README.md

# GekkoFS

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[![pipeline status](https://storage.bsc.es/gitlab/hpc/gekkofs/badges/master/pipeline.svg)](https://storage.bsc.es/gitlab/hpc/gekkofs/commits/master)
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GekkoFS is a file system capable of aggregating the local I/O capacity and performance of each compute node
in a HPC cluster to produce a high-performance storage space that can be accessed in a distributed manner.
This storage space allows HPC applications and simulations to run in isolation from each other with regards
to I/O, which reduces interferences and improves performance.

# Table of contents

- [Dependencies](#dependencies)
  - [Debian/Ubuntu](#debianubuntu)
  - [CentOS/Red Hat](#centosred-hat)
- [Step-by-step installation](#step-by-step-installation)
- [Run GekkoFS](#run-gekkofs)
  - [The GekkoFS hostsfile](#the-gekkofs-hostsfile)
  - [The GekkoFS daemon](#the-gekkofs-daemon)
    - [Manual startup and shut down](#manual-startup-and-shut-down)
    - [GekkoFS daemon orchestration via the gkfs script (recommended)](#gekkofs-daemon-orchestration-via-the-gkfs-script-recommended)
  - [The GekkoFS client library](#the-gekkofs-client-library)
    - [Interposition library via system call interception](#interposition-library-via-system-call-interception)
    - [User library via linking against the application](#user-library-via-linking-against-the-application)
  - [Logging](#logging)
- [Advanced and experimental features](#advanced-and-experimental-features)
  - [Rename](#rename)
  - [Replication](#replication)
  - [Client-side metrics via MessagePack and ZeroMQ](#client-side-metrics-via-messagepack-and-zeromq)
  - [Server-side statistics via Prometheus](#server-side-statistics-via-prometheus)
  - [GekkoFS proxy](#gekkofs-proxy)
  - [File system expansion](#file-system-expansion)
- [Miscellaneous](#miscellaneous)
  - [External functions](#external-functions)
  - [Data placement](#data-placement)
    - [Simple Hash (Default)](#simple-hash-default)
    - [Guided Distributor](#guided-distributor)
  - [Metadata Backends](#metadata-backends)
  - [CMake options](#cmake-options)
  - [Environment variables](#environment-variables)
- [Acknowledgment](#acknowledgment)

# Dependencies

- \>gcc-12 (including g++) for C++17 support
- General build tools: Git, Curl, CMake >3.6 (>3.11 for GekkoFS testing), Autoconf, Automake
- Miscellaneous: Libtool, Libconfig

### Debian/Ubuntu

GekkoFS base dependencies: `apt install git curl cmake autoconf automake libtool libconfig-dev`

GekkoFS testing support: `apt install python3-dev python3 python3-venv`

With testing

### CentOS/Red Hat

GekkoFS base dependencies: `yum install gcc-c++ git curl cmake autoconf automake libtool libconfig`

GekkoFS testing support: `python38-devel` (**>Python-3.6 required**)

# Step-by-step installation

1. Make sure the above listed dependencies are available on your machine
2. Clone GekkoFS: `git clone --recurse-submodules https://storage.bsc.es/gitlab/hpc/gekkofs.git`
    - (Optional) (Optional) If you checked out the sources using `git` without the `--recursive` option, you need to
      execute the following command from the root of the source directory: `git submodule update --init`
3. Set up the necessary environment variables where the compiled direct GekkoFS dependencies will be installed at (we
   assume the path `/home/foo/gekkofs_deps/install` in the following)
    -
   `export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:/home/foo/gekkofs_deps/install/lib:/home/foo/gekkofs_deps/install/lib64`
4. Download and compile the direct dependencies, e.g.,
    - Download example: `gekkofs/scripts/dl_dep.sh /home/foo/gekkofs_deps/git`
    - Compilation example: `gekkofs/scripts/compile_dep.sh /home/foo/gekkofs_deps/git /home/foo/gekkofs_deps/install`
    - Consult `-h` for additional arguments for each script
5. Compile GekkoFS and run optional tests
    - Create build directory: `mkdir gekkofs/build && cd gekkofs/build`
    - Configure GekkoFS: `cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=/home/foo/gekkofs_deps/install ..`
        - add `-DCMAKE_INSTALL_PREFIX=<install_path>` where the GekkoFS client library and server executable should be
          available
        - add `-DGKFS_BUILD_TESTS=ON` if tests should be build
    - Build and install GekkoFS: `make -j8 install`
    - Run tests: `make test`

