Re: [libvirt RFCv11 00/33] multifd save restore prototype

On 11/27/23 11:18, Daniel P. Berrangé wrote: > On Mon, Nov 27, 2023 at 10:43:58AM +0100, Claudio Fontana wrote: >> Hi all, >> >> I understand there has been some movement in this topic as the fixed-offset ram and multifd code evolves. >> >> I think I understood that now the idea is to pass from libvirt to QEMU two file descriptors, >> one for writing metadata, >> and a separate one for the actual memory pages, which is the one that can potentially be O_DIRECT. > > We determined that O_DIRECT changes propagate across dup'd file > descriptors, so we have only two choices > > * 1 FD, and QEMU has to take care to toggle O_DIRECT on/off > repeatedly depending on what phase it is in > * 2 FDs, one with and one without O_DIRECT > > Either is viable for libvirt. I have a mild preference for having > 1 FD, but not enough to call it a design blocker. So at the > discretion of whomever implements the QEMU part. > >> 1) I would assume that libvirt would then need to check if the user requested --parallel / --parallel-connections to enable QEMU multifd. > > Yes > >> 2) I would also assume that libvirt would check the presence of --bypass-cache as the condition to set O_DIRECT on the second (memory pages fd), and to enable QEMU "io-direct" feature. > > Yes > >> 3) I would tentatively suggest that when it comes to fixed-ram-offset, the condition to enable that one is a check like the one currently in libvirt: >> >> src/util/virfile.c::virFileDiskCopy() >> >> ie checking that we are writing to a seekable file that is not S_ISBLK . >> >> Does this match your understanding/reasoning? > > Both the io-direct and fixed-ram-offset features are dependent on > new QEMU impls, so there is a mild backwards compatibility concern. > > ie, lets say if we are running QEMU 9.0.0, but with old machine > type pc-i440fx-8.0.0, and we save the state, but want to then > restore with QEMU 8.2.0. > > Essentially we must NOT use io-direct/fixed-ram-offset if we > want the ability to migrate to older QEMU. At the same time I > would like us to be able to take advantage of this new QEMU > support to the greatest extent possible, even if not doing the > --parallel stuff which was the original motivator. > > Thus, we need some way to decide whether to use the new or the > old on disk format. > > I wonder if having a setting in '/etc/libvirt/qemu.conf' is > sufficient, or whether we must expose a flag via the API. > > With regards, > Daniel Thanks Daniel, that's an interesting point. The new fixed-ram-offset format is a QEMU format, and as such I presume that in theory there is a qemu_saveimage.h:#define QEMU_SAVE_VERSION 2 that could be bumped to 3 if this new format is used? But then again, libvirt would need to decide whether to save in "old QEMU compatibility mode" or in the new QEMU_SAVE_VERSION 3 mode that allows for fixed-ram-offset. Maybe a new libvirt option for controlling which QEMU_SAVE_VERSION format to use for the save, with the default being v2 for backward compatibility reasons?