EMC Storage Product Knowledge Sharing

Most of user asked me what are basic differences between EMC Clone/Mirror/Snapshot? This is really confusing terminology because most of things will be same logically.Only thing change that is implementation and purpose of uses. I am trying to write basic and common differences:

1) A clone is a full copy of data in a source LUN. A snapshot is a point-in time "virtual" copy that does not occupy any disk space.
2) A snapshot can be created or destroyed in seconds, unlike a clone or mirror. A clone, for example, can take minutes to hours to create depending on the size of the source LUN.
3) A clone or mirror requires exactly the same amount of disk space as the source LUN. A snapshot cache LUN generally requires approximately 10% to 20% of the source LUN size.
4) A clone is an excellent on-array solution that enables you to recover from a data corruption issue. Mirrors are designed for off-site data recovery.
5) A clone is typically fractured after it is synchronized while a mirror is not fractured but instead is actively and continuously being synchronized to any changes on the source LUN.
Clones and mirrors are inaccessible to the host until they are fractured. Clones can be easily resynchronized in either direction. This capability is not easily implemented with mirrors.
Restoring data after a source LUN failure is instantaneous using clones after a reverse synchronization is initialized. Restore time from a snapshot depends on the time it takes to restore from the network or from a backup tape.
Once a clone is fractured, there is no performance impact (that is, performance is comparable to the performance experienced with a conventional LUN). For snapshots, the performance impact is above average and constant due to copy on first write (COFW).

I left one more term EMC BCV(Business Continuity Volume). It is totally different concept thought. I will try to cover in upcoming post though I have discussed about EMC BCV in my older post. But it is more or less cloning only only implementation change.

Handling BCV and Clone Disk on Veritas Volume Manager.

Get Output of Vxdisk List command

# vxdisk list

Vxdisk list command showing that some disks are marked with the udid_mismatch flag.

Write a New UUID to Disk

You can use the following command to update the unique disk identifier (UDID) for one or more disks:

# vxdisk [-f] [-g diskgroup] updateudid disk ...

Note : The -f option must be specified if VxVM has not raised the udid_mismatch flag for a disk.

Importing a disk group containing cloned/BCV disks

You can then import the cloned disks by specifying the -ouseclonedev=on option to the vxdgimport command, as shown in this example:

# vxdg -o useclonedev=on [-o updateid] import mydg ( Group Name )

Note: This form of the command allows only cloned disks to be imported. All non-cloned disks remain unimported. .) However, the import fails if multiple copies of one or more cloned disks exist.

You can use the following command to tag all the disks in the disk group that are to be imported:

# vxdisk [-g diskgroup] settag tagname disk ...

where tagname is a string of up to 128 characters, not including spaces or tabs.
For example, the following command sets the tag, my_tagged_disks, on several disks that are to be imported together:

You can use the following command to ensure that a copy of the metadata is placed on a disk, regardless of the placement policy for the disk group:

# vxdisk [-g diskgroup] set disk keepmeta=always

Alternatively, use the following command to place a copy of the configuration
database and kernel log on all disks in a disk group that share a given tag:

# vxdg [-g diskgroup] set tagmeta=on tag=tagname nconfig=all nlog=all

To check which disks in a disk group contain copies of this configuration information, use the vxdglistmeta command:

# vxdg [-q] listmeta diskgroup

The tagged disks in the disk group may be imported by specifying the tag to the vxdgimport command in addition to the -ouseclonedev=on option:

# vxdg -o useclonedev=on -o tag=my_tagged_disks import mydg

If you have already imported the non-cloned disks in a disk group, you can use
the -n and -t option to specify a temporary name for the disk group containing
the cloned disks:

# vxdg -t -n clonedg -o useclonedev=on -o tag=my_tagged_disks import mydg