GekkoFS is now available at:

- GekkoFS daemon (server): `<install_path>/bin/gkfs_daemon`
- GekkoFS client interception library: `<install_path>/lib64/libgkfs_intercept.so`

## Spack for installing GekkoFS (alternative)

The Spack tool can be used to easily install GekkoFS and its dependencies. Refer to the
following [README](scripts/spack/README.md) for details.

# Run GekkoFS

On each node a daemon (`gkfs_daemon` binary) has to be started. Other tools can be used to execute
the binary on many nodes, e.g., `srun`, `mpiexec/mpirun`, `pdsh`, or `pssh`.

You need to decide what Mercury NA plugin you want to use for network communication. `ofi+sockets` is the default.
The `-P` argument is used for setting another RPC protocol. See below.

- `ofi+sockets` for using the libfabric plugin with TCP (stable)
- `ofi+tcp` for using the libfabric plugin with TCP (slower than sockets)
- `ofi+verbs` for using the libfabric plugin with Infiniband verbs (reasonably stable) and requires
  the [rdma-core (formerly libibverbs)](https://github.com/linux-rdma/rdma-core) library
- `ofi+psm2` for using the libfabric plugin with Intel Omni-Path (unstable) and requires
  the [opa-psm2](https://github.com/cornelisnetworks/opa-psm2>) library

## The GekkoFS hostsfile

Each GekkoFS daemon needs to register itself in a shared file (*hostsfile*) which needs to be accessible to _all_
GekkoFS clients and daemons.
Therefore, the hostsfile describes a file system and which node is part of that specific GekkoFS file system instance.
In a typical cluster environment this hostsfile should be placed within a POSIX-compliant parallel file system, such as
GPFS or Lustre.

*Note: NFS is not strongly consistent and cannot be used for the hosts file!*

## The GekkoFS daemon

The GekkoFS daemon is the server component of GekkoFS. It is responsible for managing the file system data and metadata. There are two options to run the daemons on one or several nodes: (1) manually by executing the `gkfs_daemon` binary directly or (2) by using the `gkfs` script (recommended).

### Manual startup and shut down

tl;dr example: `<install_path>/bin/gkfs_daemon -r <fs_data_path> -m <pseudo_gkfs_mount_dir_path> -H <hostsfile_path>`

Run the GekkoFS daemon on each node specifying its locally used directory where the file system data and metadata is
stored (`-r/--rootdir <fs_data_path>`), e.g., the node-local SSD;

2. the pseudo mount directory used by clients to access GekkoFS (`-m/--mountdir <pseudo_gkfs_mount_dir_path>`); and
3. the hostsfile path (`-H/--hostsfile <hostfile_path>`).

