[Qemu-devel] [PULL 01/57] Add postcopy documentation
From: "Dr. David Alan Gilbert"Signed-off-by: Dr. David Alan Gilbert Reviewed-by: Amit Shah Reviewed-by: Juan Quintela Signed-off-by: Juan Quintela --- docs/migration.txt | 191 + 1 file changed, 191 insertions(+) diff --git a/docs/migration.txt b/docs/migration.txt index f6df4be..fda8d61 100644 --- a/docs/migration.txt +++ b/docs/migration.txt @@ -291,3 +291,194 @@ save/send this state when we are in the middle of a pio operation (that is what ide_drive_pio_state_needed() checks). If DRQ_STAT is not enabled, the values on that fields are garbage and don't need to be sent. + += Return path = + +In most migration scenarios there is only a single data path that runs +from the source VM to the destination, typically along a single fd (although +possibly with another fd or similar for some fast way of throwing pages across). + +However, some uses need two way communication; in particular the Postcopy +destination needs to be able to request pages on demand from the source. + +For these scenarios there is a 'return path' from the destination to the source; +qemu_file_get_return_path(QEMUFile* fwdpath) gives the QEMUFile* for the return +path. + + Source side + Forward path - written by migration thread + Return path - opened by main thread, read by return-path thread + + Destination side + Forward path - read by main thread + Return path - opened by main thread, written by main thread AND postcopy +thread (protected by rp_mutex) + += Postcopy = +'Postcopy' migration is a way to deal with migrations that refuse to converge +(or take too long to converge) its plus side is that there is an upper bound on +the amount of migration traffic and time it takes, the down side is that during +the postcopy phase, a failure of *either* side or the network connection causes +the guest to be lost. + +In postcopy the destination CPUs are started before all the memory has been +transferred, and accesses to pages that are yet to be transferred cause +a fault that's translated by QEMU into a request to the source QEMU. + +Postcopy can be combined with precopy (i.e. normal migration) so that if precopy +doesn't finish in a given time the switch is made to postcopy. + +=== Enabling postcopy === + +To enable postcopy, issue this command on the monitor prior to the +start of migration: + +migrate_set_capability x-postcopy-ram on + +The normal commands are then used to start a migration, which is still +started in precopy mode. Issuing: + +migrate_start_postcopy + +will now cause the transition from precopy to postcopy. +It can be issued immediately after migration is started or any +time later on. Issuing it after the end of a migration is harmless. + +Note: During the postcopy phase, the bandwidth limits set using +migrate_set_speed is ignored (to avoid delaying requested pages that +the destination is waiting for). + +=== Postcopy device transfer === + +Loading of device data may cause the device emulation to access guest RAM +that may trigger faults that have to be resolved by the source, as such +the migration stream has to be able to respond with page data *during* the +device load, and hence the device data has to be read from the stream completely +before the device load begins to free the stream up. This is achieved by +'packaging' the device data into a blob that's read in one go. + +Source behaviour + +Until postcopy is entered the migration stream is identical to normal +precopy, except for the addition of a 'postcopy advise' command at +the beginning, to tell the destination that postcopy might happen. +When postcopy starts the source sends the page discard data and then +forms the 'package' containing: + + Command: 'postcopy listen' + The device state + A series of sections, identical to the precopy streams device state stream + containing everything except postcopiable devices (i.e. RAM) + Command: 'postcopy run' + +The 'package' is sent as the data part of a Command: 'CMD_PACKAGED', and the +contents are formatted in the same way as the main migration stream. + +During postcopy the source scans the list of dirty pages and sends them +to the destination without being requested (in much the same way as precopy), +however when a page request is received from the destination, the dirty page +scanning restarts from the requested location. This causes requested pages +to be sent quickly, and also causes pages directly after the requested page +to be sent quickly in the hope that those pages are likely to be used +by the destination soon. + +Destination behaviour + +Initially the destination looks the same as precopy, with a single thread +reading the migration stream; the 'postcopy advise' and 'discard' commands +are processed to change the way RAM is managed, but don't affect the stream
[Qemu-devel] [PULL 01/57] Add postcopy documentation
From: "Dr. David Alan Gilbert"Signed-off-by: Dr. David Alan Gilbert Reviewed-by: Amit Shah Reviewed-by: Juan Quintela Signed-off-by: Juan Quintela --- docs/migration.txt | 191 + 1 file changed, 191 insertions(+) diff --git a/docs/migration.txt b/docs/migration.txt index f6df4be..fda8d61 100644 --- a/docs/migration.txt +++ b/docs/migration.txt @@ -291,3 +291,194 @@ save/send this state when we are in the middle of a pio operation (that is what ide_drive_pio_state_needed() checks). If DRQ_STAT is not enabled, the values on that fields are garbage and don't need to be sent. + += Return path = + +In most migration scenarios there is only a single data path that runs +from the source VM to the destination, typically along a single fd (although +possibly with another fd or similar for some fast way of throwing pages across). + +However, some uses need two way communication; in particular the Postcopy +destination needs to be able to request pages on demand from the source. + +For these scenarios there is a 'return path' from the destination to the source; +qemu_file_get_return_path(QEMUFile* fwdpath) gives the QEMUFile* for the return +path. + + Source side + Forward path - written by migration thread + Return path - opened by main thread, read by return-path thread + + Destination side + Forward path - read by main thread + Return path - opened by main thread, written by main thread AND postcopy +thread (protected by rp_mutex) + += Postcopy = +'Postcopy' migration is a way to deal with migrations that refuse to converge +(or take too long to converge) its plus side is that there is an upper bound on +the amount of migration traffic and time it takes, the down side is that during +the postcopy phase, a failure of *either* side or the network connection causes +the guest to be lost. + +In postcopy the destination CPUs are started before all the memory has been +transferred, and accesses to pages that are yet to be transferred cause +a fault that's translated by QEMU into a request to the source QEMU. + +Postcopy can be combined with precopy (i.e. normal migration) so that if precopy +doesn't finish in a given time the switch is made to postcopy. + +=== Enabling postcopy === + +To enable postcopy, issue this command on the monitor prior to the +start of migration: + +migrate_set_capability x-postcopy-ram on + +The normal commands are then used to start a migration, which is still +started in precopy mode. Issuing: + +migrate_start_postcopy + +will now cause the transition from precopy to postcopy. +It can be issued immediately after migration is started or any +time later on. Issuing it after the end of a migration is harmless. + +Note: During the postcopy phase, the bandwidth limits set using +migrate_set_speed is ignored (to avoid delaying requested pages that +the destination is waiting for). + +=== Postcopy device transfer === + +Loading of device data may cause the device emulation to access guest RAM +that may trigger faults that have to be resolved by the source, as such +the migration stream has to be able to respond with page data *during* the +device load, and hence the device data has to be read from the stream completely +before the device load begins to free the stream up. This is achieved by +'packaging' the device data into a blob that's read in one go. + +Source behaviour + +Until postcopy is entered the migration stream is identical to normal +precopy, except for the addition of a 'postcopy advise' command at +the beginning, to tell the destination that postcopy might happen. +When postcopy starts the source sends the page discard data and then +forms the 'package' containing: + + Command: 'postcopy listen' + The device state + A series of sections, identical to the precopy streams device state stream + containing everything except postcopiable devices (i.e. RAM) + Command: 'postcopy run' + +The 'package' is sent as the data part of a Command: 'CMD_PACKAGED', and the +contents are formatted in the same way as the main migration stream. + +During postcopy the source scans the list of dirty pages and sends them +to the destination without being requested (in much the same way as precopy), +however when a page request is received from the destination, the dirty page +scanning restarts from the requested location. This causes requested pages +to be sent quickly, and also causes pages directly after the requested page +to be sent quickly in the hope that those pages are likely to be used +by the destination soon. + +Destination behaviour + +Initially the destination looks the same as precopy, with a single thread +reading the migration stream; the 'postcopy advise' and 'discard' commands +are processed to change the way RAM is managed, but don't affect the stream