Reduce unnecessary string creation when interacting with rocksdb

[wu-hanqing]

Now, we use rocksdb as metadata storage, for Get and Iterate operations, rocksdb yields rocksdb::Slice that stands for key/values.

For rocksdb::Slice, is almost same with string_view in C++17, StringPiece in chromium. And they have the same idea behind it - they are just views of strings, and don't own the underlying string. And they may give some improvement in some cases.

However, in some source code, we don't follow this design, for example, RocksDBStorageIterator::Value returns a std::string which is creating from a rocksdb::Slice and does memcpy in this progress. But, in most cases, we just parse a protobuf Message from the string which means the creation is unnecessary.

[zhanghuidinah]

/assign @ccmaxcc

[zhanghuidinah]

@ccmaxcc Are you having some trouble? If so, please let us know so that we can communicate together.

[wuhongsong]

@ccmaxcc

[liam-thunder]

Interested in CurveFS, is this PR still available to contribute? I can start with this PR to get familiar with CurveFS.

[ilixiaocui]

Interested in CurveFS, is this PR still available to contribute? I can start with this PR to get familiar with CurveFS.

@liam-thunder You are very welcome to contribute to this PR.

[liam-thunder]

address this issue in #2183

[Cyber-SiKu]

@liam-thunder Is this still going on?

[liam-thunder]

@liam-thunder Is this still going on?

Sorry for late response, this issue is blocked by E2E benchmark testing and performance analysis, and requiring some extra efforts. But I'm quite busy recently, will get back on this issue later.

[Cyber-SiKu]

@liam-thunder Are you still going on?

[liam-thunder]

@liam-thunder Are you still going on?

Yes, I'm working on deploy a single node cluster to do the benchmark.

[ilixiaocui]

@liam-thunder Is this still going on?

Sorry for late response, this issue is blocked by E2E benchmark testing and performance analysis, and requiring some extra efforts. But I'm quite busy recently, will get back on this issue later.

If you have any questions during the benchmarking process, please feel free to ask. You can also communicate in the WeChat group.