Re: [Qemu-devel] qemu and qemu.git -> Migration + disk stress introduces qcow2 corruptions

[Juan Quintela]

Kevin Wolf <address@hidden> wrote:

>> What I took from the feedback was that Kevin wanted to defer open until the 
>> device model started.  That eliminates the need to reopen or have a 
>> invalidation 
>> callback.
>> 
>> I think it would be good for Kevin to comment here though because I might 
>> have 
>> misunderstood his feedback.
>
> Your approach was to delay reads, but still keep the image open. I think
> I worried that we might have additional reads somewhere that we don't
> know about, and this is why I proposed delaying the open as well, so
> that any read would always fail.
>
> I believe just reopening the image is (almost?) as good and it's way
> easier to do, so I would be inclined to do that for 1.0.
>
> I'm not 100% sure about cases like iscsi, where reopening doesn't help.
> I think delaying the open doesn't help there either if you migrate from
> A to B and then back from B to A, you could still get old data. So for
> iscsi probably cache=none remains the only safe choice, whatever we do.

iSCSI and NFS only works with cache=none.  Even on NFS with close+open,
we have troubles if anything else has the file opened (think libvirt,
guestfs, whatever).  I really think that anynthing different of
cache=none from iSCSI or NFS is just betting (and yes, it took a while
for Christoph to convince me, I was trying to a "poor man" distributed
lock manager, and as everybody knows, it is a _difficult_ problem to
solve.).

Later, Juan.