As we all know that postcopy migration has a potential risk to lost the VM if the network is broken during the migration. This series tries to solve the problem by allowing the migration to pause at the failure point, and do recovery after the link is reconnected.
There was existing work on this issue from Md Haris Iqbal: https://lists.nongnu.org/archive/html/qemu-devel/2016-08/msg03468.html This series is a totally re-work of the issue, based on Alexey Perevalov's recved bitmap v8 series: https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg06401.html Two new status are added to support the migration (used on both sides): MIGRATION_STATUS_POSTCOPY_PAUSED MIGRATION_STATUS_POSTCOPY_RECOVER The MIGRATION_STATUS_POSTCOPY_PAUSED state will be set when the network failure is detected. It is a phase that we'll be in for a long time as long as the failure is detected, and we'll be there until a recovery is triggered. In this state, all the threads (on source: send thread, return-path thread; destination: ram-load thread, page-fault thread) will be halted. The MIGRATION_STATUS_POSTCOPY_RECOVER state is short. If we triggered a recovery, both source/destination VM will jump into this stage, do whatever it needs to prepare the recovery (e.g., currently the most important thing is to synchronize the dirty bitmap, please see commit messages for more information). After the preparation is ready, the source will do the final handshake with destination, then both sides will switch back to MIGRATION_STATUS_POSTCOPY_ACTIVE again. New commands/messages are defined as well to satisfy the need: MIG_CMD_RECV_BITMAP & MIG_RP_MSG_RECV_BITMAP are introduced for delivering received bitmaps MIG_CMD_RESUME & MIG_RP_MSG_RESUME_ACK are introduced to do the final handshake of postcopy recovery. Here's some more details on how the whole failure/recovery routine is happened: - start migration - ... (switch from precopy to postcopy) - both sides are in "postcopy-active" state - ... (failure happened, e.g., network unplugged) - both sides switch to "postcopy-paused" state - all the migration threads are stopped on both sides - ... (both VMs hanged) - ... (user triggers recovery using "migrate -r -d tcp:HOST:PORT" on source side, "-r" means "recover") - both sides switch to "postcopy-recover" state - on source: send-thread, return-path-thread will be waked up - on dest: ram-load-thread waked up, fault-thread still paused - source calls new savevmhandler hook resume_prepare() (currently, only ram is providing the hook): - ram_resume_prepare(): for each ramblock, fetch recved bitmap by: - src sends MIG_CMD_RECV_BITMAP to dst - dst replies MIG_RP_MSG_RECV_BITMAP to src, with bitmap data - src uses the recved bitmap to rebuild dirty bitmap - source do final handshake with destination - src sends MIG_CMD_RESUME to dst, telling "src is ready" - when dst receives the command, fault thread will be waked up, meanwhile, dst switch back to "postcopy-active" - dst sends MIG_RP_MSG_RESUME_ACK to src, telling "dst is ready" - when src receives the ack, state switch to "postcopy-active" - postcopy migration continued Testing: As I said, it's still an extremely simple test. I used socat to create a socket bridge: socat tcp-listen:6666 tcp-connect:localhost:5555 & Then do the migration via the bridge. I emulated the network failure by killing the socat process (bridge down), then tries to recover the migration using the other channel (default dst channel). It looks like: port:6666 +------------------+ +----------> | socat bridge [1] |-------+ | +------------------+ | | (Original channel) | | | port: 5555 +---------+ (Recovery channel) +--->+---------+ | src VM |------------------------------------>| dst VM | +---------+ +---------+ Known issues/notes: - currently destination listening port still cannot change. E.g., the recovery should be using the same port on destination for simplicity. (on source, we can specify new URL) - the patch: "migration: let dst listen on port always" is still hacky, it just kept the incoming accept open forever for now... - some migration numbers might still be inaccurate, like total migration time, etc. (But I don't really think that matters much now) - the patches are very lightly tested. - Dave reported one problem that may hang destination main loop thread (one vcpu thread holds the BQL) and the rest. I haven't encountered it yet, but it does not mean this series can survive with it. - other potential issues that I may have forgotten or unnoticed... Anyway, the work is still in preliminary stage. Any suggestions and comments are greatly welcomed. Thanks. Peter Xu (29): migration: fix incorrect postcopy recved_bitmap migration: fix comment disorder in RAMState io: fix qio_channel_socket_accept err handling bitmap: introduce bitmap_invert() bitmap: introduce bitmap_count_one() migration: dump str in migrate_set_state trace migration: better error handling with QEMUFile migration: reuse mis->userfault_quit_fd migration: provide postcopy_fault_thread_notify() migration: new property "x-postcopy-fast" migration: new postcopy-pause state migration: allow dst vm pause on postcopy migration: allow src return path to pause migration: allow send_rq to fail migration: allow fault thread to pause qmp: hmp: add migrate "resume" option migration: rebuild channel on source migration: new state "postcopy-recover" migration: let dst listen on port always migration: wakeup dst ram-load-thread for recover migration: new cmd MIG_CMD_RECV_BITMAP migration: new message MIG_RP_MSG_RECV_BITMAP migration: new cmd MIG_CMD_POSTCOPY_RESUME migration: new message MIG_RP_MSG_RESUME_ACK migration: introduce SaveVMHandlers.resume_prepare migration: synchronize dirty bitmap for resume migration: setup ramstate for resume migration: final handshake for the resume migration: reset migrate thread vars when resumed hmp-commands.hx | 7 +- hmp.c | 4 +- include/migration/register.h | 2 + include/qemu/bitmap.h | 20 ++ io/channel-socket.c | 1 + migration/exec.c | 2 +- migration/fd.c | 2 +- migration/migration.c | 465 ++++++++++++++++++++++++++++++++++++++++--- migration/migration.h | 25 ++- migration/postcopy-ram.c | 109 +++++++--- migration/postcopy-ram.h | 2 + migration/ram.c | 209 ++++++++++++++++++- migration/ram.h | 4 + migration/savevm.c | 189 +++++++++++++++++- migration/savevm.h | 3 + migration/socket.c | 4 +- migration/trace-events | 16 +- qapi-schema.json | 12 +- util/bitmap.c | 28 +++ 19 files changed, 1024 insertions(+), 80 deletions(-) -- 2.7.4