Further options are available:

```bash
Allowed options
Usage: src/daemon/gkfs_daemon [OPTIONS]

Options:
  -h,--help                   Print this help message and exit
  -m,--mountdir TEXT REQUIRED Virtual mounting directory where GekkoFS is available.
  -r,--rootdir TEXT REQUIRED  Local data directory where GekkoFS data for this daemon is stored.
  -s,--rootdir-suffix TEXT    Creates an additional directory within the rootdir, allowing multiple daemons on one node.
  -i,--metadir TEXT           Metadata directory where GekkoFS RocksDB data directory is located. If not set, rootdir is used.
  -l,--listen TEXT            Address or interface to bind the daemon to. Default: local hostname.
                              When used with ofi+verbs the FI_VERBS_IFACE environment variable is set accordingly which associates the verbs device with the network interface. In case FI_VERBS_IFACE is already defined, the argument is ignored. Default 'ib'.
  -H,--hosts-file TEXT        Shared file used by deamons to register their endpoints. (default './gkfs_hosts.txt')
  -P,--rpc-protocol TEXT      Used RPC protocol for inter-node communication.
                              Available: {ofi+sockets, ofi+verbs, ofi+psm2} for TCP, Infiniband, and Omni-Path, respectively. (Default ofi+sockets)
                              Libfabric must have enabled support verbs or psm2.
  --auto-sm                   Enables intra-node communication (IPCs) via the `na+sm` (shared memory) protocol, instead of using the RPC protocol. (Default off)
  --clean-rootdir             Cleans Rootdir >before< launching the deamon
  -c,--clean-rootdir-finish   Cleans Rootdir >after< the deamon finishes
  -d,--dbbackend TEXT         Metadata database backend to use. Available: {rocksdb, parallaxdb}
                              RocksDB is default if not set. Parallax support is experimental.
                              Note, parallaxdb creates a file called rocksdbx with 8GB created in metadir.
  --parallaxsize TEXT         parallaxdb - metadata file size in GB (default 8GB), used only with new files
  --enable-collection         Enables collection of general statistics. Output requires either the --output-stats or --enable-prometheus argument.
  --enable-chunkstats         Enables collection of data chunk statistics in I/O operations.Output requires either the --output-stats or --enable-prometheus argument.
  --output-stats TEXT         Creates a thread that outputs the server stats each 10s to the specified file.
  --enable-prometheus         Enables prometheus output and a corresponding thread.
  --prometheus-gateway TEXT   Defines the prometheus gateway <ip:port> (Default 127.0.0.1:9091).
  --version                   Print version and exit.
```

It is possible to run multiple independent GekkoFS instances on the same node. Note, that when these GekkoFS instances
are part of the same file system, use the same `rootdir` with different `rootdir-suffixe`s.

Shut it down by gracefully killing the process (SIGTERM).

### GekkoFS daemon orchestration via the `gkfs` script (recommended)

The `scripts/run/gkfs` script can be used to simplify starting the GekkoFS daemon on one or multiple nodes. To start
GekkoFS on multiple nodes, a Slurm environment that can execute `srun` is required. Users can further
modify `scripts/run/gkfs.conf` to mold default configurations to their environment.

The following options are available for `scripts/run/gkfs`:

```bash
usage: gkfs [-h/--help] [-r/--rootdir <path>] [-m/--mountdir <path>] [-a/--args <daemon_args>] [--proxy <false>] [-f/--foreground <false>]
        [--srun <false>] [-n/--numnodes <jobsize>] [--cpuspertask <64>] [-v/--verbose <false>]
        {start,expand,stop}


    This script simplifies the starting and stopping GekkoFS daemons. If daemons are started on multiple nodes,
    a Slurm environment is required. The script looks for the 'gkfs.conf' file in the same directory where
    additional permanent configurations can be set.

    positional arguments:
            COMMAND                 Command to execute: 'start', 'stop', 'expand'

    optional arguments:
            -h, --help              Shows this help message and exits
            -r, --rootdir <path>    The rootdir path for GekkoFS daemons.
            -m, --mountdir <path>   The mountdir path for GekkoFS daemons.
            -d, --daemon_args <daemon_arguments>
            --proxy                 Start proxy after the daemons are running.
                                    Add various additional daemon arguments, e.g., "-l ib0 -P ofi+psm2".
            -p, --proxy_args <proxy_arguments>
            -f, --foreground        Starts the script in the foreground. Daemons are stopped by pressing 'q'.
            --srun                  Use srun to start daemons on multiple nodes.
            -n, --numnodes <n>      GekkoFS daemons are started on n nodes.
                                    Nodelist is extracted from Slurm via the SLURM_JOB_ID env variable.
            --cpuspertask <#cores>  Set the number of cores the daemons can use. Must use '--srun'.
            -c, --config            Path to configuration file. By defaults looks for a 'gkfs.conf' in this directory.
            -e, --expand_hostfile   Path to the hostfile with new nodes where GekkoFS should be extended to (hostfile contains one line per node).
            -v, --verbose           Increase verbosity
```

