Snapshot Retention Explanation - for a 7 year old

General Topics
1

I had this conversation with ChatGPT. It covers a lot of bases. Will be of value to some as they try to wrap their head around kopia policy set (keep durations)

You:

I’m trying to understand how the snapshot retention works with kopia

Can you explain that in terms a 7 year old human would understand using values of your choice in place of X.

kopia policy set --keep-latest=X --keep-hourly=X --keep-daily=X --keep-weekly=X --keep-monthly=X --keep-annual=X

ChatGPT:

Sure! Imagine you have a special photo album that magically updates itself every time you take a new picture. This photo album has different rules for how long it keeps your pictures. Let’s break it down:

  1. –keep-latest=X: This means “Keep the last X pictures.” Imagine you just took X pictures. The album will always keep the last X pictures you took, even if you take new ones.

  2. –keep-hourly=X: This means “Keep one picture from each hour for the last X hours.” Imagine you took a picture every hour. The album will keep one picture from each hour, but only for the last X hours.

  3. –keep-daily=X: This means “Keep one picture from each day for the last X days.” Imagine you take a picture every day. The album will keep one picture from each day, but only for the last X days.

  4. –keep-weekly=X: This means “Keep one picture from each week for the last X weeks.” Imagine you take a picture every week. The album will keep one picture from each week, but only for the last X weeks.

  5. –keep-monthly=X: This means “Keep one picture from each month for the last X months.” Imagine you take a picture every month. The album will keep one picture from each month, but only for the last X months.

  6. –keep-annual=X: This means “Keep one picture from each year for the last X years.” Imagine you take a picture every year. The album will keep one picture from each year, but only for the last X years.

So, if you set these values to something like --keep-latest=5 --keep-hourly=4 --keep-daily=7 --keep-weekly=4 --keep-monthly=12 --keep-annual=2, it means:

  • Keep the last 5 pictures you took.
  • Keep one picture from each hour for the last 4 hours.
  • Keep one picture from each day for the last 7 days.
  • Keep one picture from each week for the last 4 weeks.
  • Keep one picture from each month for the last 12 months.
  • Keep one picture from each year for the last 2 years.

This way, you can always look back and see what you captured recently or even long ago!

You:

Continue addressing this thread to a 7 year old until I say otherwise.

Let us say I am taking snapshots via a cron file every 30 minutes of every day.

Does that have any effect on the keep policy?

ChatGPT:

Yes, taking snapshots every 30 minutes does have an effect on the keep policy. Let’s think of it like taking pictures every 30 minutes and how your magical photo album decides which ones to keep based on the rules we talked about.

Here’s how it works with snapshots every 30 minutes:

  1. –keep-latest=5: If you take pictures every 30 minutes, this means you’ll keep the last 5 pictures you took. So, if you took pictures at 12:00, 12:30, 1:00, 1:30, and 2:00, you’ll have those 5 pictures.

  2. –keep-hourly=4: This means you’ll keep one picture from each hour for the last 4 hours. If you took pictures every 30 minutes, you might keep the one from 12:00, 1:00, 2:00, and 3:00.

  3. –keep-daily=7: You’ll keep one picture from each day for the last 7 days. Even if you take many pictures in a day, your album will pick one from each day to save.

  4. –keep-weekly=4: You’ll keep one picture from each week for the last 4 weeks. So, out of all the pictures you take in a week, your album will save one for each week.

  5. –keep-monthly=12: You’ll keep one picture from each month for the last 12 months. The album will choose one picture from each month to keep.

  6. –keep-annual=2: You’ll keep one picture from each year for the last 2 years. Your album will save one special picture from each year.

So, even if you take snapshots (or pictures) every 30 minutes, your magical photo album (Kopia) will decide which ones to keep based on these rules. It picks the most recent ones and then keeps one from each hour, day, week, month, or year as needed. This way, you don’t end up with too many pictures, but you still have important ones from different times!

You:

When you say “your album will pick one” which one does it pick. Is it the end of the last X (hourly/daily/weekly/monthly/annual) cycle ?

