On Wed, May 11, 2022 at 07:31:45PM +0200, Claudio Fontana wrote:
> That's great, I love when things are simple.
>
> If indeed we want to remove the copy in libvirt (which will also mean
> explicitly fsyncing elsewhere, as the iohelper would not be there anymore to
> do that for us on image creation),
> with QEMU having a "file" protocol support for migration,
>
> do we plan to have libvirt and QEMU both open the file for writing
> concurrently, with QEMU opening O_DIRECT?
For non-libvirt users, I expect QEMU would open the
file directly . For libvirt usage, it is likely
preferrable to pass the pre-opened FD, because that
simplifies file permission handling.
> The alternative being having libvirt open the file with
> O_DIRECT, write some libvirt stuff in a new, O_DIRECT-
> friendly format, and then pass the fd to qemu to migrate to,
> and QEMU sending its new O_DIRECT friendly stream there.
Yep.
> In any case, the expectation here is to have a new
> "file://pathname" or "file:://fdname" as an added feature in QEMU,
> where QEMU would write a new O_DIRECT friendly stream
> directly into the file, taking care of both optional
> parallelization and compression.
I could see several distinct building blocks
* First a "file:/some/path" migration protocol
that can just do "normal" I/O, but still writing
in the traditional migration data stream
* Modify existing 'fd:' protocol so that it fstat()s
and passes over to the 'file' protocol handler if
it sees the FD is not a socket/pipe
* Add a migration capability "direct-mapped" to
indicate we want the RAM data written/read directly
to/from fixed positions in the file, as opposed to
a stream. Obviously only valid with a sub-set
of migration protocols (file, and fd: if a seekable
FD).
* Add a migration capability "bypass-cache" to
indicate we want O_DIRECT to bypass host I/O
cache. Again limited to some migration protocols
With regards,
Daniel
--
|: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org -o- https://fstop138.berrange.com :|
|: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
