Guests "thinks" that it has more space than its virtual disk really has

[Volodymyr Melnyk <v....@tucha.ua>]

Hello,

To be exact, this matter doesn't seem to be ACS-related, it's rather
VirtIO-related, though I'd like to post this message just to see if
anyone else faced the same.

There were 3 or 4 times when I encountered the following glitch
glitch: a guest operating system (it happened only to Windows Server
guests, I never saw it with Linux guest) "decides" that its primary disk
storage has more size than it actually is.

For example, an instance had a virtual drive of 200 GB, it had been
worked fine for years, but at some moment (no one knows at which exactly
one) primary partition (we mean "drive C:", which usually is the 2nd
one, as the 1st one is being used by the operating system) became the
size of ~210 GB just out of the blue. After that, the system event log
started growing with the following error messages: `The driver detected
a controller error \ Device \ Harddisk0 \ DR`. Perhaps, it happens when
the operating system tries to write pieces of data to the sectors that
don't exist.

Once I expand this virtual drive to 210 GB, the error messages don't
appear anymore. Still, after that I find some part of the data
corrupted (maybe some fragments of files are being stored to the
non-existent sectors), so it seems to be a real problem when it happens.

Alas, I didn't find a way to reproduce that. As I stated before, it's
happened only 3-4 times (the first occurrence was ~2 years ago) in the
environment of >1000 VMs, so the glitch is rare, though each time the
outcomes are quite unpleasant. :-/

Just curious, did anyone saw something similar?

Thanks in advance for any comments.