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Fix lints
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Signed-off-by: Hoverbear <[email protected]>
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@Hoverbear
Hoverbear committed Dec 20, 2018
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# Summary
## Summary

Support a `BatchSplit` feature that splits one Region into multiple Regions at
a time if the size or the number of keys exceeds a threshold. This includes
modifications of both TiKV and PD. For TiKV, every round of split-check
produces multiple split keys instead of one and changes inner split related
interface into batch style. For PD, add RPCs `AskBatchSplit` and
Support a `BatchSplit` feature that splits one Region into multiple Regions at
a time if the size or the number of keys exceeds a threshold. This includes
modifications of both TiKV and PD. For TiKV, every round of split-check
produces multiple split keys instead of one and changes inner split related
interface into batch style. For PD, add RPCs `AskBatchSplit` and
`ReportBatchSplit` to allow TiKV to ask for `region_id` and `peer_id` in batch.

# Motivation
## Motivation

Current split only splits one Region at a time. It may be very slow when a
sequential write is too fast, namely, the split speed cannot keep up with
write speed. A slow split can lead to large Regions. In this case, if a snapshot
is triggered, it will occupy a lot of I/O and make everything slow. Also, it is
hard to schedule hotspots for a large Region, so it makes performance even
Current split only splits one Region at a time. It may be very slow when a
sequential write is too fast, namely, the split speed cannot keep up with
write speed. A slow split can lead to large Regions. In this case, if a snapshot
is triggered, it will occupy a lot of I/O and make everything slow. Also, it is
hard to schedule hotspots for a large Region, so it makes performance even
worse.

# Detailed design
## Detailed design

## RPC interface
### RPC interface

### PD
#### PD

```protobuf
service PD {
// ...
rpc AskSplit(AskSplitRequest) returns (AskSplitResponse) {
// Use AskBatchSplit instead.
option deprecated = true;
}
}
rpc ReportSplit(ReportSplitRequest) returns (ReportSplitResponse) {
// Use ResportBatchSplit instead.
option deprecated = true;
Expand Down Expand Up @@ -64,20 +64,20 @@ message ReportBatchSplitResponse {
}
```

Add `AskBatchSplit` to replace `AskSplit`. It is called when TiKV produces some
split keys for one Region and asks PD to allocate new `region_id` and `peer_id`
for that Region. `split_count` in `AskBatchSplitRequest` indicates the number
of Regions to be generated, and `AskBatchSplitResponse` returns all new
Add `AskBatchSplit` to replace `AskSplit`. It is called when TiKV produces some
split keys for one Region and asks PD to allocate new `region_id` and `peer_id`
for that Region. `split_count` in `AskBatchSplitRequest` indicates the number
of Regions to be generated, and `AskBatchSplitResponse` returns all new
allocated IDs to TiKV.

Add `ReportBatchSplit` to replace `ReportBatchSplit`. It is called when TiKV
finishes splitting a Region. `ReportBatchSplitRequest` takes all metas of a new
Add `ReportBatchSplit` to replace `ReportBatchSplit`. It is called when TiKV
finishes splitting a Region. `ReportBatchSplitRequest` takes all metas of a new
generated Region for PD to update PD's related information.

For compatibility issue, the old interface is not deleted but set to
deprecated.
For compatibility issue, the old interface is not deleted but set to
deprecated.

### TiKV
#### TiKV

```protobuf
message SplitRequest {
Expand All @@ -88,7 +88,7 @@ message SplitRequest {
message BatchSplitRequest {
repeated SplitRequest requests = 1;
// If true, the last Region obtains the origin region_id,
// If true, the last Region obtains the origin region_id,
// and other Regions use new Ids.
bool right_derive = 2;
}
Expand Down Expand Up @@ -119,50 +119,50 @@ message AdminResponse {
}
```

Add a new admin command type `BatchSplit` with related request and response.
`BatchSplitRequest` wraps multiple `SplitRequest` along with `right_derive`
Add a new admin command type `BatchSplit` with related request and response.
`BatchSplitRequest` wraps multiple `SplitRequest` along with `right_derive`
which invalidates the `right_derive` in each `SplitRequest`.

When in the rolling upgrade process, new TiKVs are mixed up with old TiKVs, so
When in the rolling upgrade process, new TiKVs are mixed up with old TiKVs, so
old command type `Split` still needs to be preserved.

## Implementation in TiKV
### Implementation in TiKV

### How to produce multiple split keys
#### How to produce multiple split keys

First introducing one config `batch_split_limit` to limit the number of produced
split keys in a batch. If it is unlimited, for a once split check, it scans all
over the Region's range, and in some extreme case, this would cause performance
First introducing one config `batch_split_limit` to limit the number of produced
split keys in a batch. If it is unlimited, for a once split check, it scans all
over the Region's range, and in some extreme case, this would cause performance
issue.