ChatGPT:

Yes, when we say “your album will pick one,” it typically picks the one at the end of the last X cycle. Here’s how it works for each type of cycle:

  1. Hourly: It picks the last snapshot (picture) taken in each hour. So, if you take snapshots every 30 minutes, it will keep the snapshot taken at the end of each hour.

    Example: If you take snapshots at 1:00, 1:30, 2:00, and 2:30, and the rule is to keep one per hour, it will keep the snapshots from 1:30 and 2:30.

  2. Daily: It picks the last snapshot taken in each day. Out of all the snapshots taken during the day, it keeps the last one taken at the end of the day.

    Example: If you take snapshots throughout the day and your last snapshot is at 11:30 PM, that’s the one it will keep.

  3. Weekly: It picks the last snapshot taken in each week. It keeps the snapshot taken at the end of the week.

    Example: If you take snapshots every day and the last one of the week is on Sunday at 11:30 PM, that’s the one it will keep.

  4. Monthly: It picks the last snapshot taken in each month. It keeps the snapshot taken at the end of the month.

    Example: If you take snapshots every day, it will keep the one taken on the last day of the month.

  5. Annually: It picks the last snapshot taken in each year. It keeps the snapshot taken at the end of the year.

    Example: If you take snapshots every month, it will keep the one taken on December 31st.

So, to summarize, your album (Kopia) picks the last snapshot taken at the end of each cycle (hour, day, week, month, year) according to the keep policy. This way, it ensures you have the most recent snapshot for each time period.

You:

In each of these cases does a snapshot contain the entire repositories backed up data a that point in time?

ChatGPT:

Yes, in each of these cases, a snapshot contains the entire repository’s backed-up data at that point in time. Think of a snapshot as a complete picture of everything you have in your photo album right at that moment.

Here’s how it works:

  1. Hourly Snapshot: If you take an hourly snapshot, it captures everything in your repository (all your files and data) at the end of each hour.

  2. Daily Snapshot: A daily snapshot captures everything in your repository at the end of each day.

  3. Weekly Snapshot: A weekly snapshot captures everything in your repository at the end of each week.

  4. Monthly Snapshot: A monthly snapshot captures everything in your repository at the end of each month.

  5. Annual Snapshot: An annual snapshot captures everything in your repository at the end of each year.

So, every snapshot is like a full picture of your entire collection of data at that specific point in time. Even if you take multiple snapshots within a short period, each one will include all the data in your repository as it was when that snapshot was taken.

You:

Does a snapshot contain all the data or just what’s different between it and previous snapshots of the same time indicator (hourly/daily/weekly/monthly/annual) ?

ChatGPT:

A snapshot in Kopia is smart about saving space and time. Instead of storing all the data every time, it only saves what’s different from the previous snapshots. Here’s how it works:

  1. Full Data: The first snapshot you take contains all the data in your repository at that point in time.

  2. Changes Only (Incremental Snapshots): After the first snapshot, each new snapshot only contains the changes (differences) since the last snapshot. This means it only saves new or modified files, not everything again.

For example, if you take snapshots every 30 minutes:

  • 1st Snapshot (12:00 PM): Contains all the data.
  • 2nd Snapshot (12:30 PM): Only contains the data that changed since 12:00 PM.
  • 3rd Snapshot (1:00 PM): Only contains the data that changed since 12:30 PM.

When you restore from a snapshot, Kopia combines the initial full snapshot with all the subsequent changes to recreate the entire repository as it was at the time of that snapshot.

So, even though each snapshot seems like it might be saving everything, it actually only saves the differences after the first one, making the process much more efficient.

You:

If you are only keeping X copies of a snapshot won’t that delete the original snapshot that contained “all the data”? Doesn’t that leave a hole in the data each subsequent snapshot is built upon?

ChatGPT:

That’s a great question! It can be a bit tricky to understand how it works. Kopia is designed to handle this intelligently to avoid losing data. Here’s how:

  1. Deduplication: Kopia uses a technique called deduplication. This means that even if a snapshot is deleted, the actual data blocks that are still referenced by other snapshots are not removed. The data is stored in chunks, and these chunks are reused by subsequent snapshots if they haven’t changed.

