Tracking issues of restore performance improvement.

[3pointer]

Enhancement

Tracking issues of restore performance in different stage. basically we have 4 stages during restoartion.

1. Load Data from Storage. 
2. Create DB/Tables. 
3. Region Split & Scatter.  
4. Download & Ingest SST Files.

• (Stage 3) A smarter way to scatter batch regions #33937
  • restore: refactor split code for two phase split. #48244
  • restore: limit scan region concurency during restore #50255
• (Stage 4)Restore: new pipeline to improve performance #49886
  • restore: use token bucket to balance download requests. #49887
  • raftstore: real batch ingest sst files in different region tikv/tikv#16298

Bugfixs

Functional issues

• br restore failed due to "split region failed" #50471
  • restore: fix choose the emptry key as split key (#50478) #50518
• need disable gcmemorylimittuner when run br task #51078
• tables id cannot be reused when there are many databases not being created continously #51736
• restore: backpressure the importer producer goroutine #51777

Performance issues

• Br restore download SST throughput not stable throughout the whole restore process #50593
• one tikv has no leader and slow down br restore speed #50566
• analyze becomes slower after br restoring cluster data #50617
• br restore very slow (like it hangs) when merging files #50613
• br restore is very slowly if there are many databases(2000+) #50767
• br restore is very slow when there are many metafiles #50772
• Enhancing performance in multi-table scenarios #51685
• br restore splits too much regions if there are too many small tables whose data size is less than region size. #53532

Compatible issues

• (checkpoint) br restore reported split region error when setting granularity=coarse-grained #50665
• (checkpoint) br restore not proceeded from checkpoint and user is prompted to drop system tables #50232
• (statistic) stats is not restored if --checksum=false #50568

Enhancement

• restore: auto tune concurrency configuration when using new mode restoration #50877
• br: skip loading stats into memory if set --load-stats to false #51535
• br: stop log when full restore failed #51578
• restore: shift rewrite ranges before split ranges #51653