devel/migration/postcopy.rst

8 (or take too long to converge) its plus side is that there is an upper bound on
22 To enable postcopy, issue this command on the monitor (both source and
25 ``migrate_set_capability postcopy-ram on``
34 time later on.  Issuing it after the end of a migration is harmless.
39 separately for each vCPU. These values are calculated on destination
41 command on destination monitor:
43 ``migrate_set_capability postcopy-blocktime on``
76     Entered on receipt of the first 'discard' command; prior to
80     have discards on them to be broken.
88     on processing the blob.  With this thread able to process page
90     any access to missing pages (on Linux using the 'userfault'
98     now carries on as it would for normal precopy migration
166 - On receipt of ``CMD_PACKAGED`` (1)
173 - On receipt of 'postcopy listen' - 3 -(i.e. the 1st command in the package)
176    while the main thread carries on loading the package.   It loads normal
179    threads device load to carry on.
185    is no longer used by migration, while the listen thread carries on servicing
204 Comparing to precopy, postcopy is special on error handlings.  When any
207 QEMU instances.  On the other hand, when issue happens QEMU on both sides
218     'migrate-recover' on destination node, preparing for a resume.
220   - On source host, the admin can continue the interrupted postcopy
234     postcopy migration on both sides and go back to **POSTCOPY_ACTIVE**
240 However, if any of the missing pages got accessed on destination VM, the VM
254   a) The linux kernel on the destination must support userfault on hugepages.
255   b) The huge-page configuration on the source and destination VMs must be
256      identical; i.e. RAMBlocks on both sides must use the same page size.
269 processes that share memory and from QEMU. There are restrictions on the type of
272 The Linux kernel userfault support works on ``/dev/shm`` memory and on ``hugetlbfs``
285 fault-thread and page requests are made on behalf of the client by QEMU.
286 QEMU performs 'wake' operations on the client's userfaultfd to allow it