TiFlash OOM with high throughput write

[flowbehappy]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. A cluster with many TiKV and 4 TiFlash
2. A table with shard_row_id_bits
3. Each TiFlash instance has over 300GB
4. The GC interval is set to 5 days
5. Write throughput of each TiFlash is over 7MB/s

2. What did you expect to see? (Required)

TiFlash works normally.

3. What did you see instead (Required)

TiFlash OOM.

4. What is your TiFlash version? (Required)

v6.1.x

[flowbehappy]

It is very likely caused by delta compact works slowly. And the root cause is that ~Snapshot of Storage V2 is slow in the case of many page entries.