  2. Snapshots Are Incremental but Self-Sufficient: Although snapshots are incremental and store only changes, they reference data from previous snapshots. If an older snapshot is deleted, the data it contributed to newer snapshots is preserved as long as it’s still needed.

  3. Garbage Collection: Kopia has a garbage collection process that ensures only unreferenced data chunks are removed. This means that as long as a chunk is needed by any snapshot, it won’t be deleted.

Here’s how it works in a simple way:

  • First Snapshot (Full Snapshot): Contains all the data.
  • Second Snapshot (Incremental): Contains changes since the first snapshot but still references unchanged data from the first snapshot.
  • Third Snapshot (Incremental): Contains changes since the second snapshot but still references unchanged data from the first and second snapshots.

If you have a policy to keep only the latest X snapshots:

  • When an old snapshot (like the first one) is deleted, the actual data isn’t deleted if it’s still needed by any of the remaining snapshots.
  • The newer snapshots (second, third, etc.) continue to function correctly because they reference the required data blocks.

This ensures there are no “holes” in the data. Every snapshot, even though it stores only changes, can be used to restore the full state of the repository at the time of that snapshot. Kopia handles the underlying data blocks to make sure nothing essential is lost when older snapshots are pruned according to the retention policy.

You:

It seems that I would always want to keep the:
last 24 hours of snapshots
last 7 days of snapshots
last 4 weeks of snapshots
last 1 month of snapshots

What would be the advantage of keeping 24 recent snapshots in addition to 24 hours of snapshots? How would they differ

ChatGPT:

Great question! Let’s break down the difference between keeping the last 24 snapshots and keeping snapshots based on time intervals like the last 24 hours, 7 days, 4 weeks, and 1 month.

Keeping the Last 24 Snapshots

When you keep the last 24 snapshots, it means you are preserving the most recent 24 snapshots regardless of the time interval between them. If you are taking snapshots every 30 minutes, for example, the last 24 snapshots would cover the past 12 hours.

Advantages:

  • Frequent Changes: This is useful if you want to capture very frequent changes and have the ability to restore data from any of the last 24 snapshots.
  • Granularity: You get a high level of detail for recent changes, which can be crucial if you need to frequently revert to recent states.

Keeping Snapshots Based on Time Intervals

When you keep snapshots based on specific time intervals like the last 24 hours, 7 days, 4 weeks, and 1 month, you are ensuring that you have a snapshot at the end of each of these intervals.

Advantages:

  • Consistency Over Time: This approach ensures that you have a consistent set of snapshots over longer periods, which is useful for tracking changes over days, weeks, and months.
  • Efficient Storage: By reducing the number of snapshots over longer periods, you save storage space while still retaining essential restore points.

How They Differ

  1. Granularity:

    • Last 24 Snapshots: Provides very granular restore points, which is useful for very recent changes.
    • Time-Based Snapshots: Provides broader restore points over time, which is useful for longer-term data recovery.

  2. Coverage:

    • Last 24 Snapshots: Covers a shorter, more recent period (e.g., 12 hours if snapshots are taken every 30 minutes).
    • Time-Based Snapshots: Ensures you have snapshots from the end of each defined period, providing a longer coverage (e.g., 24 hours, 7 days, 4 weeks, and 1 month).

  3. Use Cases:

    • Last 24 Snapshots: Ideal if you need to frequently revert to recent states, such as during active development or data that changes frequently within short periods.
    • Time-Based Snapshots: Ideal for long-term recovery and auditing, ensuring you have historical data points at consistent intervals.

Example

Suppose you have the following settings:

  • –keep-latest=24: Keeps the last 24 snapshots.
  • –keep-hourly=24: Keeps one snapshot from each of the last 24 hours.
  • –keep-daily=7: Keeps one snapshot from each of the last 7 days.
  • –keep-weekly=4: Keeps one snapshot from each of the last 4 weeks.
  • –keep-monthly=1: Keeps one snapshot from the last month.

