Re: [Qemu-devel] [PATCH] bdrv_flush error handling

[Anthony Liguori <anthony@codemonkey.ws>]

Daniel P. Berrange wrote:
> On Wed, Feb 20, 2008 at 04:31:56PM +0000, Jamie Lokier wrote:
>   
>> Ian Jackson wrote:
>>     
>>> Paul Brook writes ("Re: [Qemu-devel] [PATCH] bdrv_flush error handling"):
>>>       
>>>> Disk full is a fundamentally unfriendly situation to be in. There is no good 
>>>> answer. Reporting errors back to the host has its own set of problems. Many 
>>>> guest OS effectively just lock up when this occurs.
>>>>         
>>> I think that's fine, surely ?  A locked up guest isn't very nice but
>>> it's better than a guest shot dead.
>>>       
>> Well, a guest which receives an IDE write error might do things like
>> mark parts of the device bad, to avoiding writing to those parts.  If
>> the guest is running software RAID, for example, it will radically
>> change its behaviour in response to those errors.
>>
>> Sometimes that's what you want, but sometimes it is really unwanted.
>> If the host runs out of disk space, ideally you might want to suspend
>> the guest until you can free up host disk space (or move to another
>> host), then resume the guest, perhaps manually.
>>
>> Is savevm-upon-disk-full a realistic prospect?
>>     
>
> In the 'out of disk space' scenario you wouldn't need to save the guest - merely
> stop its CPU. This gives the host admin the opportunity to hot-add storage
> to the host & then resume execution, or to kill the VM, or to free enough
> space to save the VM to disk / live migrate it to another host. 
>   

I agree.  Stopping the CPUs and spitting out a big fat warning message 
would be the best thing to do.  For the average user, this would give 
the opportunity to free up some space if possible.

Regards,

Anthony Liguori

> Shooting it dead on any I/O error doesn't give the host admin any choices
> at all
>
> Dan.
>