Sizing the system storage pool

This topic helps you determine the amount of storage that is required for the system storage pool.

Context

SAN File System stores the system and file metadata on the system storage pool. The total space required to store the metadata depends on the total number of objects in each fileset and the exact mix of the objects, such as directories, files, and symbolic links.

Generally, a local file systems requires approximately 3 MB of metadata for every 100 MB of actual data that a file system can hold, which is about three percent. However, SAN File System generally requires approximately 5 MB of metadata for every 100 MB of actual data, which is about five percent. If there are FlashCopy® images to be maintained, plan on having at least an extra four percent for every FlashCopy image to be maintained; that is, four percent of the space of the fileset or filesets used with FlashCopy images. As a rough guide, using five percent of the current file system space (which includes metadata) for the metadata storage pool will typically provide adequate space.

The System storage pool worksheet provides calculations for more accurate estimate of the space requirements for the system storage pool.

The total space required depends primarily on the amount of the data and the exact mix of objects in each fileset. For each object, there are two components to the storage requirement: a variable component and a fixed component. The variable component depends on the size of the object and the name length. The following table has the figures to be used for the calculations.

Object type Fixed storage Variable component based on object size Variable component based on name length
Files 560 bytes 415 bytes/MB 3 * name length
Directories 560 bytes 3 * name length
Symbolic links 560 bytes 3 * name length
Hard links 90 bytes 3 * name length
FIFO objects 560 bytes 3 * name length

The fixed component requires 128 MB for each metadata server.

Note: SAN File System supports the FlashCopy function, which performs a copy on write (COW) when updates occur. This require additional space in the system storage pool. Because the design is based on COW, the additional space required depends on the number of times a FlashCopy image has been taken and the percent of data that has been modified since the last FlashCopy image. For example, if a fileset requires 100 MB for its metadata component and 80 MB for its data component, and if 50 percent of the data has been modified since the last FlashCopy image was created, the additional space required would be 100 MB + 50% of 80 MB, which equals 140 MB.

Parent topic: Planning the cluster configuration

Related information
System storage pool worksheet

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