Ascp FAQs

  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. In an ascp command, use the following flags to specify transfer policies that are fixed, fair, high, or 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

    The policies have the following characteristics:

    • fixed – Attempts to transfer at the specified target rate, regardless of the actual network capacity. This policy transfers at a constant rate and finishes in a guaranteed time. This policy typically occupies most of the network's bandwidth, and is not recommended in most file transfer scenarios. In fixed mode, a maximum (target) rate value is required.
    • high – Monitors the network and adjusts the transfer rate to fully utilize the available bandwidth up to the maximum rate. When congestion occurs, a it transfers at a rate twice of a session with fair policy. In this mode, both the maximum (target) and the minimum transfer rates are required.
    • fair – Monitors the network and adjusts the transfer rate to fully utilize the available bandwidth up to the maximum rate. When other types of traffic build up and congestion occurs, it shares bandwidth fairly by transferring at an even rate. In this mode, both the maximum (target) and the minimum transfer rates are required.
    • low – Similar to fair mode, the low policy uses the available bandwidth up to the maximum rate, but is much less aggressive when sharing bandwidth with other network traffic. When congestion builds up, the transfer rate is reduced to the minimum rate until other traffic retreats.
  2. What transfer speed should I expect? 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 typically occupies most of a network's bandwidth. Aspera recommends this test be performed on a dedicated file transfer line or during a time of low network activity.

    On the client machine, start a transfer with fixed bandwidth policy. Start with a lower transfer rate and gradually increase the transfer rate toward the network bandwidth (for example, 1 MB, 5 MB, 10 MB, and so on). Monitor the transfer rate; at its maximum, it should be slighly below your available bandwidth:

    $ ascp -l 1m source-file destination 

    To improve the transfer speed, also consider upgrading the following hardware components:

    Component Description
    Hard disk The I/O throughput, the disk bus architecture (such as 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 or fails to finish, it will resume without retransferring 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 the current and original attributes match.
    • -k 2 – Check file attributes and do a sparse file checksum; resume if the current and original attributes/checksums match.
    • -k 3 – Check file attributes and do a full file checksum; resume if the current and original attributes/checksums match.

    Corruption or deletion of the .asp-meta file associated with an incomplete transfer will often result in a permanently unusable destination file even if the file transfer resumed and successfully transferred.

  4. How does Aspera handle symbolic links?
    The ascp command follows symbolic links by default. This can be changed using --symbolic-links=method with the following options:
    • follow – Follow symbolic links and transfer the linked files.
    • copy – Copy only the alias file. If a file with the same name exists on the destination, the symbolic link will not be copied.
    • copy+force – Copy only the alias file. If a file with the same name exists on the destination, the symbolic link will replace the file. If the file of the same name on the destination is a symbolic link to a directory, it will not be replaced.
    • skip – Skip the symbolic links.
    Important: On Windows, the only option is skip.
  5. What are my choices for overwriting files on the destination computer?
    In ascp, you can specify the --overwrite=method rule with the following method options:
    • never – Never overwrite the file. However, if the parent folder is not empty, its access, modify, and change times may still be updated.
    • always – Always overwrite the file.
    • diff – Overwrite the file if it is different from the source. If a complete file at the destination is the same as the source then it will not be overwritten. Partial files will be overwritten or resumed depending on the resume policy.
    • diff+older – Overwrite the file if it is older and different than the source.
    • older – Overwrite the file if its timestamp is older than the source timestamp.

    Important: If the overwrite method is diff or diff+older, difference is determined by the resume policy (-k{0|1|2|3}). If -k 0 or no -k is specified, the source and destination files are always considered different and the destination file is always overwritten. If -k 1, the source and destination files are compared based on file attributes (currently file size). If -k 2, the source and destination files are compared based on sparse checksum. If -k 3, the source and destination files are compared based on full checksum.