There is a significant distinction between VMware and Storage Snapshot, Alex. Let’s explore each one in sequence.
A snapshot is a real-time copy of the production LUN. The original data on the Production LUN at the time of creation will be referenced by the snapshot.
After the snapshot has been taken, any further data changed to the Production LUN is written to the same base LUN.
In order to retain the consistency of the snapshot data, the original data is stored in the same pool as the modified data.
This relies on the storage methods used by the storage manufacturer. Few of them employ Copy on Write (COW), while some of them use Relocate on Write (ROW)
The newly formed snapshot refers to modified and updated data.
According to the picture, the pointers for the next snapshot will be for EFCD rather than AB.
Old data is often kept in a separate store on the same pool. Again, it depends on the manufacturer of the storage.
Few Points that we need to remember when dealing with Storage snapshots are:
- Storage Snapshots by default are in R/O Mode
- The Production Lun remains R/W at all time
- New Data gets updated on Production LUN and not on Snapshot
- Deleting Storage Snapshot does not required to meagre any data to production LUN
- Storage R/O Snapshots can be discarded without any impact on Production LUN
VMware Snapshot is taken on the Virtual Machine.
Initial Snapshot gets created with 16MB of Size.
VM snapshots are always Thin Provisioned.
Based on the VMFS and the Size of the virtual disk the format of the snapshots gets very.
Default snapshot format on VMFS5 is VMFSSparse. But if the Base disk is more than 2TB of size the Snapshot gets created in SEsparse i.e. Space Efficient Sparse format.
On VMFS6 snapshot format is set to SEsparse by default and it is not dependent on base disk size.
VM snapshots are dependent on each other and they form chain dependency on each other.
For example First Snapshot will point to Base disk as it’s parent disk. Second Snapshot will point to First Snapshot as it’s Parent Disk and so on so forth.
A beautiful KB Overview of virtual machine snapshots in vSphere (1015180) talks about various aspects of the VMware snapshot.
Points that we need to remember when dealing with Storage snapshots are:
- VM Snapshots are R/W by default
- Post creating Snapshot Base/ Parent Disk becomes R/O
- All the new Writes goes on Current snapshot
- Any Modification required on existing data VMware uses Copy on Write Mechanism
- The Block gets copied to snapshot before it gets modified, as Base/ Parent Disk is in R/O mode
- Deleting VMware snapshots take more time as it has to merge all the new Data to Parent Disk
- Hence it is recommended to follow Snapshot Best Practices
I hope this gives you enough understanding to differentiate VMware and Storage Snapshots.