Practical Effect

  1. With --keep-latest=24:

    • You have 24 very recent snapshots, providing detailed restore points within a short timeframe (e.g., 12 hours).

  2. With Time-Based Policies:

    • Hourly: You ensure you have snapshots from the end of each hour for the last 24 hours.
    • Daily: You ensure you have snapshots from the end of each day for the last 7 days.
    • Weekly: You ensure you have snapshots from the end of each week for the last 4 weeks.
    • Monthly: You ensure you have a snapshot from the end of the last month.

Combined Approach

Using both approaches together, you could have a robust backup strategy:

  • –keep-latest=24: For detailed recent changes.
  • –keep-hourly=24, --keep-daily=7, --keep-weekly=4, --keep-monthly=1: For consistent, long-term restore points.

This way, you benefit from both granular recent backups and broader, consistent historical backups.

You:

If I were to take snapshots every 15 minutes, would Kopia backup have to rescan every file and folder the same way every 15 minutes to see what has changed? That would seem CPU or memory intensive. What do you think?

ChatGPT:

Taking snapshots every 15 minutes can indeed seem CPU and memory intensive if the backup solution had to rescan every file and folder each time. However, Kopia is designed to minimize this overhead using several efficient mechanisms:

  1. File System Monitoring ??:
    compumatter edit - ChatGPT was not sure if Kopia has a 'file system monitoring' feature or not. If I get conclusive feedback on this I'll notate it here

    • Kopia can use file system monitoring to keep track of changes in real-time, which reduces the need to rescan everything. It knows which files have changed since the last snapshot.

  2. Incremental Backups:

    • As mentioned before, Kopia performs incremental backups. It only backs up the files that have changed since the last snapshot. This means it doesn’t need to reprocess files that haven’t changed.

  3. Efficient Scanning:

    • Kopia uses efficient algorithms to detect changes. It relies on file metadata (like timestamps and sizes) to quickly identify which files need to be backed up. If a file hasn’t changed, it won’t be re-read.

  4. Chunking and Deduplication:

    • Kopia breaks files into chunks and uses deduplication to avoid storing duplicate data. Even if a file changes slightly, only the modified chunks are stored.

Practical Impact

  • CPU and Memory Usage: While there is some CPU and memory usage involved in detecting changes and creating snapshots, Kopia’s design aims to keep this as low as possible. Real-time file system monitoring and metadata checks are less resource-intensive than full rescans.
  • Frequency of Snapshots: Taking snapshots every 15 minutes should be feasible without significant CPU or memory overhead, thanks to these optimizations.

Example of Efficient Snapshot Process

Here’s a simplified version of how Kopia handles frequent snapshots:

  1. Initial Snapshot:

    • Scans and backs up all files.

  2. Subsequent Snapshots:

    • Checks metadata (timestamps, sizes) of files to detect changes.
    • Uses file system events (if available) to know which files changed.
    • Backs up only the modified files or chunks.

Monitoring and Optimization

  • Monitor Performance: It’s always good to monitor your system’s performance when setting up frequent snapshots. Tools like top, htop, or Kopia’s own logging can help you ensure that your system is handling the load well.
  • Adjust Frequency: If you notice performance issues, you might consider adjusting the snapshot frequency or excluding certain large or frequently changing directories.

Conclusion

Kopia is designed to handle frequent snapshots efficiently by using incremental backups, file system monitoring, and metadata checks. While there is some overhead, it’s generally minimized to avoid significant CPU or memory usage. Frequent snapshots, such as every 15 minutes, should be feasible for most systems without causing performance issues.

You:

What are the advantages of keeping backup data that is older than 30 days old? Is it just to be able to restore old or deleted versions of my data?

ChatGPT:

Keeping backup data that is older than 30 days can provide several important advantages beyond just being able to restore old or deleted versions of your data. Here are some key reasons:

1. Protection Against Long-Term Data Loss

  • Accidental Deletion: Sometimes, files might be accidentally deleted or altered and the mistake might not be noticed immediately. Older backups ensure you can recover those files even if the error is discovered after a significant delay.
  • Corruption and Ransomware: If data corruption or a ransomware attack occurs, having older backups can provide a clean version of your data from before the incident.