## The GekkoFS client library

### Interposition library via system call interception

tl;dr example:

```bash
export LIBGKFS_ HOSTS_FILE=<hostfile_path>
LD_PRELOAD=<install_path>/lib64/libgkfs_intercept.so cp ~/some_input_data <pseudo_gkfs_mount_dir_path>/some_input_data
LD_PRELOAD=<install_path>/lib64/libgkfs_intercept.so md5sum ~/some_input_data <pseudo_gkfs_mount_dir_path>/some_input_data
```

Clients read the hostsfile to determine which daemons are part of the GekkoFS instance. Because the client is an
interposition library that is loaded within the context of the application, this information is passed via the
environment variable `LIBGKFS_HOSTS_FILE` pointing to the hostsfile path. The client library itself is loaded for each
application process via the `LD_PRELOAD` environment variable intercepting file system related calls. If they are
within (or hierarchically under) the GekkoFS mount directory they are processed in the library, otherwise they are
passed to the kernel.

Note, if `LD_PRELOAD` is not pointing to the library and, hence the client is not loaded, the mounting directory appears
to be empty.

For MPI application, the `LD_PRELOAD` variable can be passed with the `-x` argument for `mpirun/mpiexec`.

### User library via linking against the application

GekkoFS offers a user library that can be linked against the application, which is built by default:
`libgkfs_user_lib.so` shared library. The corresponding API and developer headers are available in
`include/client/user_functions.hpp`. Please consult `examples/user_library` for details.

In this case, `LD_PRELOAD` is not necessary. Nevertheless, `LIBGKFS_HOSTS_FILE` is still required.

## Logging

The following environment variables can be used to enable logging in the client library: `LIBGKFS_LOG=<module>`
and `LIBGKFS_LOG_OUTPUT=<path/to/file>` to configure the output module and set the path to the log file of the client
library. If not path is specified in `LIBGKFS_LOG_OUTPUT`, the client library will send log messages
to `/tmp/gkfs_client.log`. Use `LIBGKFS_LOG_PER_PROCESS=ON` to write separate logs per client process.
When enabled, the path specified with `LIBGKFS_LOG_OUTPUT=<path/to/dir>` is used as a directory.

The following modules are available:

- `none`: don't print any messages
- `syscalls`: Trace system calls: print the name of each system call, its
  arguments, and its return value. All system calls are printed after being
  executed save for those that may not return, such as `execve()`,
  `execve_at()`, `exit()`, and `exit_group()`. This module will only be
  available if the client library is built in `Debug` mode.
- `syscalls_at_entry`: Trace system calls: print the name of each system call
  and its arguments. All system calls are printed before being executed and
  therefore their return values are not available in the log. This module will
  only be available if the client library is built in `Debug` mode.
- `info`: Print information messages.
- `critical`: Print critical errors.
- `errors`: Print errors.
- `warnings`: Print warnings.
- `mercury`: Print Mercury messages.
- `debug`: Print debug messages. This module will only be available if the
  client library is built in `Debug` mode.
- `most`: All previous options combined except `syscalls_at_entry`. This
  module will only be available if the client library is built in `Debug`
  mode.
- `all`: All previous options combined.
- `trace_reads`: Generate log line with extra information in read operations for guided distributor
- `help`: Print a help message and exit.

When tracing sytem calls, specific syscalls can be removed from log messages by
setting the `LIBGKFS_LOG_SYSCALL_FILTER` environment variable. For instance,
setting it to `LIBGKFS_LOG_SYSCALL_FILTER=epoll_wait,epoll_create` will filter
out any log entries from the `epoll_wait()` and `epoll_create()` system calls.

Additionally, setting the `LIBGKFS_LOG_OUTPUT_TRUNC` environment variable with a value different from `0` will instruct
the logging subsystem to truncate the file used for logging, rather than append to it.

For the daemon, the `GKFS_DAEMON_LOG_PATH=<path/to/file>` environment variable can be provided to set the path to the
log file, and the log module can be selected with the `GKFS_DAEMON_LOG_LEVEL={off,critical,err,warn,info,debug,trace}`
environment variable.

# Advanced and experimental features

## Rename

`-DGKFS_RENAME_SUPPORT` allows the application to rename files.
This is an experimental feature, and some scenarios may not work properly.
Support for fstat in renamed files is included.

