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


From: Anthony Liguori
Subject: Re: [Qemu-devel] QMP: RFC: I/O error info & query-stop-reason
Date: Thu, 02 Jun 2011 13:33:52 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110424 Lightning/1.0b2 Thunderbird/3.1.10

On 06/02/2011 01:09 PM, Luiz Capitulino wrote:
On Thu, 02 Jun 2011 13:00:04 -0500
Anthony Liguori<address@hidden>  wrote:

On 06/02/2011 12:57 PM, Luiz Capitulino wrote:
On Wed, 01 Jun 2011 16:35:03 -0500
Anthony Liguori<address@hidden>   wrote:

On 06/01/2011 04:12 PM, Luiz Capitulino wrote:
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", }

you can call the reason whatever you want, but don't call it stringfied
errno name :-)

In fact, just make reason "no space".

You mean, we should do:

    "reason": "no space"

Or that we should make it a boolean, like:

   "no space": true


Do we need reason in BLOCK_IO_ERROR if query-block returns this information?

True, no.

I'm ok with either way. But in case you meant the second one, I guess
we should make "reason" a dictionary so that we can group related
information when we extend the field, for example:

   "reason": { "no space": false, "no permission": true }

Why would we ever have "no permission"?

Why did it happen? It's not clear to me when read/write would return EPERM. open() should fail. In fact, EPERM is not mentioned in man 2 read.

Regards,

Anthony Liguori


It's an I/O error. I have a report from a developer who was getting
the BLOCK_IO_ERROR event and had to debug qemu to know the error cause,
it turned out to be no permission.

Part of my argument for not having reason is I don't think we actually
need to be this generic.  I think we're over abstracting.

I'm quite sure we'll want to add new errors reasons in the near future.





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