2. Regulatory and Compliance Requirements

  • Data Retention Policies: Some industries have legal or regulatory requirements to retain data for a certain period. For example, financial, medical, and legal sectors often require data to be kept for several years.
  • Audit Trails: Keeping older backups helps maintain a historical record of data, which can be crucial for audits and compliance checks.

3. Historical Reference and Analysis

  • Data Analysis: Older backups can be valuable for historical data analysis, allowing you to track changes and trends over time.
  • Version History: Accessing different versions of files can be important for projects where tracking the evolution of a document or dataset is necessary.

4. Disaster Recovery

  • Comprehensive Recovery: In the event of a disaster that affects your recent data, having older backups provides additional recovery points, ensuring you have multiple options for restoring your system.
  • Redundancy: Older backups add an extra layer of redundancy, enhancing your overall disaster recovery strategy.

5. Business Continuity

  • Minimize Downtime: By having multiple recovery points, you can quickly restore your data to a state that minimizes business disruption.
  • Client Assurance: Demonstrating that you can recover data from various points in time can build trust with clients and stakeholders, assuring them that their data is safe.

6. Recovery from Inadvertent Changes

  • Rollback Changes: If changes were made to a system or data that later prove to be problematic, older backups allow you to roll back to a stable state.
  • Configuration Recovery: Older backups can include system configurations and settings that may be needed to restore the environment to a previous state.

Summary

Keeping backup data that is older than 30 days provides a safety net for various scenarios including:

  • Recovering from accidental deletions or modifications.
  • Meeting legal and regulatory requirements.
  • Performing historical data analysis.
  • Ensuring comprehensive disaster recovery and business continuity.
  • Recovering from data corruption or ransomware attacks.

These advantages make older backups an essential part of a robust data protection strategy, ensuring that you have multiple recovery options to address a wide range of potential issues.

You:

What would you consider to be a typical retention policy for a small business that is not a law office or medical firm

ChatGPT:

For a small business that is not subject to stringent regulatory requirements like a law office or medical firm, a typical retention policy might strike a balance between data availability, storage costs, and recovery needs. Here’s a suggested retention policy:

Suggested Retention Policy

  1. Frequent Short-Term Snapshots:

    • Last 24 hours: Keep snapshots every hour.
      • Reason: Allows recovery from very recent changes or mistakes.
    • Last 7 days: Keep snapshots every 4 hours.
      • Reason: Provides granular recovery points for the past week.

  2. Moderate-Term Snapshots:

    • Last 4 weeks: Keep daily snapshots.
      • Reason: Ensures daily recovery points for the past month, useful for medium-term data recovery needs.

  3. Long-Term Snapshots:

    • Last 6 months: Keep weekly snapshots.
      • Reason: Provides weekly recovery points, balancing between too much data and too few restore points.
    • Last 1 year: Keep monthly snapshots.
      • Reason: Allows recovery of data for longer-term needs, addressing issues that are discovered well after they occur.
    • Beyond 1 year: Keep quarterly snapshots for up to 3 years.
      • Reason: Maintains historical data for annual reviews or long-term analysis without excessive storage use.

Example Retention Policy for a Small Business

  1. Hourly for 24 hours: Keep snapshots for the last 24 hours, taken every hour.
  2. 4-hourly for 7 days: Keep snapshots every 4 hours for the last 7 days.
  3. Daily for 4 weeks: Keep one snapshot per day for the last 4 weeks.
  4. Weekly for 6 months: Keep one snapshot per week for the last 6 months.
  5. Monthly for 1 year: Keep one snapshot per month for the last year.
  6. Quarterly for 3 years: Keep one snapshot every three months for the last 3 years.

Practical Considerations

  1. Storage Costs:

    • Ensure that the storage solution used (local or cloud) is cost-effective for the volume of snapshots retained.

  2. Backup Frequency:

    • Adjust the frequency of snapshots based on the volume of data changes and business needs. For example, if data changes rapidly, more frequent snapshots might be necessary.

