Commit 0c1f4036 authored by Dr. David Alan Gilbert's avatar Dr. David Alan Gilbert
Browse files

postcopy: Add doc about hugepages and postcopy 



Signed-off-by: default avatarDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: default avatarJuan Quintela <quintela@redhat.com>
Reviewed-by: default avatarLaurent Vivier <lvivier@redhat.com>
Message-Id: <20170224182844.32452-16-dgilbert@redhat.com>
Signed-off-by: default avatarDr. David Alan Gilbert <dgilbert@redhat.com>
parent 7e8cafb7

docs/migration.txt

+13 −0
Original line number Diff line number Diff line
@@ -540,3 +540,16 @@ 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.



=== Postcopy with hugepages ===



Postcopy now works with hugetlbfs backed memory:

  a) The linux kernel on the destination must support userfault on hugepages.

  b) The huge-page configuration on the source and destination VMs must be

     identical; i.e. RAMBlocks on both sides must use the same page size.

  c) Note that -mem-path /dev/hugepages  will fall back to allocating normal

     RAM if it doesn't have enough hugepages, triggering (b) to fail.

     Using -mem-prealloc enforces the allocation using hugepages.

  d) Care should be taken with the size of hugepage used; postcopy with 2MB

     hugepages works well, however 1GB hugepages are likely to be problematic

     since it takes ~1 second to transfer a 1GB hugepage across a 10Gbps link,

     and until the full page is transferred the destination thread is blocked.