Re: [Qemu-devel] [PATCH 00/14] block (mostly qcow2) changes (v6)

[Avi Kivity <avi@redhat.com>]

Gleb Natapov wrote:
> On Tue, Mar 17, 2009 at 10:40:38PM +0200, Uri Lublin wrote:
>   
>> Changes from v5:
>>    Patchset includes newly introduced qcow2 extensions.
>>    Usage of such qcow2 extensions for keeping both backing format and 
>>        highest-allocated-offset.
>>    No scanning of qcow2 images upon open.
>>     
> highest-allocated-offset is written to a disk only if block device was closed
> properly, so this value can't actually be trusted to be accurate. How
> important is to maintain it accurate? If it is important it should be
> saved on disk as part of a metadata update and if it is not it can be
> updated on a first guest write that requires new block allocation.
>   

Good catch.

I suggest a different approach.  Have a notification that triggers when 
a write past a certain offset occurs:

    (qemu) block_watermark_notify ide0-0 123564435343
    ... time passes ...
    (qemu) #block watermark exceeded: ide0-0 127454566544

This is in addition to pausing the VM on write error.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.