The Default value 1536KB for volpagemod_max_memsz is acceptable for most volume configurations. Although larger volume configurations will require the administrator/user to adjust this tunable in order to allow the vxsnap prepare command to succeed.

Per the vxtune Man Pages:

volpagemod_max_memsz

Maximum memory, measured in kilobytes, that is allocated for caching FastResync and cache object metadata by the
instant snapshots feature of VxVM. This tunable has a default value of 1536KB (1.5MB) of physical memory, which
is sufficient if the largest volume has a length of 256GB and a default region size of 64KB.

Check the FastResync region size of Data Change Object (DCO) or Cache Object with the following command:
# vxprint -g -F%regionsz

The value displayed is in 512-byte sectors. Refer to the vxsnap manual page for details.


Use this formula to calculate the minimum value of volpagemod_max_memsz:

volpagemod_max_memsz >= max(vol_size/region_size)*3/8

where the ratio of volume size to region size is the largest value for all the volumes on the system. You do not
need to consider the total size of all the volumes.

In the above formula, vol_size is in MB and the FastResync region_size is in KB.  The resulting value is the volpagemod_max_memsz in KB.

For example:
A 1TB volume with a region size of 64KB would require that volpagemod_max_memsz be set to at least
6144KB. A 512GB volume with a region size of 32KB and a 2TB volume with a region size of 128KB would also require
that volpagemod_max_memsz be set to at least 6144KB. Allocating more than the minimum amount of memory that is
required may provide better performance.

(1024*1024/64)* 3/8 = 6144
(512*1024/32)* 3/8 = 6144
(2048*1024/128)* 3/8 = 6144

The memory allocated for this cache is exclusively dedicated to it. It is not available for other processes or
applications.

Setting the value of volpagemod_max_memsz below 512KB fails if cache objects or volumes that have been prepared for
instant snapshot operations are present on the system.

If you do not use the FastResync or DRL features that are implemented using a version 20 DCO volume, the value of
volpagemod_max_memsz can be set to 0. However, if you subsequently decide to enable these features, you can use
vxtune to change the value to a more appropriate one.


The volpagemod_max_memsz value can be modified on the fly and used immediately with the command vxtune.
Note Values set with vxtune are not persistent across reboots.

Example
vxtune volpagemod_max_memsz 5M

To make the volpagemod_max_memsz persistent across reboots use the vxvoltune command.

Note the vxvoltune command values are in kilobytes.

Example:
vxvoltune volpagemod_max_memsz 5120

For additional tunable's and definitions see the man pages for vxvoltune and vxtune.