0

Google Cloud Platform

From the Google Cloud docs: https://cloud.google.com/compute/disks-image-pricing#persistent_disk_snapshots

When you delete a complete or incremental snapshot, some of its data may move to the next incremental snapshot in the snapshot chain. This additional data increases the storage cost because you are using more space in the storage system.

In the Google Cloud docs they provide an example of how incremental snapshots are stored: https://cloud.google.com/compute/docs/disks/create-snapshots

Snapshot 3 contains any new or changed data since snapshot 2 but won't contain any unchanged data from snapshot 1 or 2. Instead, snapshot 3 contains references to blocks in snapshot 1 and snapshot 2 for any unchanged data.

Amazon Web Services

However, to better help illustrate the AWS docs have an example scenarios of deleting an incremental snapshot: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-deleting-snapshot.html

Deleting a snapshot might not reduce your organization's data storage costs. Other snapshots might reference that snapshot's data, and referenced data is always preserved. If you delete a snapshot containing data being used by a later snapshot, costs associated with the referenced data are allocated to the later snapshot.

AWS states that deleting might not reduce storage usage & costs. Although it doesn't say storage & cost would increase.

Why and how would deleting a snapshot in GCE increase storage space & cost?

Rob Olmos
  • 2,220
  • 1
  • 15
  • 25
  • My initial hunch is that deleting a snapshot could cause GCE's snapshot storage system to convert one of the remaining incremental snapshots into a full snapshot (and thus increase storage overall), but that isn't spelled out from what I could find. – Rob Olmos Mar 03 '20 at 23:23

1 Answers1

1

It results from incremental nature of snapshots. Let's consider we have three snapshots:

  • snapshot1: has data blocks A, B, C
  • snapshot2: block D was added, block C removed. Due to incremental nature this snapshot does not store A and B.
  • snapshot3: block E added, and info about this block only is stored here.

Whichever snapshot you delete, the remaining ones will have all necessary info to restore full data on that moment. For example, if you delete snapshot2, info about added block D and removed block C and will be assigned to snapshot3.

Thus, you'll free no storage.

Putnik
  • 2,095
  • 3
  • 23
  • 40
  • Thank you for your response. The lack of any storage reduction is explained in the AWS docs & referenced. However, the question is regarding GCE's claim that "This additional data increases the storage cost...", whereas the way I understand it and your answer says there is no data increase or reduction. – Rob Olmos Mar 03 '20 at 23:21
  • 1
    @RobOlmos My apologies. I believe here's a typo, but I found the following here https://cloud.google.com/compute/docs/disks/snapshot-best-practices and https://cloud.google.com/compute/docs/disks/restore-and-delete-snapshots : "Important: Snapshots are incremental by default to avoid billing you for redundant data and to minimize use of storage space. However, in order to ensure the reliability of snapshot history, **a snapshot might occasionally capture a full image of the disk**." So, it may be the case. Have you asked support? – Putnik Mar 04 '20 at 06:15
  • All good. It's an odd situation so I understand the confusion. I've reached out to them on Twitter. They've forwarded it to their community support team. – Rob Olmos Mar 04 '20 at 19:03