Partial file recall mode

Edit online

In an IBM® Storage Scale environment, the space management client provides a partial file recall mode to recall a portion of a migrated file. This avoids having to recall an entire, potentially large file, when only a small portion of the file is required by an application. This recall mode is only for read access on migrated files that were transferred without compression. Files that were migrated by using compression are always recalled completely. Partial file recall mode cannot be used on executable files.

If a file is larger than the value of the minpartialrecallsize option and compression is turned on, the partial file recall mode prevails, and the file is migrated without compression.

When a vendor application makes a read request for a file that is qualified for partial file recall, and the file is migrated, the space management client calculates which portion of the file to recall based on the offsets contained in the read request. This results in time and disk space savings, because only a portion of the file is recalled.

When you use partial file recall and need to work with a large portion of a large file that has been migrated, it is possible to recall a specified portion of the file in one operation. This minimizes the number of partial file recall requests and access to the server storage based on the minpartialrecallsize option value. Use the dsmrecall command with the offset and size options to specify the data range within the file.

For example, to recall 200 MB of the file /usr/cam/video2, starting at 400 MB from the beginning of the file, issue the following command: 
 dsmrecall -offset=400m -size=200m /usr/cam/video2

You can use the following methods to specify which files HSM should recall using partial file recall:

  • Set the minpartialrecallsize option of the dsmmigfs command to the minimum size (in megabytes) that a file must have to qualify for partial file recall.
  • Set the recallmode option of the dsmattr command to partialrecall. This specifies that, regardless of its size, a file is recalled using partial file recall.

    Use this method to change the recall mode of migrated files that you normally read but you do not modify. When you set the recall mode to partial file recall, this mode remains associated with your files until you:

    • Change the recall mode
    • Modify the file
    • Recall the file selectively
    • Restore the file