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 uses 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 use 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 use 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 the bandwidth is shared 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. Use the fixed policy only for specific contexts, such as bandwidth testing, otherwise, avoid the use of this policy. The fixed policy requires a maximum target rate.
    • aggressiveness - The aggressiveness of transfers that are authorized by this access key in claiming available bandwidth. Value can be 0.00-1.00. For example, these values correspond to the policy option where a policy of high approximates to aggressiveness of 0.75, fair to 0.50 and low to 0.25. Aggressiveness can be used if you need to fine-tune the transfer policy.
  2. What transfer speed must I expect? How do I know if something is wrong with the speed?
    Aspera FASP transport has no theoretical throughput limit. Other than the network capacity, the transfer speed might 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. Perform this test 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 must 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 Fibre 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 resumes 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 – Compare file attributes and resume if they match, and retransfer if they do not.
    • -k 2 – Compare file attributes and the sparse file checksum; resume if they match, and retransfer if they do not.
    • -k 3 – Compare file attributes and the full file checksum; resume if they match, and retransfer if they do not.

    Corruption or deletion of the .asp-meta file that is associated with an incomplete transfer 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 by 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 is 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 might 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 is not overwritten. Plus, if the destination file is different than the source, but newer, it is 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 checksum. If -k 3, the source and destination files are compared based on full checksum.