Re: [Qemu-devel] [Xen-devel] [PATCHv3] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.

[Paolo Bonzini]

Il 19/03/2013 11:51, George Dunlap ha scritto:
> On 03/19/2013 10:43 AM, Paolo Bonzini wrote:
>>>> Even for successful migration, it would also be bad for downtime (QEMU
>>>> isn't exactly lightning-fast to start).  And even if failure weren't
>>>> catastrophic, it would be a pity to transfer a few gigs of memory and
>>>> then find out that QEMU isn't present in the destination. :)
>>>
>>> Well, if qemu isn't present at the destination, that's definitely user
>>> error. :-)  In any case, I know that he migrate can resume if it
>>> fails, so I suspect that the qemu is just paused on the sending side
>>> until the migration is known to complete.  As long as the last write
>>> was flushed to the NFS server before the receiver opens the file, we
>>> should be safe.
>>
>> Note that the close really must happen before the next open.  Otherwise
>> the file metadata might not be up-to-date on the destination, too.
> 
> By "file metadata" I assume you mean "metadata about the virtual disk
> within the file", not "metadata about the file within the filesystem",
> right?  That's good to know, I'll keep that in mind.

Actually especially the former (I'm calling them respectively "image
metadata" and "file metadata").  File metadata could also be a problem,
but I think it might just work except in cases like on-line resizing
during migration.

> Even if it's true that at the moment qemu doesn't write the file
> metadata until it closes the file, that just means we'd have to add a
> hook to the callback to save qemu state, to sync the metadata at that
> point, right?

Unfortunately no.  The problem is in the loading side's kernel, on which
you do not have any control.  If the loading side doesn't use O_DIRECT,
any attempt to invalidate the metadata in userspace or on the source is
futile, because there is no way to invalidate the page cache's copy of
that metadata.

Paolo