Index Cache

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Overview

Shared Index Cache

• Guidelines for Creating Shared Index Cache for Alternate Data Paths (GridStor)
• Support

Calculating the Storage Space Required for Index Cache

• Best Practices for Maintaining Index Cache

Index Retention Criteria

Other Considerations

• Audit Trail

Overview

The index cache directory is the directory in which index data resides. Each MediaAgent maintains an index cache in a user designated portion of the MediaAgent computer or a shared network drive. The index data maintained in the index cache is accessed by the system during the data protection, browse and data recovery operations.

During the MediaAgent installation, the install program prompts for an index cache location for the specific MediaAgent. You can change this location and designate a new directory for the index cache at any time from the MediaAgent Properties in the CommCell Console. When you change the directory for index cache, the existing cache contents are moved and all new cache entries are directed to the specified directory. However, note that on Windows MediaAgents, the existing cache contents are copied and hence the original directory may have to be manually deleted to free the disk space.

Similarly, shared index cache must also be created from the CommCell Console, after the installing all the MediaAgents that share the index.

The index cache is maintained on a least recently used (LRU) basis. As the capacity of the cache is reached, those index data files that have been least recently accessed are overwritten with the new index data. If a data protection, browse and data recovery operation requires index data that is either no longer in the cache or not accessible to the operation, the index data is recovered from the media.

To ensure that other files do not use up disk space that is needed for index data, you can create a partition specifically for the index cache directory. The partition should be large enough to accommodate four percent of the estimated amount of data managed by the MediaAgent. Specifically, this means four percent of the maximum amount of backed-up data that can be immediately available for restoration via this MediaAgent. (See Calculating the Storage Space Required for the Index Cache Directory below for more information.)

A MediaAgent’s Index cache information be viewed or modified from the Index Cache tab of the MediaAgent Properties dialog box.

Shared Index Cache

The index cache can be shared between multiple MediaAgents for the following reasons:

• You have a dedicated partition created exclusively for index cache and hence want to share this partition between multiple MediaAgents.
• You wish to define Alternate Data Paths (GridStor) for Storage Policies and therefore wish to create a shared index cache.

Shared index cache can be configured from the MediaAgent Properties in the CommCell Console. See Create Shared Index Cache for step-by-step instructions. Note that all the MediaAgents that share the index cache must be installed before sharing the index cache. Also, all the MediaAgents that share the index cache must be of the same release version.

Guidelines for Creating Shared Index Cache for Alternate Data Paths (GridStor)

To create Alternate Data Paths in the CommCell, you must create an index cache directory in a network share that can be accessed by all the MediaAgents that are configured with alternate data paths. (See Alternate Data Paths (GridStor) for a detailed description of the topic.)

Follow the guidelines listed below while creating a Shared Index Cache Directory for alternate data paths:

• Ensure that all the MediaAgents that will be configured as data path candidates, point to the same network share and the same folder within this network share for its index cache location.
• If you have both the Unix and Windows MediaAgents pointing to the same folder, make sure that the files and folders are accessible to both these operating systems.
• For MediaAgents that access the Shared Index Cache Directory across domains, make sure that two-way trust relationships between the domains are established.
• If you reboot a Unix MediaAgent, verify and ensure that the mount point to the index cache folder is mounted in same location specified in this Index Cache tab of the MediaAgent Properties dialog box.
• If the location for the shared index cache is changed, then the changed index cache folder must be reflected in all the MediaAgents that share the index cache.

Support

Shared index cache is supported on Windows and Unix Operating Systems.

Calculating the Storage Space Required for Index Cache

The storage space required for the index cache entries of a MediaAgent is estimated between 2 to 3 percentage of the total data protected using the MediaAgent. This recommendation is applicable for most CommCell deployments.

However, if you are interested in an in-depth understanding on the index cache space requirements, you can use the following information.

The estimation of index cache size is based on a number of assumptions, which include the following:

• Average base name (i.e., last level of path) length or the number of characters in the file/folder name (e.g., the file expense_report.xls has a length of 18 characters, the folder jan_sales has a length of 9 characters.)
• Average file size
• Average incremental percentage of the files being backed up
• The length of the backup cycle

Examples:

For file system or file system-like data if the above mentioned assumptions are applied you may require the index cache size to be .72% of the total data, using the following assumptions:

• The average base name size of the files are 24 bytes (UTF-8 encoding, which means non-English language characters are more than one byte each)
• The average file size is 50000 bytes
• On an average, 10% of the data gets backed up by the incremental backups in a cycle
• Using a one-week cycle of 1 Full backup followed by 6 Incremental backups
• Keep 2 backup cycles in the index cache

For database objects, such as Exchange Mailbox this could be significantly larger and hence you may require the index cache size to be 2.5% of the total data, using the following assumptions:

• The average base name size of the files are 90 bytes
• The average file size is 20000 bytes
• On an average, 10% of the data gets backed up by the incremental backups in a cycle
• Using a one-week cycle of 1 Full backup followed by 6 incremental backups
• Keep 2 backup cycles in the index cache

The index size can be much larger, for example, if the following conditions are different:

• Base names are extremely long
• Average file size is much smaller (meaning more files per gigabyte backed up)
• The Create new index on full backup option is not selected for the subclient (but this would tend to mean fewer index versions in the cache)
• The full backup cycle is much longer than 7 backups
• The incremental rate of change is greater than 10%
• The number of cycles in the cache is larger than 2
• Only Differential backups are run instead of the Incremental backups

Note that the above example describes a very simple configuration including uniform requirements across client computers. In reality, client computers can have more than one agent, and the storage space required by each type of data must be added into the total. Different computers can have widely varying storage requirements.

Best Practices for Maintaining Index Cache

As the calculation of index cache is based on several assumptions which could in turn change over a period of time in any given environment, we recommend the following as best practices for maintaining index cache:

• If possible, use a file system that is dedicated to index cache data only (so that non index data does not grow and encroach on the index cache capacity.
• Use a more liberal than a conservative estimate while allocating space for index cache.
• Make sure to consider the space allocated for index cache in the following cases:
  • When clients are added to a MediaAgent
  • When the backup cycle is modified
  • Factors that affect the composition of backups, such as the data size, file names, etc.
• If you are maintaining backward-compatible content indexes generated by a previous software version, additional disk space may be required in the index cache folder. For information, see Books Online for the previous software version.

Index Retention Criteria

The amount of index data that can be cached is based on the following:

• Index retention time in days

  Whenever a backup is initiated, the system removes all the index files which have not been accessed within the specified number of days.

• Index cleanup percent

  Whenever a backup is initiated, the system checks for the available disk space in the disk housing the index cache. If the amount of used disk space exceeds the specified percentage, the system removes index files in the index cache, based on the Least Recently Used (LRU) basis until the disk space is equal to the specified percentage. However, keep in mind that the index retention time will not be taken into consideration during this process.

• Minimum Free Space (MB)

  The system also checks the total amount of free space that must be available at all times in the index cache.

• Free Space Warning for Index Cache Using Alerts

  If the amount of free space falls below the specified amount in the volume in which the Index Cache is stored, the MediaAgent generates an event message and generates the MediaAgents (Disk Space Low) alert, if configured.

  See Available Alerts and Alert Descriptions and Space Check Thresholds for the Software Installation and System Directories for detailed information on setting up the alert.

  The above parameters are not applicable for NetWare MediaAgents. Ensure that you have sufficient disk space on the NetWare volume that hosts the index cache before performing data protection operations.

Other considerations

Audit Trail

Operations performed with this feature are recorded in the Audit Trail. See Audit Trail for more information.

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