This is disabled by default.

## Replication

The user can enable the data replication feature by setting the replication environment variable:
`LIBGKFS_NUM_REPL=<num repl>`.
The number of replicas should go from `0` to the `number of servers - 1`. The replication environment variable can be
set up for each client independently.

## Client-side metrics via MessagePack and ZeroMQ

GekkoFS clients support capturing the I/O traces of each individual process and periodically exporting them to a given
file or ZeroMQ sink via the TCP protocol.
To use this feature, the corresponding ZeroMQ (`libzmq` and `cppzmq`) dependencies are required which can be found in
the `default_zmq` dependency profile.
In addition, GekkoFS must be compiled with client metrics enabled (disabled by default) via the CMake argument
`-DGKFS_ENABLE_CLIENT_METRICS=ON`.

Client metrics are individually enabled per GekkoFS client process via the following environment variables:

- `LIBGKFS_ENABLE_METRICS=ON` enables capturing client-side metrics.
- `LIBGKFS_METRICS_FLUSH_INTERVAL=10` sets the flush interval to 10 seconds (defaults to 5). All outstanding client
  metrics are flushed when the process ends.
- `LIBGKFS_METRICS_PATH=<path>` sets the path to flush client-metrics (defaults to `/tmp/gkfs_client_metrics`).
- `LIBGKFS_METRICS_IP_PORT=127.0.0.1:5555` enables flushing to a set ZeroMQ server. This option disables flushing to a
  file.

The ZeroMQ export can be tested via the `gkfs_clientmetrics2json` application which is built when enabling the CMake
option `-DGKFS_BUILD_TOOLS=ON`:

- Starting the ZeroMQ server: `gkfs_clientmetrics2json tcp://127.0.0.1:5555`
- `gkfs_clientmetrics2json <path>` can also be used to unpack the Messagepack export from a file.
  Examplarily output with the ZeroMQ sink enabled when running:
  `LD_PRELOAD=libgkfs_intercept.so LIBGKFS_ENABLE_METRICS=ON LIBGKFS_METRICS_IP_PORT=127.0.0.1:5555 gkfs cp testfile /tmp/gkfs_mountdir/testfile`:

```bash
~ $ gkfs_clientmetrics2json tcp://127.0.0.1:5555
Binding to: tcp://127.0.0.1:5555
Waiting for message...

Received message with size 68
Generated JSON:
[extra]avg_thruput_mib: [221.93,175.87,266.81,135.69]
end_t_micro: [8008,12396,16006,18454]
flush_t: 18564
hostname: "evie"
io_type: "w"
pid: 1259304
req_size: [524288,524288,524288,229502]
start_t_micro: [5755,9553,14132,16841]
total_bytes: 1802366
total_iops: 4
```

## Server-side statistics via Prometheus

GekkoFS daemons are able to output general operations (`--enable-collection`) and data chunk
statistics (`--enable-chunkstats`) to a specified output file via `--output-stats <FILE>`. Prometheus can also be used
instead or in addition to the output file. It must be enabled at compile time via the CMake
argument `-DGKFS_ENABLE_PROMETHEUS` and the daemon argument `--enable-prometheus`. The corresponding statistics are then
pushed to the Prometheus instance.

## GekkoFS proxy

The GekkoFS proxy is an additional (alternative) component that runs on each client and acts as gateway between the
client and daemons. It can improve network stability, e.g., for opa-psm2, and provides a basis for future asynchronous
I/O as well as client caching techniques to control file system semantics.

The `gkfs` script fully supports the GekkoFS proxy and an example can be found in `scripts/run`. When using the proxy
manually additional arguments are required on the daemon side, i.e., which network interface and protocol should be
used:

```bash
<daemon args> --proxy-listen eno1 --proxy-protocol ofi+sockets
```

The proxy is started thereafter:

```bash
./gkfs_proxy -H ./gkfs_hostfile --pid-path ./vef_gkfs_proxy.pid -p ofi+sockets
```

The shared hostfile was generated by the daemons whereas the pid_path is local to the machine and is
detected by clients. The pid-path defaults to `/tmp/gkfs_proxy.pid`.

