Re: [Qemu-devel] [PATCH] add reopen to blockdev-transaction

[Paolo Bonzini]

Il 02/03/2012 14:00, Kevin Wolf ha scritto:
> Am 01.03.2012 17:52, schrieb Paolo Bonzini:
>>>> But you can even keep from your first patch the drive-reopen command and
>>>> not make it atomic, that shouldn't be a problem.
>>>
>>> I'm not sure whether it makes sense for a separate drive-reopen or
>>> whether to just add this to blockdev-transaction (or even both); I can
>>> make libvirt use whichever color bikeshed we pick.  There's definitely a
>>> transaction aspect here
>>
>> It's not so much atomicity, it's just safety.  The drive-reopen command
>> must be implemented in a similar way to bdrv_append; it must not do a
>> close+reopen in the same way as the existing blockdev-snapshot-sync
>> command, but that's just that blockdev-snapshot-sync was implemented
>> poorly.
> 
> For reopen this is a bit harder because you deal with already opened
> images and you must never have the same image opened twice at the same time.

This is only for read-write images, and the backing files are read-only,
so this shouldn't be a problem, no?

Hmm, actually there could one problem.  Say you switch from base->old to
base->new; if old is the outcome of a live snapshot operation, base will
still be open read-write.

So perhaps we do need a new method like bdrv_freeze; after bdrv_freeze
you know that bdrv_close will not need to write to disk.  So e.g. for
QED bdrv_freeze will turn off the need-check bit.

Paolo