Local storage: store files in subsubdirectories

[Mic92]

Currently it seems like all chunks end up in one big directory:

impl LocalBackend {                                                                    
▏   pub async fn new(config: LocalStorageConfig) -> ServerResult<Self> {               
▏   ▏   fs::create_dir_all(&config.path)                                               
▏   ▏   ▏   .await                                                                     
▏   ▏   ▏   .map_err(ServerError::storage_error)?;                                     
▏   ▏                                                                                  
▏   ▏   Ok(Self { config })                                                            
▏   }                                                                                  
▏                                                                                      
▏   fn get_path(&self, p: &str) -> PathBuf {
▏   ▏   self.config.path.join(p)                                                       
▏   }                                                                                  
}

This seems bad from a performance standpoint. Many file operations in the kernel require locking of the parent directory which makes them slow when having a lot of concurrency. Also thinks like listing a directory becomes quite slow this way. What are your thoughts on taking the first few bytes of the directory to compute a subdirectory instead?

[Mic92]

https://github.com/zhaofengli/attic/pull/98/files

[zhaofengli]

Fixed by #98.