Under default operation, clients detect automatically whether to use the proxy. If another proxy path is used, the
environment variable `LIBGKFS_PROXY_PID_FILE` can be set for the clients.

Alternatively, the `gkfs` automatically sets all required arguments:

```bash
scripts/run/gkfs -c scripts/run/gkfs.conf -f start --proxy
* [gkfs] Starting GekkoFS daemons (1 nodes) ...
* [gkfs] GekkoFS daemons running
* [gkfs] Startup time: 2.013 seconds
* [gkfs] Starting GekkoFS proxies (1 nodes) ...
* [gkfs] GekkoFS proxies running
* [gkfs] Startup time: 5.002 seconds
Press 'q' to exit
```

Please consult `include/config.hpp` for additional configuration options. Note, GekkoFS proxy does not support
replication.

## File system expansion

GekkoFS supports extending the current daemon configuration to additional compute nodes. This includes redistribution of
the existing data and metadata and therefore scales file system performance and capacity of existing data. Note,
that it is the user's responsibility to not access the GekkoFS file system during redistribution. A corresponding
feature that is transparent to the user is planned. Note also, if the GekkoFS proxy is used, they need to be manually
restarted, after expansion.

To enable this feature, the following CMake compilation flags are required to build the `gkfs_malleability` tool:
`-DGKFS_BUILD_TOOLS=ON`. The `gkfs_malleability` tool is then available in the `build/tools` directory. Please consult
`-h` for its arguments. While the tool can be used manually to expand the file system, the `scripts/run/gkfs` script
should be used instead which invokes the `gkfs_malleability` tool.

The only requirement for extending the file system is a hostfile containing the hostnames/IPs of the new nodes (one line
per host). Example starting the file system. The `DAEMON_NODELIST` in the `gkfs.conf` is set to a hostfile containing
the initial set of file system nodes.:

```bash
~/gekkofs/scripts/run/gkfs -c ~/run/gkfs_verbs_expandtest.conf start
* [gkfs] Starting GekkoFS daemons (4 nodes) ...
* [gkfs] GekkoFS daemons running
* [gkfs] Startup time: 10.853 seconds
```

... Some computation ...

Expanding the file system. Using `-e <hostfile>` to specify the new nodes. Redistribution is done automatically with a
progress bar. When finished, the file system is ready to use in the new configuration:

```bash
~/gekkofs/scripts/run/gkfs -c ~/run/gkfs_verbs_expandtest.conf -e ~/hostfile_expand expand
* [gkfs] Starting GekkoFS daemons (8 nodes) ...
* [gkfs] GekkoFS daemons running
* [gkfs] Startup time: 1.058 seconds
Expansion process from 4 nodes to 12 nodes launched...
* [gkfs] Expansion progress:
[####################] 0/4 left
* [gkfs] Redistribution process done. Finalizing ...
* [gkfs] Expansion done.
```

Stop the file system:

```bash
~/gekkofs/scripts/run/gkfs -c ~/run/gkfs_verbs_expandtest.conf stop
* [gkfs] Stopping daemon with pid 16462
srun: sending Ctrl-C to StepId=282378.1
* [gkfs] Stopping daemon with pid 16761
srun: sending Ctrl-C to StepId=282378.2
* [gkfs] Shutdown time: 1.032 seconds
```

# Miscellaneous

## External functions

GekkoFS allows to use external functions on your client code, via LD_PRELOAD.
Source code needs to be compiled with -fPIC. We include a pfind io500 substitution,
`examples/gfind/gfind.cpp` and a non-mpi version `examples/gfind/sfind.cpp`

## Data placement

The data distribution can be selected at compilation time, we have 2 distributors available:

### Simple Hash (Default)

Chunks are distributed randomly to the different GekkoFS servers.

