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5.4.14.2. Extending a Mirrored Volume

As of the Red Hat Enterprise Linux 6.3 release, it is possible to grow mirrored logical volumes with the lvextend command without performing a synchronization of the new mirror regions.
If you specify the --nosync option when you create a mirrored logical volume with the lvcreate command, the mirror regions are not synchronized when the mirror is created, as described in Section 5.4.3, “Creating Mirrored Volumes”. If you later extend a mirror that you have created with the --nosync option, the mirror extensions are not synchronized at that time, either.
You can determine whether an existing logical volume was created with the --nosync option by using the lvs command to display the volume's attributes. A logical volume will have an attribute bit 1 of "M" if it is a mirrored volume that was created without an initial synchronization, and it will have an attribute bit 1 of "m" if it was created with initial synchronization.
The following command displays the attributes of a mirrored logical volume named lv that was created without initial synchronization, showing attribute bit 1 as "M". Attribute bit 7 is "m", indicating a target type of mirror. For information on the meaning of the attribute bits, see Table 5.4, “lvs Display Fields”. 
# lvs vg
  LV   VG   Attr     LSize Pool Origin Snap%  Move Log     Copy%  Convert
  lv   vg   Mwi-a-m- 5.00g                         lv_mlog 100.00
If you grow this mirrored logical volume with the lvextend command, the mirror extension will not be resynchronized.
If you created a mirrored logical volume without specifying the --nosync option of the lvcreate command, you can grow the logical volume without resynchronizing the mirror by specifying the --nosync option of the lvextend command.
The following example extends a logical volume that was created without the --nosync option, indicated that the mirror was synchronized when it was created. This example, however, specifies that the mirror not be synchronized when the volume is extended. Note that the volume has an attribute of "m", but after executing the lvextend commmand with the --nosync option the volume has an attribute of "M". 
# lvs vg
  LV   VG   Attr     LSize  Pool Origin Snap%  Move Log     Copy%  Convert
  lv   vg   mwi-a-m- 20.00m                         lv_mlog 100.00        
# lvextend -L +5G vg/lv --nosync
  Extending 2 mirror images.
  Extending logical volume lv to 5.02 GiB
  Logical volume lv successfully resized
# lvs vg
  LV   VG   Attr     LSize Pool Origin Snap%  Move Log     Copy%  Convert
  lv   vg   Mwi-a-m- 5.02g                         lv_mlog 100.00
If a mirror is inactive, it will not automatically skip synchronization when you extend the mirror, even if you create the mirror with the --nosync option specified. Instead, you will be prompted whether to do a full resync of the extended portion of the logical volume.

Note

If a mirror is performing recovery, you cannot extend the mirrored logical volume if you created or extended the volume with the --nosync option specified. If you did not specify the --nosync option, however, you can extend the mirror while it is recovering.