On 11/27/23 11:50, Daniel P. Berrangé wrote: > On Mon, Nov 27, 2023 at 11:40:29AM +0100, Claudio Fontana wrote: >> 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? > > What makes me reluctant to add public API, is that generally I feel that > things like file format versions are internal/private impl details that > mgmt apps should not need to care about. > > Even the compatibility problem should only be a short term issue, since > there's only limited version combinations that end up getting used in > practice, and so called "backwards" version compat usage is even rarer. > > I guess if we stick with a qemu.conf setting initially, we can revisit > later if we find an API control knob is requird. > > With regards, > Daniel I think qemu.conf also works; I'd still default to v2 for a while, we can document that in order to get these additional features setting v3 in qemu.conf is required to save in the new format. Thanks, Claudio _______________________________________________ Devel mailing list -- devel@xxxxxxxxxxxxxxxxx To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxx
