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Add postcopy documentation 

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
master
Dr. David Alan Gilbert 2015-11-05 18:10:28 +00:00 committed by Juan Quintela
parent a8b4f9585a
commit 2bfdd1c8a6
1 changed files with 191 additions and 0 deletions
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docs/migration.txt
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@ -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
processing.
------------------------------------------------------------------------------
1 2 3 4 5 6 7
main -----DISCARD-CMD_PACKAGED ( LISTEN DEVICE DEVICE DEVICE RUN )
thread | |
| (page request)
| \___
v \
listen thread: --- page -- page -- page -- page -- page --
a b c
------------------------------------------------------------------------------
On receipt of CMD_PACKAGED (1)
All the data associated with the package - the ( ... ) section in the
diagram - is read into memory (into a QEMUSizedBuffer), and the main thread
recurses into qemu_loadvm_state_main to process the contents of the package (2)
which contains commands (3,6) and devices (4...)
On receipt of 'postcopy listen' - 3 -(i.e. the 1st command in the package)
a new thread (a) is started that takes over servicing the migration stream,
while the main thread carries on loading the package. It loads normal
background page data (b) but if during a device load a fault happens (5) the
returned page (c) is loaded by the listen thread allowing the main threads
device load to carry on.
The last thing in the CMD_PACKAGED is a 'RUN' command (6) letting the destination
CPUs start running.
At the end of the CMD_PACKAGED (7) the main thread returns to normal running behaviour
and is no longer used by migration, while the listen thread carries
on servicing page data until the end of migration.
=== Postcopy states ===
Postcopy moves through a series of states (see postcopy_state) from
ADVISE->DISCARD->LISTEN->RUNNING->END
Advise: Set at the start of migration if postcopy is enabled, even
if it hasn't had the start command; here the destination
checks that its OS has the support needed for postcopy, and performs
setup to ensure the RAM mappings are suitable for later postcopy.
The destination will fail early in migration at this point if the
required OS support is not present.
(Triggered by reception of POSTCOPY_ADVISE command)
Discard: Entered on receipt of the first 'discard' command; prior to
the first Discard being performed, hugepages are switched off
(using madvise) to ensure that no new huge pages are created
during the postcopy phase, and to cause any huge pages that
have discards on them to be broken.
Listen: The first command in the package, POSTCOPY_LISTEN, switches
the destination state to Listen, and starts a new thread
(the 'listen thread') which takes over the job of receiving
pages off the migration stream, while the main thread carries
on processing the blob. With this thread able to process page
reception, the destination now 'sensitises' the RAM to detect
any access to missing pages (on Linux using the 'userfault'
system).
Running: POSTCOPY_RUN causes the destination to synchronise all
state and start the CPUs and IO devices running. The main
thread now finishes processing the migration package and
now carries on as it would for normal precopy migration
(although it can't do the cleanup it would do as it
finishes a normal migration).
End: The listen thread can now quit, and perform the cleanup of migration
state, the migration is now complete.
=== Source side page maps ===
The source side keeps two bitmaps during postcopy; 'the migration bitmap'
and 'unsent map'. The 'migration bitmap' is basically the same as in
the precopy case, and holds a bit to indicate that page is 'dirty' -
i.e. needs sending. During the precopy phase this is updated as the CPU
dirties pages, however during postcopy the CPUs are stopped and nothing
should dirty anything any more.
The 'unsent map' is used for the transition to postcopy. It is a bitmap that
has a bit cleared whenever a page is sent to the destination, however during
the transition to postcopy mode it is combined with the migration bitmap
to form a set of pages that:
a) Have been sent but then redirtied (which must be discarded)
b) Have not yet been sent - which also must be discarded to cause any
transparent huge pages built during precopy to be broken.
Note that the contents of the unsentmap are sacrificed during the calculation
of the discard set and thus aren't valid once in postcopy. The dirtymap
is still valid and is used to ensure that no page is sent more than once. Any
request for a page that has already been sent is ignored. Duplicate requests
such as this can happen as a page is sent at about the same time the
destination accesses it.