mscp.rst

MSCP

Date:	v0.2.1

NAME

mscp - copy files over multiple SSH connections

SYNOPSIS

mscp [-46vqDpdNh] [ -n NR_CONNECTIONS ] [ -m COREMASK ] [ -u MAX_STARTUPS ] [ -I INTERVAL ] [ -W CHECKPOINT ] [ -R CHECKPOINT ] [ -s MIN_CHUNK_SIZE ] [ -S MAX_CHUNK_SIZE ] [ -a NR_AHEAD ] [ -b BUF_SIZE ] [ -L LIMIT_BITRATE ] [ -l LOGIN_NAME ] [ -P PORT ] [ -F SSH_CONFIG ] [ -o SSH_OPTION ] [ -i IDENTITY ] [ -J DESTINATION ] [ -c CIPHER ] [ -M HMAC ] [ -C COMPRESS ] [ -g CONGESTION ] source ... target

DESCRIPTION

mscp copies files over multiple SSH (SFTP) connections by multiple threads. It enables transferring (1) multiple files simultaneously and (2) a large file in parallel, reducing the transfer time for a lot of/large files over networks.

The usage of mscp follows the scp command of OpenSSH, for example:

$ mscp srcfile user@example.com:dstfile

Remote hosts only need to run standard sshd supporting the SFTP subsystem, and users need to be able to ssh to the hosts as usual. mscp does not require anything else.

mscp uses libssh as its SSH implementation. Thus, supported SSH features, for example, authentication, encryption, and various options in ssh_config, follow what libssh supports.

OPTIONS

-n NR_CONNECTIONS

Specifies the number of SSH connections. The default value is calculated from the number of CPU cores on the host with the following formula: floor(log(nr_cores)*2)+1.

-m COREMASK

Configures CPU cores to be used by the hexadecimal bitmask. For example, -m 0x25 pins threads onto CPU cores 0, 2, and 5. The default value is not specified: all CPU cores are used and no threads are pinned to any cores.

-u MAX_STARTUPS

Specifies the number of concurrent unauthenticated SSH connection attempts. sshd limits the number of simultaneous SSH connection attempts by MaxStartups in sshd_config. The default MaxStartups is 10; thus, we set the default MAX_STARTUPS 8.

-I INTERVAL

Specifies the interval (in seconds) between SSH connection attempts. Some firewall products treat SSH connection attempts from a single source IP address for a short period as a brute force attack. This option inserts intervals between the attempts to avoid being determined as an attack. The default value is 0.

-W CHECKPOINT

Specifies a checkpoint file to save the state of a failed transfer. When transferring fails due to, for example, connection disruption or user interrupt, mscp writes the information about the remaining files and chunks to the specified checkpoint file. -W option with -D (dry-run mode) only writes a checkpoint file and exits.

-R CHECKPOINT

Specifies a checkpoint file to resume a transfer. When a checkpoint file is passed, mscp reads the checkpoint to load a remote host, copy direction, and files and their chunks to be transferred. Namely, mscp can resume a past failed transfer from the checkpoint. Resuming with a checkpoint does not require source ... target arguments. Other SSH connection options, such as port number and config file, should be specified as with the failed run. In addition, checkpoint files have file paths as relative paths. Thus, you must run mscp in the same working directory as the failed run. You can see the contents of a checkpoint file with the mscp -vv -D -R CHECKPOINT command (Dry-run mode). Note that the checkpoint file is not automatically removed after the resumed transfer ends successfully. Users should check the return value of mscp and remove the checkpoint if it returns 0.

-s MIN_CHUNK_SIZE

Specifies the minimum chunk size. mscp divides a single file into chunks and copies the chunks in parallel. The default value is 16M bytes.

-S MAX_CHUNK_SIZE

Specifies the maximum chunk size. The default is file size divided by the number of connections and devided by 4. If the calculated value is smarller than the MIN_CHUNK_SIZE value, MIN_CHUNK_SIZE is used.

-a NR_AHEAD

Specifies the number of inflight SFTP commands. The default value is 32.

-b BUF_SIZE

Specifies the buffer size for I/O and transfer over SFTP. The default value is 16384. Note that the SSH specification restricts buffer size delivered over SSH. Changing this value is not recommended at present.

