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On Sun, Jan 10, 2016 at 06:07:33PM +0800, Luo Gangyi wrote:
> Hi devs,
> 
> 
> Do you test the difference between within and without
> VIR_MIGRATE_NON_SHARED_INC ?
> 
> 
> When I add VIR_MIGRATE_NON_SHARED_INC in block_migration_flags in
> nova, nova block migration behaves more like a full copy of disk.  The
> disk in destination nodes is large(10GB+) and the process of live
> migration is slow.
> 
> 
> However, when I remove  VIR_MIGRATE_NON_SHARED_INC in
> block_migration_flags in nova, nova block migration behaves more like
> a incremental copy of disk.  The disk in destination nodes is
> small(10MB-) and the process of live migration is very fast.
> 
> 
> So I was confused about what VIR_MIGRATE_NON_SHARED_INC really means.

There are two flags that are related, that might be helpful to be clear
about: VIR_MIGRATE_NON_SHARED_DISK and VIR_MIGRATE_NON_SHARED_INC.

Below is an explanation[1] with example, by Eric Blake, from
libvirt-users list.

NB: It is talking in terms of `virsh` commands, where
'--copy-storage-all' means VIR_MIGRATE_NON_SHARED_DISK and
'--copy-storage-inc' means VIR_MIGRATE_NON_SHARED_INC:

    "If you use qcow2 backing chains for thin provisioning, as in:
    
    base.qcow2 <- active.qcow2
    
    then --copy-storage-all copies the entire disk over to the
    destination (both base.qcow2 and active.qcow2 contents), while
    --copy-storage-inc copies only active.qcow2 (and assumes you have
    manually copied base.qcow2 in some other means - since it is
    read-only, it won't change from the last migration, and there are
    some storage arrays where you can very efficiently copy files
    around).  Thus, --copy-storage-inc can be MUCH faster if
    active.qcow2 represents a small delta in relation to base.qcow2."


[1] https://www.redhat.com/archives/libvirt-users/2015-April/msg00172.html
    Semantics of "virsh migrate --copy-storage-all" vs.
    --copy-storage-inc



-- 
/kashyap