Ascp FAQs

Answers to some common questions about controlling transfer behavior, such as bandwidth usage, resuming files, and overwriting files.

  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:

    • high - Adjust the transfer rate to fully utilize the available bandwidth up to the maximum rate. When congestion occurs, the transfer rate is twice as fast as a fair-policy transfer. The high policy requires maximum (target) and minimum transfer rates.
    • fair - Adjust the transfer rate to fully utilize the available bandwidth up to the maximum rate. When congestion occurs, bandwidth is shared fairly by transferring at an even rate. The fair policy requires maximum (target) and minimum transfer rates.
    • low - Adjust the transfer rate to use the available bandwidth up to the maximum rate. Similar to fair mode, but less aggressive when sharing bandwidth with other network traffic. When congestion occurs, the transfer rate is reduced to the minimum rate until other traffic decreases.
    • fixed - Attempt to transfer at the specified target rate, regardless of network or storage capacity. This can decrease transfer performance and cause problems on the target storage. Aspera discourages using the fixed policy except in specific contexts, such as bandwidth testing. The fixed policy requires a maximum (target) rate.
  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 computer to connect to a server, 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 computer, 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 slightly 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 re-transferring the files?
    Use the -k flag to enable resume, and specify a resume rule:
    • -k 0 – Always re-transfer the entire file.
    • -k 1 – Compare file attributes and resume if they match, and re-transfer if they do not.
    • -k 2 – Compare file attributes and the sparse file checksums; resume if they match, and re-transfer if they do not.
    • -k 3 – Compare file attributes and the full file checksums; resume if they match, and re-transfer if they do not.

    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 followsskips 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. (Default)
    • copy - Copy only the alias file. If a file with the same name is found at the destination, the symbolic link is not copied.
    • copy+force - Copy only the alias file. If a file (not a directory) with the same name is found at the destination, the alias replaces the file. If the destination is a symbolic link to a directory, it's not replaced.
    • skip - Skip symbolic links. Do not copy the link or the file it points to.
    Important: On Windows, the only option is skip.

    Symbolic link handling also depends on the server configuration and the transfer direction. For more information, see Symbolic Link Handling.

  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 different from the source. If a complete file at the destination is the same as a file on the source, it is not overwritten. Partial files are overwritten or resumed depending on the resume policy.
    • diff+older - Overwrite the file if older and also different than the source. For example, if the destination file is the same as the source, but with a different timestamp, it will not be overwritten. Plus, if the destination file is different than the source, but newer, it will not be overwritten.
    • older - Overwrite the file if its timestamp is older than the source timestamp.

    Interaction with resume policy (-k): 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 checksums. If -k 3, the source and destination files are compared based on full checksums.