Prioritizing Preview Requests

Both the archiver and stager processes request that media is loaded and unloaded. If the number of requests exceeds the number of drives available for media loads, the excess requests are sent to the preview queue.

The number of entries that can be in the preview queue is determined by the previews= directive in the defaults.conf file. For information about changing the value of this directive, see defaults.conf(4) in Sun QFS and Sun Storage Archive Manager Reference Manual.

By default, preview requests are satisfied in first-in-first-out (FIFO) order.

The overall priority of preview requests is determined by the combination of static and dynamic factors. Higher numbers correspond to higher priority. A static priority factor is set when the request is generated. Its effect does not change the overall priority after the request is generated and is waiting to be satisfied. A dynamic priority factor can increase or decrease the overall priority of a request while the request is waiting to be satisfied.

You can override the FIFO default by entering directives in the /etc/opt/SUNWsamfs/preview.cmd command file.

About the preview.cmd File

The sam-amld daemon reads the preview.cmd file at startup. This file orders the requests in the preview queue according to whether the request is for staging or archiving. You can increase the priority for specific VSNs and you can control the priority of preview requests for specific file systems.

The following rules apply to the preview.cmd file:

• Place one directive per line.

• If you change this file while the sam-amld daemon is running, restart the daemon to have your changes take effect.

• Begin comment lines with a hash character (#).

  For more information about this file, see preview.cmd(4) in Sun QFS and Sun Storage Archive Manager Reference Manual.

The following types of directives are used in the preview.cmd file:

• Global directives, which apply to all file systems

• File-system directives, specific to individual file systems

Global directives are placed at the top of the file and their settings apply to all file systems.

File system directives begin with the fs = directive, which names the file system to which all subsequent directives apply. More than one block of file directives can appear in a file. File system directives apply until the next fs = line is encountered or until the end of file is encountered.

When multiple directives affect a file system, the directives that are specific to that file system override the global directives.

The following sections describe how to edit the preview.cmd file to control the preview queue:

• How to Set the Global VSN and Age Priority Directives

• How to Set Global or File System-Specific Water Mark Directives

• Setting Up a Preview Request Priority Scheme

How to Set the Global VSN and Age Priority Directives

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Caution - The VSN and age priority directives are global directives, so they are placed before any file-system-specific directives in the preview.cmd file.

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1. Update the vsn_priority directive.

   vsn_priority = value

   This directive is a static priority factor that indicates the value by which the total priority increases when there is a high-priority volume. The default value for is 1000.0. To use this priority factor, a volume must have its priority flag set before it is scheduled as a preview request. Use the chmed(1M) command to set the priority flag with the -p option (for example, chmed +p lt.AAA123).

2. Update the age_priority directive.

   age_priority = factor

   This directive is a static priority factor, although its effect is dynamic. This factor is multiplied by the number of seconds for which a request is a preview request. The result is then added to the overall priority of the request. The longer a request waits, the higher the priority becomes. Setting this factor ensures that older requests are not indefinitely superseded by newer requests with other higher-priority factors.

   Setting this factor to more than 1.0 increases the importance of the time factor in calculation of the total priority and setting it to less than 1.0 decreases the importance of the time factor. Setting the factor to 0.0 eliminates the time factor from the overall priority calculation.

   A volume whose priority flag is not set increases in priority based on the time it remains in the queue. Its priority can become higher than a VSN that comes into the queue later with the priority flag already set.

How to Set Global or File System-Specific Water Mark Directives

• The water mark preview request directives can be used as either global or file-system-specific directives. These directives determine the water mark priority of the preview requests, as shown in the following equation.

  lwm_priority + lhwm_priority + hlwm_priority + hwm_priority = water mark priority

  Together, the four water mark settings create a dynamic priority factor that includes a percentage value indicating how full the file system is and the levels at which the HWM and LWM are set. The value assigned to a preview request is determined by whether a factor is global, specific to a file system, or not set.

  The water mark priorities are used to calculate only requests for archiving. They are not used to calculate media requests for staging. When the water mark priority factor is a positive number, the result on the overall calculated priorities is to raise archiving requests over staging requests. In contrast, when the water mark priority factor is a negative number, the overall priority for archiving requests is reduced, which tends to favor staging requests over archival requests. A water mark priority factor of 0.0 (or no specified directive) indicates that no special action occurs.

  For more information, see Example 1 - Scheme for Enforcing Stage Requests.

