDER BY `date` DESC, `id` DESC LIMIT 0, 6

Mysql error: Can't create/write to file '/tmp/#sql_482_1.MAD' (Errcode: 5 "Input/output error")

Also, this looks amiss: deflate_module - UNDEFINED -AND- expires_module - UNDEFINED

Much thanks for your consideration!