Re: [Qemu-devel] [RFC Patch 0/7]Qemu: Dynamic host pagecache change

[Eric Blake <eblake@redhat.com>]

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On 01/31/2012 08:05 PM, Supriya Kannery wrote:
>     For changing host pagecache setting of a running VM, it is
> important to have a safe way of reopening its image file.
> 
> Following patchset introduces:
>  * a generic way to reopen image files safely. 
>         In this approach, before reopening an image, for each
>     block driver, its state will be stashed. Incase preparation
>     (bdrv_reopen_prepare) for reopening returns success, the stashed 
>     state will be cleared (bdrv_reopen_commit) and reopened state will 
>     be used further. Incase preparation of reopening returns failure, 
>     the state of the driver will be rolled back (bdrv_reopen_abort) 
>     to the stashed state.
>         This approach is extended to raw-posix, raw-win32 and vmdk block 
>     drivers in this patchset. Once this is reviewed and finalised, I will 
>     extend the implementation to other drivers like qcow2, qed etc..

How will this interplay with the goal of passing images in by fd rather
than by name?  I'd really like to start thinking about how we plan on
coordinating situations where an fd has to be reopened in order to
switch flags (such as O_RDONLY becoming O_RDWR, or adding or subtracting
O_DIRECT), but where SELinux or other isolation means that the
management app (such as libvirt) has to do the open and pass the fd via
'getfd' monitor command.

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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