migrating from an old OpenVZ (Centos 5) to new OpenVZ (Centos 6)
Also if migrating from 32-bit HN to 64-bit your RAM will probably be much bigger than it should be!
16x bigger

eg. 32bit HN:

             total       used       free     shared    buffers     cached
Mem:           256         16        239          0          0          0
-/+ buffers/cache:         16        239
Swap:            0          0          0


eg. 64bit HN with same config:


free -m
             total       used       free     shared    buffers     cached
Mem:          3906         42       3863          0          0         33
-/+ buffers/cache:          9       3897
Swap:        28614          0      28614



Error: required UB parameter swappages not set

 vzctl start 27000
Starting container...
Error: required UB parameter swappages not set
Saving VE layout 'simfs' to the VE config file
CT configuration saved to /etc/vz/conf/27000.conf
[root@server18 conf]# vzlist
Container(s) not found
[root@server18 conf]# vzctl start 27000
Starting container...
Error: required UB parameter swappages not set

vzctl set 27000 --swappages 0:unlimited --save

 vzctl set 27000 --swappages 0:unlimited --save
CT configuration saved to /etc/vz/conf/27000.conf
[root@server18 conf]# vzctl start 27000
Starting container...
stat(/vz/root/27000): No such file or directory
stat(/vz/root/27000): No such file or directory
stat(/vz/root/27000): No such file or directory
Initializing quota ...
Container is mounted
Adding IP address(es): 98.158.135.57 98.158.135.58
Setting CPU units: 1000
Setting devices
Container start in progress...