sst_importer: add wire extra bytes into the packed size

[YuJuncen]

What is changed and how it works?

Issue Number: Close #14313

What's Changed:
This PR added some extra bytes for each modification, so the unpacked_size would be nearer to the real raft entry size.

Check List

Tests

• Manual test (add detailed scripts or steps below)

I have tested it in the internal cluster, if you are a PingCAP employee, you can check it at here. There aren't suitable screenshots to be published, sorry for that.

And I guess this can also be checked by any table with average row size less than 70B (see the comments), but for now I don't have suitable cluster for running such tests...

Release note

None

(This is a fix over the master branch.)

[ti-chi-bot]

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• tabokie
• tonyxuqqi

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[tabokie]

Better add a test.

[BusyJay]

Maybe we should let the engine to tell the size of modification. In raftkv2, writes are encoded using more simple codec and can batch up more modifications than v1.

[YuJuncen]

@BusyJay I agree the idea of letting the engine provide some way to calculate "real size" of some write, and I think before we figuring out some ideal interface definition, perhaps we can merge this for solving the problem we have encountered now?

[BusyJay]

LGTM

[tabokie]

/merge

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 7385494

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #14328.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #14329.