### Guided Distributor

The guided distributor allows defining a specific distribution of data on a per directory or file basis.
The distribution configurations are defined within a shared file (called `guided_config.txt` henceforth) with the
following format:
`<path> <chunk_number> <host>`

To enable the distributor, the following CMake compilation flags are required:

* `GKFS_USE_GUIDED_DISTRIBUTION` ON
* `GKFS_USE_GUIDED_DISTRIBUTION_PATH` `<path_guided_config.txt>`

To use a custom distribution, a path needs to have the prefix `#` (e.g., `#/mdt-hard 0 0`), in which all the data of all
files in that directory goes to the same place as the metadata.
Note, that a chunk/host configuration is inherited to all children files automatically even if not using the prefix.
In this example, `/mdt-hard/file1` is therefore also using the same distribution as the `/mdt-hard` directory.
If no prefix is used, the Simple Hash distributor is used.

#### Guided configuration file

Creating a guided configuration file is based on an I/O trace file of a previous execution of the application.
For this the `trace_reads` tracing module is used (see above).

The `trace_reads` module enables a `TRACE_READS` level log at the clients writing the I/O information of the client
which is used as the input for a script that creates the guided distributor setting.
Note that capturing the necessary trace records can involve performance degradation.
To capture the I/O of each client within a SLURM environment, i.e., enabling the `trace_reads` module and print its
output to a user-defined path, the following example can be used:
`srun -N 10 -n 320 --export="ALL" /bin/bash -c "export LIBGKFS_LOG=trace_reads;LIBGKFS_LOG_OUTPUT=${HOME}/test/GLOBAL.txt;LD_PRELOAD=${GKFS_PRLD} <app>"`

Then, the `examples/distributors/guided/generate.py` scrpt is used to create the guided distributor configuration file:

* `python examples/distributors/guided/generate.py ~/test/GLOBAL.txt >> guided_config.txt`

Finally, modify `guided_config.txt` to your distribution requirements.

## Metadata Backends

There are two different metadata backends in GekkoFS. The default one uses `rocksdb`, however an alternative based
on `PARALLAX` from `FORTH`
is available. To enable it use the `-DGKFS_ENABLE_PARALLAX:BOOL=ON` option, you can also disable `rocksdb`
with `-DGKFS_ENABLE_ROCKSDB:BOOL=OFF`.

Once it is enabled, `--dbbackend` option will be functional.

## CMake options

#### Core
- `GKFS_BUILD_TOOLS` - Build tools (default: OFF)
- `GKFS_BUILD_TESTS` - Build tests (default: OFF)
- `GKFS_CREATE_CHECK_PARENTS` - Enable checking parent directory for existence before creating children (default: ON)
- `GKFS_MAX_INTERNAL_FDS` - Number of file descriptors reserved for internal use (default: 256)
- `GKFS_MAX_OPEN_FDS` - Maximum number of open file descriptors supported (default: 1024)
- `GKFS_RENAME_SUPPORT` - Enable support for rename (default: OFF)
- `GKFS_FOLLOW_EXTERNAL_SYMLINKS` - Enable support for following external links into the GekkoFS namespace (default: OFF)
- `GKFS_USE_LEGACY_PATH_RESOLVE` - Use the legacy implementation of the resolve function, deprecated (default: OFF)
- `GKFS_USE_GUIDED_DISTRIBUTION` - Use guided data distributor (default: OFF)
- `GKFS_USE_GUIDED_DISTRIBUTION_PATH` - File Path for guided distributor (default: /tmp/guided.txt)

#### Logging
- `GKFS_ENABLE_CLIENT_LOG` - Enable logging messages in clients (default: ON)
- `GKFS_CLIENT_LOG_MESSAGE_SIZE` - Maximum size of a log message in the client library (default: 1024)

#### Statistics
- `GKFS_ENABLE_PROMETHEUS` - Enable pushing daemon statistics to a Prometheus Gateway (default: OFF)
- `GKFS_ENABLE_CLIENT_METRICS` - Enable client metrics via MSGPack (default: OFF)

#### Backends
- `GKFS_ENABLE_ROCKSDB` - Enable RocksDB metadata backend (default: ON)
- `GKFS_ENABLE_PARALLAX` - Enable Parallax metadata support (default: OFF)

## Environment variables
The GekkoFS daemon, client, and proxy support a number of environment variables to augment its functionality:

