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From OpenVZ Wiki

Basic disk quota management:

To set disk space, run the following commands:

  vzctl set CTID --diskspace $SoftLimit$:$HardLimit$ --save

Example:

[host-node]# vzctl set 101 --diskspace 6G:7G --save

You could verify the space available with this command (ie : CTID =101)

[host-node]# vzctl exec 101 df -h

If you want remove disk quota:

DISK_QUOTA=no