SizeChecker and KeysChecker can be rewritten to produce multiple keys, and the
general logic of SizeChecker and KeysChecker are similar. The only difference
between them is one splits Region based on size and the other splits Region
based on the number of keys. So here we mainly describe the logic of
SizeChecker and KeysChecker can be rewritten to produce multiple keys, and the
general logic of SizeChecker and KeysChecker are similar. The only difference
between them is one splits Region based on size and the other splits Region
based on the number of keys. So here we mainly describe the logic of
SizeChecker:

- before: it scans key-value pairs in a Region's range sequentially to
accumulate their size as `total_size` and stops once the size reaches
`region_max_size` or scans to the end of the range. If `total_size` is smaller
than `region_max_size` at the end, checker wouldn't produce any split key; if
not, it regards the very key at which `total_size` reaches `region_split_size`
as split key.
- after: it scans key-value pairs in a Region's range sequentially to
accumulate their size as `total_size` and stops once the size reaches
`region_split_size * (batch_split_limit-1) + region_max_size` or scans to the
end of the range. During the scan process, it records the key as a split key
every `region_split_size`, but after finishing scanning, it may discard the
last split key if the size of the rest is not bigger than `region_max_size -
region_split_size`. With this algorithm, if `batch_split_limit` is set to 1,
TiKV can perfectly behave the same way as the split without batch.

### Compatibility concern

The general process in raftstore changes a little. It mainly replaces `Split`
with `BatchSplit`. But one thing should be noted that during the rolling
upgrade, PD version control will refuse the `AskBatchSplit` request, thus split
can't be performed during this process until all TiKVs bump to a new version.
To let TiKV know whether `AskBatchSplit` fails for compatibility or not, we
- **Before:** it scans key-value pairs in a Region's range sequentially to
accumulate their size as `total_size` and stops once the size reaches
`region_max_size` or scans to the end of the range. If `total_size` is
smaller than `region_max_size` at the end, checker wouldn't produce any split
key; if not, it regards the very key at which `total_size` reaches
`region_split_size` as split key.
- **After:** it scans key-value pairs in a Region's range sequentially to
accumulate their size as `total_size` and stops once the size reaches
`region_split_size * (batch_split_limit-1) + region_max_size` or scans to the
end of the range. During the scan process, it records the key as a split key
every `region_split_size`, but after finishing scanning, it may discard the
last split key if the size of the rest is not bigger than `region_max_size -
region_split_size`. With this algorithm, if `batch_split_limit` is set to 1,
TiKV can perfectly behave the same way as the split without batch.

#### Compatibility concern

The general process in raftstore changes a little. It mainly replaces `Split`
with `BatchSplit`. But one thing should be noted that during the rolling
upgrade, PD version control will refuse the `AskBatchSplit` request, thus split
can't be performed during this process until all TiKVs bump to a new version.
To let TiKV know whether `AskBatchSplit` fails for compatibility or not, we
introduce a new error type for `ResponseHeader`:

```protobuf
Expand All @@ -172,41 +172,40 @@ enum ErrorType {
}
```

So once TiKV gets `AskBatchSplitResponse` with
`ErrorType::INCOMPATIBLE_VERSION`, it uses the original `AskSplit` instead of
`AskBatchSplit`, and all the following processes will degrade to the original
So once TiKV gets `AskBatchSplitResponse` with
`ErrorType::INCOMPATIBLE_VERSION`, it uses the original `AskSplit` instead of
`AskBatchSplit`, and all the following processes will degrade to the original
way. So the original code path is not deleted.

### Approximate split key
#### Approximate split key

What we said above can ease the problem. However, scanning a large Region can
also consume a lot of time and CPU. The test shows that large Regions can still
What we said above can ease the problem. However, scanning a large Region can
also consume a lot of time and CPU. The test shows that large Regions can still
easily show up even with batch split implemented, although split is speeded up.

When a Region becomes large enough, it's more practical to divide it into
smaller chunks quickly. This can be achieved via size estimation, which can be
calculated from SST properties. Although it may not be accurate enough, it's
When a Region becomes large enough, it's more practical to divide it into
smaller chunks quickly. This can be achieved via size estimation, which can be
calculated from SST properties. Although it may not be accurate enough, it's
okay for a large Region.

So if the size of Region is larger than `region_max_size * batch_split_limit *
2`, TiKV uses an approximate way to produce split keys. The approximate way is
quite similar to the algorithm we describe above, but to estimate TiKV uses
approximate size of the Region and the number of keys in the Region's range to
calculate the average distance between two SST property keys, and produces a
So if the size of Region is larger than `region_max_size * batch_split_limit *
2`, TiKV uses an approximate way to produce split keys. The approximate way is
quite similar to the algorithm we describe above, but to estimate TiKV uses
approximate size of the Region and the number of keys in the Region's range to
calculate the average distance between two SST property keys, and produces a
split key every `region_split_size / distance` keys.

# Drawbacks
## Drawbacks

- When the approximate way is used, Region may split into several
- When the approximate way is used, Region may split into several
disproportional Regions due to size estimation.

# Alternatives
## Alternatives

None

## Unresolved questions

# Unresolved questions

Generally, it is more urgent to split a large Region, so we can change the
split check queue from a naive FIFO queue to a priority queue so that a large
Generally, it is more urgent to split a large Region, so we can change the
split check queue from a naive FIFO queue to a priority queue so that a large
Region can be split early and quickly.

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