[libvirt RFCv8 00/27] multifd save restore prototype

[Daniel P. Berrangé]

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 :|