### Client
#### Core
- `LIBGKFS_HOSTS_FILE` - Path to the hostsfile (created by the daemon and mandatory for the client).
#### Logging
- `LIBGKFS_LOG` - Log module of the client. 
Available modules are: `none`, `syscalls`, `syscalls_at_entry`, `info`, `critical`, `errors`, `warnings`, `mercury`, `debug`, `most`, `all`, `trace_reads`, `help`.
- `LIBGKFS_LOG_OUTPUT` - Path to the log file of the client.
- `LIBGKFS_LOG_PER_PROCESS` - Write separate logs per client process.
- `LIBGKFS_LOG_SYSCALL_FILTER` - Filter out specific system calls from log messages.
- `LIBGKFS_LOG_OUTPUT_TRUNC` - Truncate the file used for logging.
#### Client-metrics
Client-metrics require the CMake argument `-DGKFS_ENABLE_CLIENT_METRICS=ON` (see above).
- `LIBGKFS_ENABLE_METRICS` - Enable capturing client-side metrics.
- `LIBGKFS_METRICS_FLUSH_INTERVAL` - Set the flush interval for client metrics.
- `LIBGKFS_METRICS_PATH` - Path to flush client metrics.
- `LIBGKFS_METRICS_IP_PORT` - Enable flushing to a set ZeroMQ server (replaces `LIBGKFS_METRICS_PATH`).
- `LIBGKFS_PROXY_PID_FILE` - Path to the proxy pid file (when using the GekkoFS proxy).
- `LIBGKFS_NUM_REPL` - Number of replicas for data.
#### Caching
##### Dentry cache
Improves performance for `ls -l` type operations by caching file metadata for subsequent `stat()` operations during
`readdir()`. Dependening on the size of the directory, this can avoid a signficant number of stat RPCs.
- `LIBGKFS_DENTRY_CACHE` - Enable caching directory entries until closing the directory (default: OFF).
  Further compile-time settings available at `include/config.hpp`.

##### Write size cache
During write operations, the client must update the file size on the responsible metadata daemon. The write size cache
can reduce the metadata load on the daemon and reduce the number of RPCs during write operations, especially for many
small I/O operations.

Note that this cache may impact file size consistency in which stat operations may not reflect the actual file size
until the file is closed. The cache does not impact the consistency of the file data itself.

- `LIBGKFS_WRITE_SIZE_CACHE` - Enable caching the write size of files (default: OFF).
- `LIBGKFS_WRITE_SIZE_CACHE_THRESHOLD` - Set the number of write operations after which the file size is synchronized
  with the corresponding daemon (default: 1000). The file size is further synchronized when the file is `close()`d or
  when `fsync()` is called.

### Daemon
#### Logging
- `GKFS_DAEMON_LOG_PATH` - Path to the log file of the daemon.
- `GKFS_DAEMON_LOG_LEVEL` - Log level of the daemon. Available levels are: `off`, `critical`, `err`, `warn`, `info`, `debug`, `trace`.
### Proxy
#### Logging
- `GKFS_PROXY_LOG_PATH` - Path to the log file of the proxy.
- `GKFS_PROXY_LOG_LEVEL` - Log level of the proxy. Available levels are: `off`, `critical`, `err`, `warn`, `info`, `debug`, `trace`.

# Acknowledgment

This software was partially supported by the EC H2020 funded NEXTGenIO project (Project ID: 671951, www.nextgenio.eu).

This software was partially supported by the ADA-FS project under the SPPEXA project (http://www.sppexa.de/) funded by
the DFG.

This software is partially supported by the FIDIUM project funded by the DFG.

This work was partially funded by the European Union’s Horizon 2020 and the German Ministry of Education and Research (
BMBF) under the ``Adaptive multi-tier intelligent data manager for Exascale (ADMIRE)''
project (https://www.admire-eurohpc.eu/); Grant Agreement number:
956748-ADMIRE-H2020-JTI-EuroHPC-2019-1. Further, this work was partially supported by the Spanish Ministry of Economy
and Competitiveness (MINECO) under grants PID2019-107255GB, and the Generalitat de Catalunya under contract
2021-SGR-00412. This publication is part of the project ADMIRE PCI2021-121952, funded by MCIN/AEI/10.13039/501100011033.