[Qemu-devel] QMP: RFC: I/O error info & query-stop-reason

[Luiz Capitulino <lcapitulino@redhat.com>]

Hi there,

There are people who want to use QMP for thin provisioning. That's, the VM is
started with a small storage and when a no space error is triggered, more space
is allocated and the VM is put to run again.

QMP has two limitations that prevent people from doing this today:

1. The BLOCK_IO_ERROR doesn't contain error information

2. Considering we solve item 1, we still have to provide a way for clients
   to query why a VM stopped. This is needed because clients may miss the
   BLOCK_IO_ERROR event or may connect to the VM while it's already stopped

A proposal to solve both problems follow.

A. BLOCK_IO_ERROR information
-----------------------------

We already have discussed this a lot, but didn't reach a consensus. My solution
is quite simple: to add a stringfied errno name to the BLOCK_IO_ERROR event,
for example (see the "reason" key):

{ "event": "BLOCK_IO_ERROR",
   "data": { "device": "ide0-hd1",
             "operation": "write",
             "action": "stop",
             "reason": "enospc", }
   "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }

Valid error reasons could be: "enospc", "eio", etc.

B. query-stop-reason
--------------------

I also have a simple solution for item 2. The vm_stop() accepts a reason
argument, so we could store it somewhere and return it as a string, like:

-> { "execute": "query-stop-reason" }
<- { "return": { "reason": "user" } }

Valid reasons could be: "user", "debug", "shutdown", "diskfull" (hey,
this should be "ioerror", no?), "watchdog", "panic", "savevm", "loadvm",
"migrate".

Also note that we have a STOP event. It should be extended with the
stop reason too, for completeness.