Before creating a snapshot schedule, choose a start time and a snapshotfrequency. The start time is when you want the snapshot schedule to begin.The frequency is defined as hourly, daily, or weekly.

A snapshot schedule begins within the hour of the specified start time.For example, if the scheduled start time is at 12:00, snapshotting startsanytime between 12:00 and 12:59. The schedule uses the UTC time standard.

Snapshot schedules are impacted by daylight savings time (DST) shifts.At the beginning and end of DST in the US, you might see the start times ofyour snapshot schedule jobs move forward or backward by an hour, respectively,regardless of whether your local timezone uses DST.

• Setting an hourly schedule: To set up an hourly schedule, choose the starttime and choose a snapshot interval using an integer between 1 and 23. Asnapshot interval defines how often the snapshot is taken. When setting theinterval, choose a number that evenly divides 24 (1, 2, 3, 4, 6, 8, or 12) sothat the snapshot starts at the same time interval every day. For example,setting a start time of 9:00 with an interval of 6 indicates the snapshotschedule starts between 9:00 and 10:00 with a snapshot interval of every 6hours.

• Setting a daily schedule: Set the frequency to daily for the snapshot tooccur once a day. For example, you can set a snapshot scheduleso that a snapshot is generated every day at 9:00.

• Setting a weekly schedule: Set the frequency to weekly for snapshottingon one or more days every week. For example, a start time of9:00 PM with a weekly schedule set to Monday indicates thesnapshot schedule occurs at 9:00 PM on Monday every week.You can also set a weekly schedule that generates snapshots on differentdays of the week. For example, you might create a schedule for every week onMonday at 14:00 (2:00 PM) and Thursday at 10:00 AM.

Names for scheduled snapshots

Snapshots captured as a part of your snapshot schedule have automaticallygenerated names. The snapshot names are generated according to the followingpattern:

[disk-name-prefix]-[scope-name]-[timestamp]-[random-chars]

• disk-name-prefix: The name of the source disk. This field can be up to 20characters.
• scope-name: The name of the zone or region of the source disk.
• timestamp: The UTC creation timestamp inYYYYMMDDHHMMSS format.
• random-chars: Randomly generated alphanumeric characters to avoid namingconflicts with other snapshots.

Snapshot retention policy

You can specify how long Compute Engine retains the automatically createdsnapshots by setting a snapshot retention policy.

As a best practice, set up a retention policy to minimize snapshotstorage costs. If you don't set a retention policy, Compute Engineretains the snapshots indefinitely. You incur storage costs for those snapshots until youmanually delete them.

If you choose to set up a snapshot retention policy, you must do so when you createthe snapshot schedule.

If you don't set a retention policy, all your auto-generated snapshotswill be retained indefinitely. You will incur storage costs for these snapshotsuntil you delete them manually.

Automatic snapshot deletion

You can specify a maximum retention time period for your snapshot.The maximum retention time periodspecifies the maximum number of days for which Compute Engine retainsyour disk's snapshot after its creation. After the maximum retention time periodis over, Compute Engine attempts to automatically delete that disk snapshot.Note that Compute Engine completes the auto-deletion of that snapshotonly if you have a newer snapshot of the disk available. If you don't have anewer snapshot available, then Compute Engine continues to retain thatsnapshot until you create a newer snapshot.

For example, if you set its value as3, Compute Engine retains thesnapshot for a maximum of three days before attempting to delete it. After threedays, Compute Engine checks if there is a newer snapshot created foryour disk. If such a snapshot is available, Compute Engine automaticallydeletes your snapshot.

• The snapshot retention policy applies only to automatic snapshots, not tothose you created manually.
• You cannot apply a retention policy retroactively. If you create a snapshotschedule without a retention policy and then switch to a snapshot schedulewith a retention policy, the retention policy will only apply to thenew snapshots.
• Regardless of the retention policy, you can always manually deleteautomatically generated and manual snapshots.
• If you want Compute Engine to automatically delete your disk snapshotafter a specified maximum retention period, then you must have a newersnapshot created for that disk before the auto-deletion time.
• If you set a maximum retention time for your disk snapshot, ensurethat you use a minimum value of 1 day.

Source disk deletion rule

When you create a snapshot schedule, you can also set a source diskdeletion rule. The source disk deletion rule controls what happens to yoursnapshots if the source disk is deleted. By default, if your source disk isdeleted, snapshots automatically generated from your snapshot schedule areretained permanently unless you delete them manually, regardless of the originalretention policy you set, unless you specify otherwise.

Alternatively, you can set the rule so that your snapshot retention policy isapplied to your snapshots if the source disk is deleted. For example, you mightapply a retention policy that keeps your snapshots for one week after the sourcedisk is deleted.

For example, assume you created a snapshot schedule with a retention policy of 3days but choose not to apply the retention policy on source disk deletion. Afteryou apply the policy to your Persistent Disk or Hyperdisk,Compute Engine automatically generates snapshots but will alsoautomatically delete snapshots after 3 days, per the retention policy. However,if you then delete the source disk, the retention policy no longer appliesand all existing snapshots that were created by this schedule are retainedindefinitely.

Scheduling snapshots for Windows instances