About Partial Releasing and Partial Staging

Releasing and staging are complementary processes. Files can be completely released from online disk cache after they are archived, or a site can specify that the beginning of a file (the stub) remain in disk cache while the remainder of the file is released. Partially releasing a file provides immediate access to data in the file stub without requiring that the file be staged. You can specify both the default partial release size and the maximum size of the stub to remain online when a file system is mounted.

You can set the through the values for partial releasing and staging using the mount command or in the SAM-QFS Manager. See the SAM-QFS Manager online help for more information.

The mount command options are as follows:

-o partial= n option - Sets the default size (n) of a file stub to remain online. The -o partial= n setting must be less than or equal to the -o maxpartial= n setting.
-o maxpartial= n option - Sets the maximum size (n) of a file stub to remain online.

You can specify the default stub size for a file by specifying the --p option on the release command or the -p option on the sam_release library routine. To specify different-sized file stubs for different types of files or different applications, specify the --s option on the release command or the -s option on the sam_release library routine. The -s and s values must be less than the -o maxpartial value used with the mount command when the file system was mounted.

Note - A partially released file takes up space on the disk equal to one DAU. For example, if the partial release file stub is set to 16K and the DAU size is 256K, the actual space consumed by the file on the disk is 256K.

Use the mount option, -o partial_stage= n, to establish how much of a partial release stub must be read before the rest of the file is staged. Reading past the -o partial_stage= n size specification initiates the stage of the file.

By default, the -o partial_stage= n option is set to the size of the partial release stub. Changing this value affects file staging as follows:

If the -o partial_stage= n option is set to the size of the partial release stub, the default behavior prevents the file from being staged until the application reaches the end of the partial release stub. Waiting until the end of the stub is reached causes a delay in accessing the rest of the file.
If the -o partial_stage= n option is set to a value smaller than the partial release stub, the file is staged after the application crosses the threshold set by the -o partial_stage= n option. This reduces the chance of a delay in accessing the rest of the file data.

Example 14-1 Partial Staging

In this example, a site has set the following options:

-o partial_stage=16 (16 kilobytes)
-o partial=2097152 (2 gigabytes)
-o maxpartial=2097152 (2 gigabytes)

The filemgr program reads the first 8 kilobytes of a file. The file is not staged.

A video-on-demand application reads the same file. After it reads past the first 16 kilobytes of the file, the file is staged. The application continues reading while the archive tape is mounted and positioned.

When the video-on-demand application reads past 2 gigabytes of file data, it is reading immediately behind the staging activity. The application does not wait, because the tape mounting and positioning is done while the application reads the partial file data.

Several command-line options affect whether a file can be marked for partial release. Some options are enabled by the system administrator, and others can be enabled by individual users. The following sections describe the release characteristics that can be set by the various types of users.

Summary of System Administrator Options
Summary of User Options

Summary of System Administrator Options

As a system administrator, you can change the maximum value and default value for partial release when the file system is mounted. The mount options in the following table affect partial release. For more information about the mount command, see mount_samfs(1M) in Sun QFS and Sun Storage Archive Manager Reference Manual.

Option	Effect
`-o maxpartial=` n	Determines the maximum amount of space in kilobytes that can remain in disk cache when a file is marked for partial release. The maximum value is 2,097,152 kilobytes, which is 2 gigabytes. The minimum value is 0, which disables the partial release feature is disabled so that released files are released completely, and no portion of a file remains in disk cache. Users cannot override the value specified on this option after the file system is mounted. By default, the n argument is set to 16.
`-o partial=` n	Sets a default amount of space in kilobytes that remains in disk cache when a user marks a file for partial release by using the `release` command's `-p` option. The n argument must be at least 8 and it can be as great as the value specified for the `-o maxpartial=` `n` option. Because some applications do not need access to the entire file to complete their operations, this option ensures that applications have the beginnings of files available to them. Also, this option prevents files from being staged unnecessarily. By default, n is 16. A file that has been partially released from a disk takes up space on the disk equal to one DAU.
`-o partial_stage=` n	Specifies that when a partially released file is accessed, n bytes of the file must be read before the entire file is staged from the archive media. Set this value lower than the amount of the `-o partial` setting. For n, specify an integer from 0 to the `-o maxpartial` specification. By default, this value is 16 or the value specified for the `--o partial` option.
`-o stage_n_window=` n	Specifies the amount of data to be staged at any one time to n. Specify an integer from 64 to 2,048,000. The default is 256 kilobytes. This option applies only to files that have the `stage -n` attribute set.

Summary of User Options

As a user, you can set maximum and default values for the size of a file stub that can remain in disk cache after the file is released. You can also determine whether the partial release feature is enabled for a particular file system.

By using the release command and the sam_release library routines, however, a user can set other release attributes and can specify the files to be marked for partial release. The command and library options that determine partial release attributes are shown in the following table. For more information, see release(1) in Sun QFS and Sun Storage Archive Manager Reference Manual and sam_release(3) in Sun QFS and Sun Storage Archive Manager Reference Manual.

Table 14-1 User Release Options

Options

Effect

release command and --p option or sam_release library routine and -p option

The --p and -p options mark the named file for partial release. If these options are used, the amount of the file remaining in online disk cache after the file is released depends on the value of the -o partial= n option that was set when the file system in which the file resides was mounted. These options cannot be used to specify the number of bytes to remain online.

release command and --s partial_size option or sam_release library routine and -s option

The -s and s options mark the named file for partial release, and they specify the amount of the file to remain in online disk cache. The arguments to the -s or s options specify the amount, in kilobytes, to remain online. A user cannot specify that the amount of a file remaining online be greater than the amount specified for the -o maxpartial= n value when the file system was mounted. If the user's value is greater than the value for the file system, the value for the file system is used, and the user's specification is ignored.

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	Sun Storage Archive Manager 5.3 Configuration and Administration Guide Sun QFS and Sun Storage Archive Manager 5.3 Information Library