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RFC:TiKV RawKV MVCC GC #90

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RFC:TiKV RawKV MVCC GC #90

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RFC:TiKV RawKV MVCC GC
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RFC:TiKV RawKV MVCC GC
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RFC:TiKV RawKV MVCC GC. add design details and flow chart
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feature RawKV GC,modify pd interface design
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feature RawKV GC,modify interface response
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feature RawKV GC,modify interface response.
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# RFC: TiKV RawKV MVCC GC


## Summary
Move TiKV MVCC GC worker from TiDB into a group of independent GC worker node role and implement a new GC process in TiKV for RawKV.
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As the subject of this RFC is for RawKV, I think we are not discussing about "move GC worker from TiDB".

I suggest the main contents to be:

  1. Design of TiKV procedures for RawKV GC.
  2. PD meta data & interfaces.
  3. The new standalone GC worker component.
  4. Discuss whether or how TiDB can migrate to this design.

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"node role" seems to be not easy to understand.
Suggest to use "component", as TiUP using this word. See tiup.io.


## Motivation
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The description of GC worker would be better to place in "Background" section.
I think the motivation includes:

  1. We change RawKV encoding to MVCC, so the GC is necessary.
  2. No GC for TxnKV scenario when TiDB is not deployed.
  3. The GC safe point & Txn safe point is easy to be misunderstand.

GC worker is an important component for TiKV that deletes outdated MVCC data so as to not explode the storage. But currently, the GC worker is implemented in TiDB, which makes TiKV not usable without TiDB.And current GC process is just for transaction of TiDB,it's not usable for RawKV.

## Background
According to the documentation for the current GC worker in a TiDB cluster, the GC process is as follows:

In TiDB GC worker leader:
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@nolouch nolouch Mar 23, 2022 •
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Currently, TiDB GC worker is not standardized use client-go. I think there are some problems that also need to be solved. details: https://docs.google.com/document/d/1jA3lK9QbYlwsvn67wGsSuusD1Dzx7ANq_vya384RBIg/edit#

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I have read this document and discussed with @MyonKeminta .For implement this proposal, it need a clearer design and more development.
But due to scheduling reasons, we hope to make changes to GC as small as possible to support TiKV API V2.
So I submit this RFC.

1. Regularly calculates a new timestamp called "safe point", and push the safe point to PD.
2. Get the minimal Service safe point among all services from the response of step 2, which is GC safe point .
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Suggested change
2. Get the minimal Service safe point among all services from the response of step 2, which is GC safe point .
2. Get the minimal Service safe point among all services from the response of step 1, which is GC safe point .

3. Txn GC process: resolve locks and record delete ranges information.

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Refer to RunDistributedGCJob, where are two more steps:

  1. Save Txn safe point.
  2. Upload GC safe point to PD.

In PD leader:
1. Receive update safe point requests from TiDB or other tools (e.g. CDC, BR).
2. Calculate the minimal timestamp = min(all service safe point, now - gc_life_time).

In every TiKV nodes：
1. Get GC safe point from PD regularly.
2. Deletion will be triggered in CompactionFilter and GcTask thread;

## New GC worker architecture
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Maybe it is not a new architecture. We are utilizing the same architecture but another implementation.

In a TiKV cluster without TiDB nodes , there are a few different points as follows:
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Suggested change
In a TiKV cluster without TiDB nodes , there are a few different points as follows:
In a TiKV cluster without TiDB nodes, there are a few different points as follows:

1. We need to move GC worker into another node role.
2. For [API V2](https://github.com/tikv/rfcs/blob/master/text/0069-api-v2.md) .It need gc the earlier version in default cf. But Txn GC worker process will be triggered by WriteCompactionFilter of write cf.
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Suggested change
2. For [API V2](https://github.com/tikv/rfcs/blob/master/text/0069-api-v2.md) .It need gc the earlier version in default cf. But Txn GC worker process will be triggered by WriteCompactionFilter of write cf.
2. For [API V2](https://github.com/tikv/rfcs/blob/master/text/0069-api-v2.md), it need gc the earlier version in default cf. But Txn GC worker process will be triggered by WriteCompactionFilter of write cf.

3. RawKV encoded code of RawValue is different with Txn in TiDB.

So we designed a new GC architecture and process for TiKV cluster.

## Detailed design
For support TiKV cluster deploy without TiDB nodes.
1. Add a new node role instead of GC worker in TiDB nodes.
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We need to decide exactly which role before merging the RFC.

The code of new GC worker,It will be added into [tikv/migration](https://github.com/tikv/migration)
2. And for API V2, we need add new CompactionFilter which is named RawGCcompactionFilter, and add a new GCTask type implementation.
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There is already a CompactionFilter for RawKV. Are we going to add a new one or just modify it?

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TTLCompactionFilter is just for APIV1TTL,In API V2 ,There are several differences with APIV1TTL.I think add a new CompactionFilter is better, @pingyu What do you think?

3. GC conditions in RawGCcompactionFilter is: (ts < GCSafePoint) && ( ttl-expired || deleted-mark || not the newest version ).
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@andylokandy andylokandy Mar 22, 2022 •
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Need to clarify the GCSafePoint. For example (1) Do RawKV, TiDB, and TxnKV share the same GCSafePoint? (2) Who will push GCSafePoint forward?

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We may want to remove a key that ts > GCSafePoint but ttl-expired = true. @pingyu What do you think?

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We should keep such entries. Expired entries are just the same as logical deleted. Miss to capture these entries will cause downstream to return an earlier version, and being inconsistent to upstream.

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@pingyu So there will be this case: the CDC may sync a key that is already expired with downstream, right?

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(2) Who will push GCSafePoint forward?
the new node role will instead of GC Worker in TiDB, It will push GCSafePoint forward.

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@pingyu So there will be this case: the CDC may sync a key that is already expired with downstream, right?

Yes.

1. If the newest version is earlier than GC safe point and it's delete marked or expired ttl,those keys and earlier versions of the same userkey will be sent to a gc scheduler thread to gc asynchronous.

## Reference
https://docs.google.com/document/d/1jA3lK9QbYlwsvn67wGsSuusD1Dzx7ANq_vya384RBIg/edit#heading=h.rr3hcmc7ejb8
https://docs.pingcap.com/tidb/stable/garbage-collection-overview