Faster IO500

## [Unreleased]

### New

  - Metadata batching ([!305](https://storage.bsc.es/gitlab/hpc/gekkofs/-/merge_requests/305))

    - Added client-side metadata batching for file/node creation to reduce metadata RPC bottlenecks.

    - Introduced new environment variables: `LIBGKFS_METADATA_BATCH` and `LIBGKFS_METADATA_BATCH_THRESHOLD`.

  - directory optimization with compression and reattemp ([!270](https://storage.bsc.es/gitlab/hpc/gekkofs/-/merge_requests/270))

    - Refactor sfind so it can use SLURM_ environment variables to ask to different servers.

    - Create a sample bash script to gather all the info (map->reduce)

- `LIBGKFS_PROTECT_FILES_GENERATOR=1` enables the application as generator, so a open will create (and increase) a file .lockgekko and close will remove it (or decrease its value). The behaviour only uses metadata at server side.

- `LIBGKFS_PROTECT_FILES_CONSUMER=1` enables the application as consumer, so a open will wait until the .lockgekko dissapears. The wait is limited to ~40 seconds. 

##### Metadata batching

During file/node creation (e.g., `open` with `O_CREAT`), the client sends a separate RPC for each file creation. The metadata batching feature allows buffering these creation operations on the client-side and sending them in batches to the daemons, reducing network RPC overhead during massive file creations.

Remaining buffered creation requests are automatically flushed when the application exits.

- `LIBGKFS_METADATA_BATCH=ON` - Enable client-side metadata batching for file creation (default: OFF).

- `LIBGKFS_METADATA_BATCH_THRESHOLD` - Set the number of file creation operations per host after which the batch is flushed (default: 64).

### Daemon

#### Core

- `GKFS_DAEMON_CREATE_CHECK_PARENTS` - Enable checking parent directory for existence before creating children.

        ADD_PREFIX("CREATE_WRITE_OPTIMIZATION");

static constexpr auto READ_INLINE_PREFETCH = ADD_PREFIX("READ_INLINE_PREFETCH");

static constexpr auto ENABLE_FORK = ADD_PREFIX("ENABLE_FORK");

static constexpr auto METADATA_BATCH = ADD_PREFIX("METADATA_BATCH");

static constexpr auto METADATA_BATCH_THRESHOLD =

        ADD_PREFIX("METADATA_BATCH_THRESHOLD");

} // namespace gkfs::env

#define GEKKOFS_PRELOAD_CTX_HPP

#include <map>

#include <unordered_map>

#include <thallium.hpp>

#include <memory>

#include <vector>

    std::shared_ptr<thallium::engine> rpc_engine_;

    std::shared_ptr<thallium::engine> ipc_engine_;

    bool use_metadata_batch_{false};

    size_t metadata_batch_threshold_{64};

    std::unordered_map<uint64_t, std::vector<std::pair<std::string, mode_t>>>

            metadata_batch_buffer_;

    mutable std::mutex metadata_batch_mutex_;

public:

    static PreloadContext*

    void

    ipc_engine(std::shared_ptr<thallium::engine> engine);

    bool

    use_metadata_batch() const;

    void

    use_metadata_batch(bool use_metadata_batch);

    size_t

    metadata_batch_threshold() const;

    void

    metadata_batch_threshold(size_t threshold);

    void

    flush_metadata_batches();

    void

    flush_metadata_batch(uint64_t host_id);

    void

    add_metadata_batch_entry(uint64_t host_id, const std::string& path,

                             mode_t mode);

};

} // namespace preload

int

forward_create(const std::string& path, mode_t mode, const int copy);

int

forward_batch_create(uint64_t host_id, const std::vector<std::string>& paths,

                     const std::vector<uint32_t>& modes);

int

forward_create_write_inline(const std::string& path, mode_t mode,

                            const std::string& data, uint64_t count,