Forum Discussion

martinfrancis's avatar
martinfrancis
Level 3
2 years ago

instant snapshot (vxsnap) - multiple volumes - are the snapshots consistent between each other ?

Using tradition 3rd mirror break off snapshot using vxassist, when multiple volumes are specified on the snapshot command, all the volumes are split atomically and all the volumes are split exactly at same point in time.
This is a very very useful feature. vxassist man page clearly document this behavior (quoted below)

"The snapshot command accepts more than one volume. All the given volumes are split atomically (in a single transaction). That is, all the given volumes are relatively consistent as a result of taking the snapshot"

However when it comes to Instant snapshot (space optimized on full size instant snapshot), the man page for vxsnap does not describe this behavior when multiple volumes are snapped in one command.
1) Is the behavior of atomic split still same as legacy third mirror breakoff?
2) Are the volumes relatively consistent between each other?

 

atomic
instant snap
pit
point in time
vxsnap
  • sdighe1's avatar
    sdighe1
    2 years ago

    Hi Martin, 

    Your understanding is write about freeze and thaw. You can certainly recommend using vxsnap command for snapshots as I mentioned with multiple tuples of "vol/snap" entries. This works fine and we test this functional in our pipelines in almost daily fashion.

    vxsnap manpage is missing this information, which needs to be taken care. 

  • sdighe1's avatar
    sdighe1
    Level 3
    2 years ago

    Hi Martin, 

    Your understanding is write about freeze and thaw. You can certainly recommend using vxsnap command for snapshots as I mentioned with multiple tuples of "vol/snap" entries. This works fine and we test this functional in our pipelines in almost daily fashion.

    vxsnap manpage is missing this information, which needs to be taken care. 

  • martinfrancis's avatar
    martinfrancis
    Level 3
    2 years ago

    No it does not answer the question I posted.
    The link you posted basically says, on a volume type fsgen, a snapshot operation will flush the dirty buffers of vxfs file system so that the resulting snapshot will be consistent on the file system level.
    My question (to rephrase ) is what is the snapshot consistency behavior with vxsnap command when multiple volumes are snapshotted together as part of a single vxsnap command.
    In previous (legacy mode) of vxassist using third mirror breakoff - all the volumes that are snapshotted together were consistent between each other (i.e a point in time snapshot of the volumes). The man page of vxassist clearly documents this behavior.
    On the vxnap manpage or documentation, it doesn't whether a vxsnap operation on multiple volumes will create a snapshot which are consistent between each other .
    Can someone clarify how vxsnap behaves in case of multiple volume snapshot ?

  • sdighe1's avatar
    sdighe1
    Level 3
    2 years ago

    Hi Martin,

    With vxsnap command, if you take snapshot of multiple volumes in single invocation , all the file-systems involved in operation will be freezes one-by-one and then volumes snapshot is created.

    This way the snapshot are exact point-in-time copy with all volumes involved.

    Typically one can do this by specifying multiple tuples of "source=volname/snap=volname_snapid" or "source=volname/new=volname_snapid/cachesize=<>" or "source=volname/new=volname_snapid/cache=cache-obj-name" tuples in single command line.

    e.g. vxsnap -g dg1 make source=vol1/new=vol1_snap/cache=cache_obj_1 source=vol2/new=vol2_snap/cache=cache_obj_1 .....

     

     

    • martinfrancis's avatar
      martinfrancis
      Level 3
      2 years ago

      When you say the file systems will be frozen one by one, do you mean they will be frozen one by one but they will not be thawed until the snapshot of ALL the volumes are taken - correct?
      If this is how it is done, then it is great for my use case because then ALL the volumes will be consistent from a vxfs point of view between each other.

      The reason for my original query is because on vxassist manpage it clearly documents the legacy third mirror snapshot feature: Quoting from vxassist man page.
      "The snapshot command accepts more than one volume. All the given volumes are split atomically (in a single transaction). That is, all the given volumes are relatively consistent as a result of taking the snapshot"
      The key word is "Single transaction"
      https://sort.veritas.com/public/documents/vie/8.0/linux/manualpages/html/man/volume_manager/html/man1m/vxassist.1m.html
      On the vxsnap man page or vxsnap documenation, this key piece of information is missing