mirror of
https://github.com/Motorhead1991/qemu.git
synced 2025-08-02 15:23:53 -06:00
8b2395970a
Migration bandwidth is a very important value to live migration. It's because it's one of the major factors that we'll make decision on when to switchover to destination in a precopy process. This value is currently estimated by QEMU during the whole live migration process by monitoring how fast we were sending the data. This can be the most accurate bandwidth if in the ideal world, where we're always feeding unlimited data to the migration channel, and then it'll be limited to the bandwidth that is available. However in reality it may be very different, e.g., over a 10Gbps network we can see query-migrate showing migration bandwidth of only a few tens of MB/s just because there are plenty of other things the migration thread might be doing. For example, the migration thread can be busy scanning zero pages, or it can be fetching dirty bitmap from other external dirty sources (like vhost or KVM). It means we may not be pushing data as much as possible to migration channel, so the bandwidth estimated from "how many data we sent in the channel" can be dramatically inaccurate sometimes. With that, the decision to switchover will be affected, by assuming that we may not be able to switchover at all with such a low bandwidth, but in reality we can. The migration may not even converge at all with the downtime specified, with that wrong estimation of bandwidth, keeping iterations forever with a low estimation of bandwidth. The issue is QEMU itself may not be able to avoid those uncertainties on measuing the real "available migration bandwidth". At least not something I can think of so far. One way to fix this is when the user is fully aware of the available bandwidth, then we can allow the user to help providing an accurate value. For example, if the user has a dedicated channel of 10Gbps for migration for this specific VM, the user can specify this bandwidth so QEMU can always do the calculation based on this fact, trusting the user as long as specified. It may not be the exact bandwidth when switching over (in which case qemu will push migration data as fast as possible), but much better than QEMU trying to wildly guess, especially when very wrong. A new parameter "avail-switchover-bandwidth" is introduced just for this. So when the user specified this parameter, instead of trusting the estimated value from QEMU itself (based on the QEMUFile send speed), it trusts the user more by using this value to decide when to switchover, assuming that we'll have such bandwidth available then. Note that specifying this value will not throttle the bandwidth for switchover yet, so QEMU will always use the full bandwidth possible for sending switchover data, assuming that should always be the most important way to use the network at that time. This can resolve issues like "unconvergence migration" which is caused by hilarious low "migration bandwidth" detected for whatever reason. Reported-by: Zhiyi Guo <zhguo@redhat.com> Reviewed-by: Joao Martins <joao.m.martins@oracle.com> Reviewed-by: Juan Quintela <quintela@redhat.com> Signed-off-by: Peter Xu <peterx@redhat.com> Signed-off-by: Juan Quintela <quintela@redhat.com> Message-ID: <20231010221922.40638-1-peterx@redhat.com> |
||
|---|---|---|
| .. | ||
| block-dirty-bitmap.c | ||
| block.c | ||
| block.h | ||
| channel-block.c | ||
| channel-block.h | ||
| channel.c | ||
| channel.h | ||
| colo-failover.c | ||
| colo.c | ||
| dirtyrate.c | ||
| dirtyrate.h | ||
| exec.c | ||
| exec.h | ||
| fd.c | ||
| fd.h | ||
| file.c | ||
| file.h | ||
| global_state.c | ||
| meson.build | ||
| migration-hmp-cmds.c | ||
| migration-stats.c | ||
| migration-stats.h | ||
| migration.c | ||
| migration.h | ||
| multifd-zlib.c | ||
| multifd-zstd.c | ||
| multifd.c | ||
| multifd.h | ||
| options.c | ||
| options.h | ||
| page_cache.c | ||
| page_cache.h | ||
| postcopy-ram.c | ||
| postcopy-ram.h | ||
| qemu-file.c | ||
| qemu-file.h | ||
| ram-compress.c | ||
| ram-compress.h | ||
| ram.c | ||
| ram.h | ||
| rdma.c | ||
| rdma.h | ||
| savevm.c | ||
| savevm.h | ||
| socket.c | ||
| socket.h | ||
| target.c | ||
| threadinfo.c | ||
| threadinfo.h | ||
| tls.c | ||
| tls.h | ||
| trace-events | ||
| trace.h | ||
| vmstate-types.c | ||
| vmstate.c | ||
| xbzrle.c | ||
| xbzrle.h | ||
| yank_functions.c | ||
| yank_functions.h | ||