  The water mark directives have the following format:

  wmtype_priority = value

  
  

  Water Mark Directive Argument lwm_priority = value Specify the amount by which you want the water mark priority factor to change for archiving requests when the file system is below the LWM level. The default is 0.0. lhwm_priority = value Specify the amount by which you want the water mark priority factor to change for archiving requests when the file system crosses from below to above the LWM level but remains below the HWM level. This shift indicates that the file system is filling up. The default is 0.0. hlwm_priority = value Specify the amount by which you want the water mark priority factor to change for archiving requests when the file system has crossed from above the HWM level to below it, but remains above the LWM level. This shift indicates that the releaser was not able to free enough disk space to leave the file system below the LWM level. The default is 0.0. hwm_priority = value Specify the amount by which you want the water mark priority factor to change for archiving requests when the file system is above the HWM level. The default is 0.0.

  When a file system crosses from one condition to another, the priority of each volume associated with that file system is recalculated based on the appropriate water mark priority setting, with or without the chmed command's -p option.

  The following example frees enough disk space so that the file system goes below the LWM level.

  lhwm_priority = -200.0
  hlwm_priority = 100.0

Setting Up a Preview Request Priority Scheme

The total priority for a preview request is the sum of all priority factors:

total priority = vsn_priority + wm_priority + (age_priority * time_in_sec_as_preview_request)

Change the default FIFO scheme only for reasons such as the following:

• Ensure that staging requests are processed before archive requests.

• Ensure that archive requests gain top priority when a file system is about to fill up.

• Push requests that use a specific group of media to the top of the preview request list.

  The following example shows a preview.cmd file that addresses these three conditions.

Example 15-6 Sample preview.cmd File

# condition 1
lwm_priority = -200.0
lhwm_priority = -200.0
hlwm_priority = -200.0
# condition 2
hwm_priority = 500.0
# condition 3
age_priority = 1.0

For environments in which user access to data is of paramount importance, the VSN drives are limited, or file archiving is performed as a background function, use the preview.cmd file to influence how the storage system resources handle staging requests. You can customize the settings in the preview.cmd file to support any of the preceding scenarios and influence the configured SAM-QFS environment.

Because data is not affected by the settings in this file, you are encouraged to experiment and adjust the directive settings to achieve the proper balance between archiving and staging requests when weighed against the priorities of each preview request.

Example 15-7 Scheme for Enforcing Stage Requests

The following example calculations show how you can use a negative value for wm_priority to ensure that stage requests have priority over archive requests. This example assumes the following:

• Several requests are sitting in the queue for 100 seconds.

• The default value vsn_priority is 1000.

The following table shows how the total request priorities are calculated as follows:

Example 15-8 Scheme for Enforcing Archive Requests

When the environment is balanced between the importance of staging a file for the user and the importance of getting new files archived to media, the biggest concern is exceeding the HWM level. In this situation, if not enough files have met their archive requirements to lower the percentage of the file system that is full, meeting the pending archive requests is the best way to keep the file system from reaching its limit.

In this situation, the preview.cmd file can be as simple as the following example:

hwm_priority = 500.0

Example 15-9 Scheme for Ranking Requests by Media Type

Assume that a site has an environment in which users are working on groups of files that use specific volumes and are segregated from other users. In this environment, certain projects might have higher priorities at certain times; therefore, greater priority is required from the available system storage resources. The following example gives users and their media the appropriate priority:

vsn_priority = 5000.0

Then, for every volume in the priority user's group, include the following information:

# chmed +p lt. VSN

Now every request that requires the specified VSN is placed above other pending mount requests in the preview queue.

Example 15-10 Scheme for Complex Priorities

Assume two file systems that have the following requirements:

• No request must wait too long in the queue (age_priority).

• When one of the file systems is below the LWM level, staging requests take precedence.

• When one of the file systems is above the LWM level but below the HWM level, do not prioritize requests.

  The following example shows the affected directives.

  lwm_priority = -200.0
  lhwm_priority = 0.0
  hlwm_priority = 0.0

  When one of the file systems goes over the HWM level, archive requests take priority.

Assume both file systems are over the HWM level but the second file system (samfs2) must be prevented from reaching its limit. The following example shows a preview.cmd file that prioritizes requests according to these requirements.

age_priority = 100.0
vsn_priority = 20000.0
lhwm_priority = -200.0
hlwm_priority = -200.0
fs = samfs1
hwm_priority = 1000.0
fs = samfs2
hwm_priority = 5000.0