[PATCH RFC 0/9] qemu: Support mapped-ram migration capability

This series is a RFC for support of QEMU's mapped-ram migration capability [1] for saving and restoring VMs. It implements the first part of the design approach we discussed for supporting parallel save/restore [2]. In summary, the approach is 1. Add mapped-ram migration capability 2. Steal an element from save header 'unused' for a 'features' variable and bump save version to 3. 3. Add /etc/libvirt/qemu.conf knob for the save format version, defaulting to latest v3 4. Use v3 (aka mapped-ram) by default 5. Use mapped-ram with BYPASS_CACHE for v3, old approach for v2 6. include: Define constants for parallel save/restore 7. qemu: Add support for parallel save. Implies mapped-ram, reject if v2 8. qemu: Add support for parallel restore. Implies mapped-ram. Reject if v2 9. tools: add parallel parameter to virsh save command 10. tools: add parallel parameter to virsh restore command This series implements 1-5, with the BYPASS_CACHE support in patches 8 and 9 being quite hacky. They are included to discuss approaches to make them less hacky. See the patches for details. The QEMU mapped-ram capability currently does not support directio. Fabino is working on that now [3]. This complicates merging support in libvirt. I don't think it's reasonable to enable mapped-ram by default when BYPASS_CACHE cannot be supported. Should we wait until the mapped-ram directio support is merged in QEMU before supporting mapped-ram in libvirt? For the moment, compression is ignored in the new save version. Currently, libvirt connects the output of QEMU's save stream to the specified compression program via a pipe. This approach is incompatible with mapped-ram since the fd provided to QEMU must be seekable. One option is to reopen and compress the saved image after the actual save operation has completed. This has the downside of requiring the iohelper to handle BYPASS_CACHE, which would preclude us from removing it sometime in the future. Other suggestions much welcomed. Note the logical file size of mapped-ram saved images is slightly larger than guest RAM size, so the files are often much larger than the files produced by the existing, sequential format. However, actual blocks written to disk is often lower with mapped-ram saved images. E.g. a saved image from a 30G, freshly booted, idle guest results in the following 'Size' and 'Blocks' values reported by stat(1) Size Blocks sequential 998595770 1950392 mapped-ram 34368584225 1800456 With the same guest running a workload that dirties memory Size Blocks sequential 33173330615 64791672 mapped-ram 34368578210 64706944 Thanks for any comments on this RFC! [1] https://gitlab.com/qemu-project/qemu/-/blob/master/docs/devel/migration/m... [2] https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/message/K... [3] https://mail.gnu.org/archive/html/qemu-devel/2024-05/msg04432.html Jim Fehlig (9): qemu: Enable mapped-ram migration capability qemu_fd: Add function to retrieve fdset ID qemu: Add function to get migration params for save qemu: Add a 'features' element to save image header and bump version qemu: conf: Add setting for save image version qemu: Add support for mapped-ram on save qemu: Enable mapped-ram on restore qemu: Support O_DIRECT with mapped-ram on save qemu: Support O_DIRECT with mapped-ram on restore src/qemu/libvirtd_qemu.aug | 1 + src/qemu/qemu.conf.in | 6 + src/qemu/qemu_conf.c | 8 ++ src/qemu/qemu_conf.h | 1 + src/qemu/qemu_driver.c | 25 ++-- src/qemu/qemu_fd.c | 18 +++ src/qemu/qemu_fd.h | 3 + src/qemu/qemu_migration.c | 99 ++++++++++++++- src/qemu/qemu_migration.h | 11 +- src/qemu/qemu_migration_params.c | 20 +++ src/qemu/qemu_migration_params.h | 4 + src/qemu/qemu_monitor.c | 40 ++++++ src/qemu/qemu_monitor.h | 5 + src/qemu/qemu_process.c | 63 +++++++--- src/qemu/qemu_process.h | 16 ++- src/qemu/qemu_saveimage.c | 187 +++++++++++++++++++++++------ src/qemu/qemu_saveimage.h | 20 ++- src/qemu/qemu_snapshot.c | 12 +- src/qemu/test_libvirtd_qemu.aug.in | 1 + 19 files changed, 455 insertions(+), 85 deletions(-) -- 2.44.0