Vmstorage: Duplicates Can Be Created Even When The Data Is Inserted Sequentially

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Introduction

In this article, we will discuss a rare corner case that has been detected in the vmstorage system. This issue arises when running tests on a system with 1 CPU, and it involves the creation of duplicates even when data is inserted sequentially. We will delve into the details of this bug, its reproduction, and the potential solutions.

Describe the Bug

The bug in question is a rare corner case that has been detected when running tests on a system with 1 CPU. This issue is related to the creation of duplicates in the vmstorage system, even when data is inserted sequentially. To understand this bug, let's consider the following scenario:

Assumptions

We assume the following data:

metric 123 1971-01-01 
metric 456 1971-01-02
metric 789 1971-01-03

In this scenario, we insert three samples for the time series metric and each sample has a different date. We insert this data in the same goroutine on a system with 1 CPU.

The Bug

Once the samples are inserted, we rotate indexDB and insert the same samples again. The database will create a duplicate for the first record. This is because after the first insertion, the metric will be recorded in the tsidCache. After the indexDB is rotated and the second insertion is performed, the metric will be found in tsidCache but because indexDB generations do not match, the database will create a new entry for that metric in the new indexDB.

The Problem

The problem arises when the updatePerDateData() function is called. This function is responsible for updating the data in the new indexDB. However, it does not find the date+metricID in the dateMetricIDCache because it only keeps the two most recent dates. In our case, the date 1971-01-01 is discarded. As a result, the updatePerDateData() function creates a new entry in the new indexDB, even though it should not.

The Cause

The cause of this issue is the way the dateMetricIDCache works. It keeps only the two most recent dates, which means that older dates are discarded. This is why the updatePerDateData() function does not find the date+metricID in the cache, even though it has been added by the add() function.

The Solution

To solve this issue, we need to modify the dateMetricIDCache to keep all dates, not just the two most recent ones. This will ensure that the updatePerDateData() function can find the date+metricID in the cache and avoid creating duplicates.

To Reproduce

To reproduce this bug, you can follow these steps:

Remove the Condition

Remove the condition introduced in the PR that fixes this issue.

Run the Test

Run the following command:

GOEXPERIMENT=synctest taskset -c 0 go test ./lib/storage -run=TestStorageAddRowsForVariousDataPatternsConcurrently/perDayIndexes/serial/sameBatchMetrics/sameRowMetrics/sameBatchDates/diffRowDates

This command will run the test on system with 1 CPU and reproduce the bug.

Version

The latest version of the vmstorage system is affected by this bug.

Logs

No logs are available for this issue.

Screenshots

No screenshots are available for this issue.

Used Command-line Flags

No command-line flags are used to reproduce this issue.

Additional Information

No additional information is available for this issue.

Conclusion

Q: What is the vmstorage system?

A: The vmstorage system is a data storage system designed to handle large amounts of data efficiently. It is a distributed system that allows for concurrent data insertion and retrieval.

Q: What is the bug in the vmstorage system?

A: The bug in the vmstorage system is a rare corner case that involves the creation of duplicates even when data is inserted sequentially. This issue arises when running tests on a system with 1 CPU.

Q: What causes the bug?

A: The bug is caused by the way the dateMetricIDCache works. It keeps only the two most recent dates, which means that older dates are discarded. This is why the updatePerDateData() function does not find the date+metricID in the cache, even though it has been added by the add() function.

Q: How can the bug be reproduced?

A: To reproduce the bug, you can follow these steps:

  1. Remove the condition introduced in the PR that fixes this issue.
  2. Run the following command:
GOEXPERIMENT=synctest taskset -c 0 go test ./lib/storage -run=TestStorageAddRowsForVariousDataPatternsConcurrently/perDayIndexes/serial/sameBatchMetrics/sameRowMetrics/sameBatchDates/diffRowDates

This command will run the test on system with 1 CPU and reproduce the bug.

Q: What is the impact of the bug?

A: The bug can cause duplicates to be created in the vmstorage system, even when data is inserted sequentially. This can lead to data inconsistencies and errors.

Q: How can the bug be fixed?

A: To fix the bug, you need to modify the dateMetricIDCache to keep all dates, not just the two most recent ones. This will ensure that the updatePerDateData() function can find the date+metricID in the cache and avoid creating duplicates.

Q: What is the current status of the bug?

A: The bug is currently being investigated and fixed. A patch is being developed to address the issue.

Q: How can I get more information about the bug?

A: You can get more information about the bug by checking the vmstorage system's issue tracker or by contacting the development team directly.

Q: What are the next steps to fix the bug?

A: The next steps to fix the bug are:

  1. Develop a patch to modify the dateMetricIDCache to keep all dates.
  2. Test the patch to ensure it fixes the bug.
  3. Deploy the patch to the production environment.

Q: What is the estimated time to fix the bug?

A: The estimated time to fix the bug is currently unknown. However, the development team is working to resolve the issue as soon as possible.

Q: How can I stay updated on the bug's status?

A: You can stay updated on the bug's status by checking the vmstorage system's issue tracker or by contacting the development team directly.