Tran,
First, I suggest you read through SiteGround's notes on inode usage (unless you have already done so).
https://www.siteground.com/kb/what_is_an_inode/
http://kb.siteground.com/How_to_reduce_the_number_of_inodes_my_account_uses/
On the server side, clear the cache files (located in the moodledata folder). See this Moodle forum discussion on cache files,
Is a Moodle cron job running regularly on your server? That should be cleaning up files. See this thread on cron, and this documentation on cron.
I would also look at Moodle settings:
- Reduce the length of time log files are kept.
- Check to see if automated course backup is enabled. If so, reduce the number of backup versions that are stored.
Your Moodle version, 2.6, is very old. Core bug support for that ended in November 2014, and security issue support ended in May 2015. I strongly recommend upgrading Moodle.
Floyd