WordPress: Tags, Categories disappearing at random, Slow performance or database problem

I've got a Linode Virtual Private Server that's been running pretty well for a couple of years. Sometimes WordPress overwhelms the memory and processor resources a bit and we get these automated notifications that we've "gone over 110% usage for the past 2 hours...."

But this week, we began having problems with our server. And I started getting reports about random problems from different people.

One was a problem was on single posts, where an Error object could not be converted to a string for the breadcrumbs function of the theme.

Another problem was saying that "object from empty value in /wp-admin/includes/post.php on line 632, and WordPress would say only, "You are currently editing the page that shows your latest posts."

Then the tags disappeared. And the categories list was blank. And the Most Popular widget gave us all kinds of errors. And when I tried to do administration through the website, there was these really long delays.

The funny thing was, that the problems would come and go every few minutes.

After several hours, I tried to do a sudo apt-get update, and sudo apt-get upgrade. I thought maybe there was a bug in the server software. Maybe the MySQL database was damaged?

The upgrade process stopped at the MySQL portion. It said there wasn't enough room to do the upgrade.

After fiddling with phpMyAdmin to get a backup, I tried going through Putty to work with MySQL directly. It came back with an error that said: "Got error 28 from storage engine."

I looked it up and lo-and-behold! The disk didn't have enough room left on it. It took me a while to identify exactly what that meant.

To get a view of your Linux disk drive usage, type this disk free command:

df -h

For me, it came back saying:
/dev/root
Size = 27G
Used = 27G
Avail = 0
Use% = 100

Hmmm... my hard drive is full!?!

So, I did some clumsy attempts to clean it off, but the real solution was to go to the Linode administration panel. There, I saw that the drive itself was nearly 2 terabytes. However, I was working on an old configuration where the disk partition for my Linux/Apache/MySQL/PHP + Wordpress installation was a paltry 27 gigabytes -- 14% of the total allowance.

I doubled the partition size and, viola! Back to working like new in an instant.