Reduce the block cache configurations into a single one

[Yangsx-1]

Close #1495

[git-hulk]

@Yangsx-1 Another point is that needs to ignore the below configurations when rewriting:

• rocksdb.share_metadata_and_subkey_block_cache
• rocksdb.metadata_block_cache_size
• rocksdb.subkey_block_cache_size

to make sure they won't appear in the config file.

[Yangsx-1]

@Yangsx-1 Another point is that needs to ignore the below configurations when rewriting:

• rocksdb.share_metadata_and_subkey_block_cache
• rocksdb.metadata_block_cache_size
• rocksdb.subkey_block_cache_size

to make sure they won't appear in the config file.

This means that these configurations shouldn't be in the config file? If it appears, there may be some warning messages or error?

[git-hulk]

@Yangsx-1 Another point is that needs to ignore the below configurations when rewriting:

• rocksdb.share_metadata_and_subkey_block_cache
• rocksdb.metadata_block_cache_size
• rocksdb.subkey_block_cache_size

to make sure they won't appear in the config file.

This means that these configurations shouldn't be in the config file? If it appears, there may be some warning messages or error?

What I mean is to remove them from the kvrocks.conf and keep the config.cc as it is, since the rocksdb.block_cache_size is expected to calculate from them if it's not set.

[Yangsx-1]

@Yangsx-1 Another point is that needs to ignore the below configurations when rewriting:

• rocksdb.share_metadata_and_subkey_block_cache
• rocksdb.metadata_block_cache_size
• rocksdb.subkey_block_cache_size

to make sure they won't appear in the config file.

This means that these configurations shouldn't be in the config file? If it appears, there may be some warning messages or error?

What I mean is to remove them from the kvrocks.conf and keep the config.cc as it is, since the rocksdb.block_cache_size is expected to calculate from them if it's not set.

OK

[git-hulk]

Others are generally good for me, thanks for your efforts.

[PragmaTwice]

@git-hulk rocksdb.share_metadata_and_subkey_block_cache is completely ignored after the change (it will always be ON, despite whether the option is ON or OFF). Is it OK?

[git-hulk]

@git-hulk rocksdb.share_metadata_and_subkey_block_cache is completely ignored after the change (it will always be ON, despite whether the option is ON or OFF). Is it OK?

Yes, it's intentional. And it's a historically wrong decision to separate the block cache between column families. From the user side, it's really hard to balance the cache capacity between column families, the best way is to let LRU determine. To avoid the change being too aggressive, @ShooterIT added this option to allow users to use the old way. But from the performance, sharing the block cache is always better than the separate, so I think it's time to ignore this configuration and remove it in next major version.

[PragmaTwice]

Thanks for your contribution!