Have You Checked Your Snapshots Lately?
Mon, 28 Mar 2022 21:44:38 -0000
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While this question may sound like a line from a scary movie, it is a serious one. Have you checked your snapshots lately?
In many regions of the world, seasonal time changes occur to maximize daylight hours and ensure darkness falls later in the day. This time shift is commonly known as Daylight Time, Daylight Saving Time, or Summer Time. Regions that observe this practice often change their clocks by 30 minutes or 1 hour depending on the region and time of year. At the time of this publication, multiple regions of the world have recently experienced a time change, while others occur shortly after this publication.
Some storage systems use Coordinated Universal Time (UTC) internally for logging purposes and to run scheduled jobs. Users typically create a schedule to run a task based on their local time, but the storage system then adjusts this time and runs the job based on the internal UTC time. When a regional change in time occurs, scheduled tasks that run on a UTC schedule “shift” when compared to wall clock time. Something that used to run at one time locally may seem to run at another, but only because the wall clock time in the region has changed. While this shift in schedule may not be an issue to most, with some customers the change is noticeable. Some have found that jobs such as snapshot creations and deletions are now occurring during other scheduled tasks such as backups or the snapshots are now missing the intended time, such as the beginning or end of the business workday.
To show what I mean, let’s use the Eastern US time zone as an example. Let’s say a user has created a rule to take a snapshot daily at 12:00 AM midnight local time. When Daylight Saving Time is not in observance, 12:00 AM US EST is equivalent to 5:00 AM UTC and the snapshot schedule will be configured to run at 5:00 AM UTC daily within the system. On Sunday, March 13, 2022 at 2:00 AM the regions of the United States that observe time changes altered their clocks 1 hour forward. The 2:00 AM hour instantaneously became 3:00 AM and an hour of time was seemingly lost.
As the figure below shows, a scheduled job that is configured to run at 5:00 AM UTC daily was taking snapshots at 12:00 AM local time but now runs at 1:00 AM local time, due to the UTC schedule of the storage system and the time change. A similar shift also occurs when the time change occurs again later in the year.
Within PowerStore, protection policies, snapshot rules, and replication rules are used to apply data protection to a resource. A snapshot rule is created to tell the system when to create a snapshot on a resource. The snapshot rule is then added to a protection policy, and the protection policy is assigned to a resource. When creating a snapshot rule, the user can either choose a fixed interval based on several hours or provide a specific time to create a snapshot.
For systems running PowerStoreOS 2.0 or later, when specifying the exact time to create a snapshot, the user also selects a time zone. The time zone drop-down list defaults to the user’s local time zone, but it can be adjusted if the system is physically located in a different time zone. Specifying a specific time with a time zone ensures that seasonal time changes do not impact the creation time of a snapshot.
For systems that were configured with a code prior to the 2.0 release and later upgraded, it is a great idea to review the snapshot rules and ensure that ones that are configured for a particular time of day are set correctly.
So, I ask again: Have you checked your snapshots lately?
Resources
Technical Documentation
- To learn more about the different features that PowerStore provides, see the PowerStore Info Hub.
- For additional information about PowerStore snapshots, see the PowerStore: Snapshots and Thin Clones white paper.
Demos and Hands-on Labs
- To see how PowerStore’s features work and integrate with different applications, see the PowerStore Demos YouTube playlist.
- To gain firsthand experience with PowerStore, see our many Hands-On Labs.
Author: Ryan Poulin