-L LIMIT_BITRATE

Limits the bitrate, specified with k (K), m (M), and g (G), e.g., 100m indicates 100 Mbps.

-4

Uses IPv4 addresses only.

-6

Uses IPv6 addresses only.

-v

Increments the verbose output level.

-q

Quiet mode: turns off all outputs.

-D

Dry-run mode: it scans source files to be copied, calculates chunks, resolves destination file paths, and exits. Dry-run mode with -vv option can confirm files to be copied and their destination paths.

-r

No effect. mscp copies recursively if a source path is a directory. This option exists for just compatibility.

-l LOGIN_NAME

Specifies the username to log in on the remote machine as with ssh(1).

-P PORT

Specifies the port number to connect to on the remote machine as with scp(1).

-F SSH_CONFIG

Specifies an alternative per-user ssh configuration file. Note that acceptable options in the configuration file are what libssh supports.

-o SSH_OPTION

Specifies ssh options in the format used in ssh_config. Note that acceptable options are what libssh supports.

-i IDENTITY

Specifies the identity file for public key authentication.

-J DESTINATION

A shortcut to define a ProxyJump configuration directive. Each SFTP session of mscp connects to the target host by first making an ssh connection to the jump host described by destination.

-c CIPHER

Selects the cipher to use for encrypting the data transfer. See mscp -h or Ciphers in libssh features.

-M HMAC

Specifies MAC hash algorithms. See mscp -h or MAC hashes in libssh features.

-C COMPRESS

Enables compression: yes, no, zlib, zlib@openssh.com. The default is none. See libssh features.

-g CONGESTION

Specifies the TCP congestion control algorithm to use (Linux only). See sysctl net.ipv4.tcp_allowed_congestion_control for available values.

-p

Preserves modification times and access times (file mode bits are preserved by default).

-d

Increments the ssh debug output level.

-N

Enables Nagle's algorithm. It is disabled by default.

-h

Prints help.

EXIT STATUS

Exit status is 0 on success, and >0 if an error occurs.

ENVIRONMENT

mscp recognizes the following environment variables.

MSCP_SSH_AUTH_PASSWORD

This environment variable passes a password for password authentication to establish SSH connections.

MSCP_SSH_AUTH_PASSPHRASE

This environment variable passes a passphrase for public-key authentication for establishing SSH connections.

NOTES

mscp uses glob(3) for globbing pathnames, including matching patterns for local and remote paths. However, globbing on the remote side does not work with musl libc (used in Alpine Linux and the single-binary version of mscp) because musl libc does not support GLOB_ALTDIRFUNC.

mscp does not support remote-to-remote copy, which scp supports.

EXAMPLES

Copy a local file to a remote host with different name:

$ mscp ~/src-file 10.0.0.1:copied-file

Copy a local file and a directory to /tmp at a remote host:

$ mscp ~/src-file dir1 10.0.0.1:/tmp

Save a checkpoint if transfer fails:

$ mscp -W mscp.checkpoint many-large-files 10.0.0.1:dst/

Check the remaining files and chunks, and resume the failed transfer:

# Dump the content of a checkpoint and exit (dry-run mode)
$ mscp -vv -D -R mscp.checkpoint

# resume transferring from the checkpoint
$ mscp -R mscp.checkpoint

In a long fat network, following options might improve performance:

$ mscp -n 64 -m 0xffff -a 64 -c aes128-gcm@openssh.com src 10.0.0.1:

-n increases the number of SSH connections than default, -m pins threads to specific CPU cores, -a increases asynchronous inflight SFTP WRITE/READ commands, and -c aes128-gcm@openssh.com will be faster than the default chacha20-poly1305 cipher, particularly on hosts that support AES-NI.

SEE ALSO

scp(1), ssh(1), sshd(8).

PAPER REFERENCE

Ryo Nakamura and Yohei Kuga. 2023. Multi-threaded scp: Easy and Fast File Transfer over SSH. In Practice and Experience in Advanced Research Computing (PEARC '23). Association for Computing Machinery, New York, NY, USA, 320–323. DOI.

CONTACT INFORMATION

For patches, bug reports, or feature requests, please open an issue on GitHub.

AUTHORS

Ryo Nakamura <upa@haeena.net>