Ascp FAQs

This topic lists frequently asked questions regarding ascp command:

  1. How do I control the transfer speed?

    You can specify a transfer policy that determines how a fasp transfer utilizes the network resource, and you can specify target and minimum transfer rates where applicable. With the ascp command, use the following flags to specify transfer policies that are fixed, fair, high, and low:

    Policy Command template
    Fixed
    --policy=fixed -l target_rate
    Fair
    --policy=fair -l target_rate -m min_rate
    High
    --policy=high -l target_rate -m min_rate
    Low
    --policy=low -l target_rate -m min_rate
  2. What should I expect in terms of transfer speed? How do I know if something is "wrong" with the speed?

    Aspera's fasp transport has no theoretical throughput limit. Other than the network capacity, the transfer speed may be limited by rate settings and resources of the computers. To verify that your system's fasp transfer can fulfill the maximum bandwidth capacity, prepare a client machine to connect to this computer, and test the maximum bandwidth.

    Note: This test will typically occupy the majority of a network's bandwidth. It is recommended that this test be performed on a dedicated file transfer line or during a time of very low network activity.

    On the client machine, start a transfer with fixed policy. Start with a lower transfer rate and increase gradually toward the network bandwidth (e.g. 1m, 5m, 10m...). Monitor the transfer rate and make sure that it fulfills your bandwidth:

    $ ascp -l 1m source-file destination

    To improve the transfer speed, you may also upgrade the following hardware components:

    Component Description
    Hard disk The I/O throughput, the disk bus architecture (e.g. RAID, IDE, SCSI, ATA, and Fiber Channel).
    Network I/O The interface card, the internal bus of the computer.
    CPU Overall CPU performance affects the transfer, especially when encryption is enabled.
  3. How do I ensure that if the transfer is interrupted / fails to finish, it will resume the transfer without re-transferring the files?

    Use the -k flag to enable resume, and specify a resume rule:

    • -k 0 Always retransfer the entire file.
    • -k 1 Check file attributes and resume if they match.
    • -k 2 Check file attributes and do a sparse file checksum; resume if they match.
    • -k 3 Check file attributes and do a full file checksum; resume if they match.
  4. How does Aspera handle symbolic links?

    ascp command follows symbolic links by default. There is a -o SymbolicLink flag that offers handling options:

    • --symbolic-links=follow: Follow symbolic links and transfer the linked files.
    • --symbolic-links=copy: Copy only the alias file.
    • --symbolic-links=skip: Skip the symbolic links.
  5. What are my choices regarding file overwrites on the destination computer?

    In ascp, you can specify the overwriting rule with the following flags:

    • --overwrite=always: Always overwrite the file.
    • --overwrite=never: Never overwrite the file.
    • --overwrite=diff: Overwrite if file is different from the source.
    • --overwrite=older: Overwrite if file is older than the source.

    Note: For --overwrite=diff, if a complete file exists on the destination computer (i.e., no .aspx file) and is the same as the source file, then the destination file will remain unmodified (no change on timestamp/attributes either). Otherwise the entire source file will be retransferred. Note this policy interacts with the resume policy.