Veritas Access Administrator's Guide

File system mount-time memory usage

Mounting a file system on a computer system allocates system memory that is not freed until the file system is unmounted. The amount of memory allocated at mount time is directly proportional to the size of the file system being mounted. The amount of memory that is allocated at mount-time is therefore important information to help determine the system memory requirements for a Veritas Access environment. The mount-time memory requirement is different if you expect to mount a total of 1 PB of storage or 2 PBs of storage. The number of files currently in the file system does not affect the amount of memory allocated at mount-time. The amount of memory allocated at mount-time is also inversely proportional to the file system block size.

The information required to determine the amount of memory allocated at mount time is the total size of all the file systems that are mounted on the same computer system at the same time and the block size of each file system.

The amount of memory allocated at mount time can therefore be estimated by obtaining the total size of all the file systems that are mounted on a system according to the file system block size. So four totals in all, one for each file system block size of 1 KB, 2 KB, 4 KB, and 8 KB.

The mount-time memory requirement is therefore:

((a*w) + (b*x) + (c*y) + (d*z))

A file system using a 1 KB block size (the smallest file system block size) allocates approximately eight times more memory at mount time than a file system of the same size using a 8 KB block size (the largest file system block size). For this reason, the Veritas Access file system defaults to a block size of 8 KB if a block size is not specified when creating a file system.

Some customers might like to create small file systems using a 1 KB file system block size and subsequently grow the file system size significantly, as the file system block size cannot be changed after the file system is created. This procedure can result in very large file systems using a 1 KB block size that can result in an unexpectedly large allocation of system memory at mount time.

A Clustered File System (CFS) primary mount requires slightly more memory allocated at mount-time than a CFS secondary. The performance team recommends that the memory utilization of a CFS primary be used as the guideline for calculating the file system mount-time memory requirement.

Figure: Mount-time memory consumption for 32 TB file systems provides the guideline for the system memory utilization at mount time.

Figure: Mount-time memory consumption for 32 TB file systems