  3. Data Sensitivity:

    • Assess the sensitivity of data and the potential impact of data loss. More critical data might warrant more frequent and longer retention.

  4. Disaster Recovery:

    • Ensure that older snapshots are stored in a secure and reliable manner to facilitate disaster recovery. This may include off-site backups or cloud storage.

Implementation Using Kopia

Here’s how you might configure this policy using Kopia:

kopia policy set --global \
  --keep-latest=24 \
  --keep-hourly=42 \
  --keep-daily=28 \
  --keep-weekly=24 \
  --keep-monthly=12 \
  --keep-annual=3
  • –keep-latest=24: Keeps the last 24 snapshots (hourly for 24 hours).
  • –keep-hourly=42: Keeps hourly snapshots for the next 18 hours (combining with the latest 24 hours to cover 7 days).
  • –keep-daily=28: Keeps daily snapshots for the last 4 weeks.
  • –keep-weekly=24: Keeps weekly snapshots for the last 6 months.
  • –keep-monthly=12: Keeps monthly snapshots for the last year.
  • –keep-annual=3: Keeps annual snapshots for up to 3 years.

This policy provides a good balance between having enough restore points for various needs and managing storage costs effectively. Adjustments can be made based on specific business requirements and storage capacity.

You:

Can you provide an estimate in percentages as to how much more data this policy would use:

kopia policy set --global
–keep-latest=24
–keep-hourly=42
–keep-daily=28
–keep-weekly=24
–keep-monthly=12
–keep-annual=3

Vs this policy:

kopia policy set --global
–keep-latest=24
–keep-hourly=24
–keep-daily=7
–keep-weekly=4
–keep-monthly=12
–keep-annual=1

ChatGPT:

To estimate the percentage increase in data usage between the two policies, we need to consider the number of snapshots each policy retains. The data usage is roughly proportional to the number of snapshots because each snapshot will store changes since the last snapshot.

Here are the two policies:

Policy 1

kopia policy set --global \
  --keep-latest=24 \
  --keep-hourly=42 \
  --keep-daily=28 \
  --keep-weekly=24 \
  --keep-monthly=12 \
  --keep-annual=3

Policy 2

kopia policy set --global \
  --keep-latest=24 \
  --keep-hourly=24 \
  --keep-daily=7 \
  --keep-weekly=4 \
  --keep-monthly=12 \
  --keep-annual=1

Number of Snapshots

  1. Policy 1:

    • Latest: 24
    • Hourly: 42
    • Daily: 28
    • Weekly: 24
    • Monthly: 12
    • Annual: 3
    • Total: 24 + 42 + 28 + 24 + 12 + 3 = 133 snapshots

  2. Policy 2:

    • Latest: 24
    • Hourly: 24
    • Daily: 7
    • Weekly: 4
    • Monthly: 12
    • Annual: 1
    • Total: 24 + 24 + 7 + 4 + 12 + 1 = 72 snapshots

Calculating the Increase in Snapshots

To find the percentage increase in the number of snapshots between Policy 2 and Policy 1:

  1. Find the difference in the number of snapshots:

    • $133 - 72 = 61$

  2. Calculate the percentage increase based on Policy 2:

    • $\left( \frac{61}{72} \right) \times 100 \approx 84.72%$

Conclusion

Policy 1 uses approximately 84.72% more data than Policy 2 because it retains significantly more snapshots. This estimation assumes that the incremental changes captured in each snapshot are relatively consistent. If the incremental changes vary significantly, the actual data usage could be more or less than this estimate.

2

Does it? Can you point us into source of this “news”? Ideally it should be present somewhere in kopia source code…

3

This was not “news”. This is a conversation I had with ChatGPT A.I. The goal for me was to try to make snapshot retention more digestible for people. One of it’s responses included the phrase Kopia “can” use file system monitoring. The word “can” was an indicator that it wasn’t sure. So I have edited / restructured that portion of the conversation to make clear this particular feature is an unknown unknown unless someone with knowledge of it further qualifies it. Thanks for highlighting that. It needed to be highlighted.