By default, the vxassist command creates a snapshot volume with the same layout as the original volume.  But there are times when it is not desired or practical to create snapshot volumes with the same volume layout as the original volume.

The following example shows how to create a snapshot volume with a different volume layout.

In this example, vol6 is the volume that needs to create a snapshot volume.  This volume (vol6) is a two-column striped volume (see vxprint output below):


root@phoenix / [306]:vxprint -htg robertdg vol6

...

dg robertdg    default      default  80000    1053001473.1156.milan.veritas.com

dm disk01       c1t3d0s2     sliced  3535     17914356 -
dm disk02       c1t4d0s2     sliced  3535     17914356 -
dm disk03       c1t8d0s2     sliced  3535     17914356 -
dm disk04       c1t10d0s2    sliced  2843     17915304 -
dm disk05       c1t2d0s2     sliced  3535     17914356-

v  vol6        -            ENABLED  ACTIVE  204800  SELECT    vol6-01  fsgen
pl vol6-01      vol6        ENABLED  ACTIVE   206756  STRIPE    2/128    RW
sd disk05-01    vol6-01      disk05  0        103332   0/0      c1t2d0   ENA
sd disk04-03    vol6-01      disk04  1101576  103332   1/0      c1t10d0  ENA

The above output shows the size of the volume vol6 is 100MB and the volume layout is 2-column stripe with stripe unit128.

Here the goal is to create a snapshot volume using only one disk:disk05.  That is, the snapshot volume layout will not be striped.  

The first step is to make sure that disk05 has sufficient free disk space for creating a new sub-disk of size 206756 sectors for a snapshot volume.  To check for free disk space, run the following command:

root@phoenix / [321]:vxdg -g robertdg free disk05
DISK        DEVICE      TAG          OFFSET    LENGTH    FLAGS
disk05      c1t2d0s2     c1t2d0       0        17914356  -

The above vxdg command output shows that disk05 has sufficient free disk space for creating a sub-disk of size 100MB.

If you run the following command tocreate a snapshot volume:

root@phoenix /[320]:vxassist -g robertdg snapstart vol6 alloc="disk05"

it fails with the following error:

vxvm:vxassist: ERROR: Cannot allocate space to mirror 204800 block volume

This happens because, by default, the vxassist snapstart command uses the volume layout of the original volume (vol6) to create the snapshot volume. To make a snapshot volume with a volume layout different than the original volume, one more vxassist command attribute called "layout" needs to be used.  

The following command creates a snapshot volume with a concat plex layout:

root@phoenix /[322]:vxassist -g robertdg snapstart vol6 layout=concat alloc="disk05"

The vxprint -htg output shows the snapshot volume plex has been created with the layout as expected:

root@phoenix /[323]:vxprint -htg robertdg vol6

...

v  vol6        -            ENABLED  ACTIVE  204800  SELECT    vol6-01  fsgen
plvol6-01      vol6        ENABLED  ACTIVE   206756  STRIPE    2/128    RW
sddisk01-04    vol6-01      disk01  1282644  103332   0/0       c1t3d0  ENA
sddisk02-04    vol6-01      disk02  2126364  103332   1/0       c1t4d0  ENA
plvol6-02      vol6        ENABLED  SNAPDONE 205716  CONCAT    -        WO
sddisk05-01    vol6-02      disk05  0        205716   0        c1t2d0   ENA

The next step is to run the vxassist snapshot command below to separate the plex vol6-02 from vol6 so a new snapshot volume is created.

root@phoenix / [325]:vxassist -g robertdg snapshot vol6

The vxprint -htg output below shows that there are  two volumes(with different volume layouts) containing the same data (at the time the snap was taken):

root@phoenix / [326]:vxprint -htg robertdg vol6 SNAP-vol6
...

v  SNAP-vol6    -            ENABLED  ACTIVE  204800   ROUND    -        fsgen
plvol6-02      SNAP-vol6    ENABLED  ACTIVE  205716  CONCAT    -        RW
sddisk05-01    vol6-02      disk05  0        205716   0        c1t2d0  ENA

v  vol6        -            ENABLED  ACTIVE  204800  SELECT    vol6-01  fsgen
plvol6-01      vol6        ENABLED  ACTIVE   206756  STRIPE    2/128    RW
sddisk01-04    vol6-01      disk01  1282644  103332   0/0       c1t3d0  ENA
sddisk02-04    vol6-01      disk02  2126364  103332   1/0       c1t4d0  ENA

Note if the layout of the volume to be snapshot is Raid5,a special license called Raid5 Snapshot is required.

To check if you are licensed for RAID5 snapshot, run vxdctl license and check for licensed features.

Example:

#vxdctl license | grep -i raid
RAID-5
RAID-5 Snapshot

or

run vxlicrep and look forRAID5SNAP.

Example:

#vxlicrep | grep -i raid
 RAID5SNAP                          = Enabled